Resina

RESPIRATION

THE continued existence of an amoeba in a pool of water, or of a white blood-cell in the liquor sanguinis, depends upon a continual interchange of substances between the organism and the surrounding medium. The substances in question pass from the medium into the organism in a certain chemical form; they pass from the organism into the surrounding medium with their chemical form modified. Regarding merely the initial and final stages of this reconstitution of chemical form, we may speak of it as being of the nature of an oxidation. This view does not profess to be comprehensive; nevertheless, it is true that the metabolic and anabolic processes of cells, taken as a whole, resemble combustion at least to this extent that oxygen and oxidizable carbon take part in them, and that carbon dioxide results from them. Partly as a matter of tradition, and partly as a matter of convenience, physiologists have described the introduction of oxygen into the organism and the emission of carbon dioxide from it as the comρlemental portions of one process of respiration. Although such a combined consideration is not strictly philosophical, inasmuch as it leaves out of view the introduction of the carbon into the organism, yet it is extremely convenient because the two processes referred to do, in all classes of the animal kingdom from the highest to the lowest, involve the same organs and tissues in their performance. Respiration may therefore be defined as the aggregate of those processes which are concerned in the introduction of oxygen into the system and the escape of carbon dioxide from it.

Respiration in such an organism as an amoeba is extremely simple. The medium surrounding it contains a practically unlimited supply of oxygen, and is so vast that the carbon dioxide put out into the medium is quickly removed from the neighbourhood of the organism. The interchange of oxygen and carbon dioxide takes place at the surface of the organism, so far as we know, continuously. In the higher animals, constituted as they are of a vast number of structural units accurately packed together, each resembling more or less in its physiological instincts the unicellular amoeba, respiration presents a much more intricate problem. The fine interstices which exist between the structural elements do indeed contain a small quantity of a fluid medium which serves the function of the water bathing the amoeba; but the store of oxygen in the medium would speedily become exhausted, and the emitted carbon dioxide would quickly accumulate to a dangerous degree, if the medium were not continually restored to its original purity. This revival is effected by the circulating blood which is brought by its capillaries into the neighbourhood of the remotest cells of the body. But even the mass of the blood is small compared with that of the cells it nourishes; unless it be itself purified and restored in turn the interstitial juices which depend upon it for their purification must soon fail to support the respiration of the cells. Such restoration of the blood takes place in certain organs called lungs or gills, where the blood acquires a fresh store of oxygen and parts with its excess of carbon dioxide.

Respiration in the higher animals may therefore be divided into (1) internal respiration, or the interchange of oxygen and carbon dioxide between the cells of the body and the fluid drenching them, and (2) external respiration, or the gaseous interchange taking place in the special respiratory organs (lungs, gills). The first is really a part of Nutrition (g.v.} ; the second, or respiration proper, is the subject of the present article.

It will be evident on reflection that the process of respiration naturally falls to be described under two divisions, the first of which is concerned with the movements of the chest in inspiration and expiration and the manner in which they are brought about, and the second with the interchange of gases which takes place between the blood and the air in the lungs.

The Movements op Respiration. Structure of the Organs of Respiration.

In order to understand the movements it is necessary first to know the structure of the air passages and thorax.

Anatomy of the Air Passages.—The essential organs of respiration consist of an air tube called the trachea, communicating at its upper end with the mouth and bifurcating below into two bronchi, one on the right hand and one on the left. Each bronchus divides and subdivides, diminishing in calibre at every division until a diameter of about 1 mm. is attained; such a diminutive bronchial tube is called a bronchiole. Every bronchiole is a cylindrical tube which divides dichotomously and rapidly several times, and finally terminates in irregular alveolar passages. The sides of the alveolar passages, and of the subdividing bronchioles in less abundance, arc studded with hemispherical dilatations called air cells or alveoli. The terminal portion of an alveolar passage, with its air cells, is sometimes spoken of as an iηfundibιdnιm—a term we may wisely forget at once, since it points to a distinction where no essential difference exists. The alveoli cluster in great abundance about the alveolar passages, and, although we have spoken of them as hemispherical, they are in reality made polygonal by mutual compression. They are surrounded by connective tissue of a very elastic quality, which gives to their delicate walls a firm support, and is so disposed about them that all the alveoli derived from one bronchiole are more closely knit together than they are bound to those of a neighbouring bronchiole; hence we may speak of a bronchiole with the assemblage of its members as a lobule, a term of peculiar importance since it will be evident on reflection that each lobule contains all the essential parts of a lung—is in fact a lung in miniature. By connecting tissue the lobules are compacted to form lobes, of which two on the left side and three on the right go to make up the respective lungs.

The trachea and larger bronchi are composed of a series of superposed crescentic pieces or imperfect rings of hyaline cartilage which with a tough fibrous and elastic membrane form a tube; in the trachea the incomplete cartilage-rings are so placed that their hiatuses are at the back. In the bronchial tubes, especially the smaller ones', the cartilages are fewer, and less regular in shape, being in fact mere nodules in the bronchial walls. In the bronchioles there is no cartilage. There is an external fibrous and elastic layer investing the trachea and bronchi; and stretching from tip to tip of the imperfect cartilaginous lings are bands of involuntary muscular fibres. The function of the cartilages is doubtless to maintain the patency of the primary air passages by resisting external compression; that of the muscular fibres is probably to resist unusual distension of the tubes, as in the strain of coughing, &c. The mucous membrane of the trachea consists of a mτιcosa of fine connective tissue, mixed with some tissue of the adenoid sort, and with elastic fibres, disposed in longitudinal bundles. Beneath the mucosa is the slight submucosa, which supports large blood-vessels, lymphatics, and mucous glands, and unites the mucosa to the cartilages. Above the mucosa lies the basement membrane, supporting a stratified epithelium, the upper cells of which are columnar and ciliated and may include amongst them the well-known goblet-cells, while the deeper layers are squamous and capable of regenerating the upper stratum if this be lost. The epithelium is pierced by the ducts of glands. Blood-capillaries, lymphatics, and fine nerves have all been shown to exist in the tracheal mucosa.

The same structural elements as are found in the trachea continue to be found in all the series of bronchial tubes above 1 mm. in diameter; they are arranged in much the same order, save that the muscular fibres are relatively increased and that they surround the whole tube (within the zone occupied by the cartilaginous nodules) somewhat like the tunica media of an artery. The mucous membrane is still ciliated, but its surface-cells are squat. On the other hand the bronchioles and alveolar passages have a different structure: cartilage and glands are no longer found; the epithelium is flat and ciliated; and the muscular fibres gradually thin out on the walls of the alveolar passages.

It is, however, the air-cells which have most interest for the physiologist. They are about rτ⅛th inch in diameter, and their walls are made up of a delicate film-like basement membrane, on tho outside of which are numerous elastic fibres. Elastic connective tissue intervenes between neighbouring alveoli, and mingled with it are not a few involuntary muscular fibres. The alveoli are lined with flat cells in a single layer, some cells being large, clear, and polygonal, while others are small and granular, and are found singly or in groups of two or three inserted between the edges of the clear sort. The cells are united together by cement in which fine holes are sometimes to be seen; through these holes, or pscudo-stomata, migratory leucocytes can make their way into and out of the alveoli.

The blood-vessels of the lung are of two sorts, nutritive and functional, i.e., concerned in the function of the organs. The former are called bronchial, and arise from the aorta or intercostal arteries. They serve to nourish the tissues of the lung, and the blood they contain finds its way, in part into the bronchial veins and thence into the vena azygos, intercostal vein, or superior vena cava, and in part into veins of the functional system. The latter system of vessels consists of the pulmonary arteries, which arise in the right ventricle and run through the lung substance pari passu with the bronchial tubes to the lobules. Here they branch into a dense network of capillaries which spread over the outside of the alveoli, enmeshing them so tightly ns to indent their walls. From the capillaries the blood flows into pulmonary venules which run together to form the large pulmonary veins, which open into the left auricle. The pulmonary venules may anastomose freely, but the arteries never do so. The veins possess no valves.

The lymphatic vessels of the lung abound in all parts, but are usually described as having a threefold distribution,—(a) in the layer of tissue beneath the membrane investing the whole lung, (fi) in the perivascular tissue, and (c) in the peribronchial tissue. When fine carbon particles are introduced into the alveoli of the lung they find their way with the greatest ease into the interalveolar tissues, and finally come to lie in the three positions just referred to, as may be demonstrated in the lung of any coal miner. The lymphatics of the membrane investing the lung communicate with the free surface of that membrane by means of openings not unlike the stomata of leaves.

The lungs, with the heart and great vessels, are the chief organs contained in the thorax, or that division of the great body cavity which lies above the diaphragm. Each lung is invested with a membrane called pleura, which plays a most important part in the mechanism of respiration.

The Pleural Membrane.—Let us imagine a bag shaped like an hour glass, and let us suppose one half of this bag to be packed up small and pushed through the constricted portion of the hour glass into the interior of the other half. We should then have a bag shaped like one half of an hour glass but provided with double walls —a more or less globular double-walled bag with a narrow opening into it. If further we imagine the interior of this double-walled bag (no< the interior of the hour glass) to be entirely filled with one lung, we shall have an exact conception of the relationship of the lung to the pleura. The lung so covered is placed in its proper half of the thorax, which in circumstances of health it accurately fills. The outer layer of the double-walled bag is closely glued to the inner wall of the thorax. The inner layer is as firmly adherent to the surface of the lung. The space between the double walls, i.e., the interior of the original hour glass, is called the pleural cavity; it is vacuous in health, being moistened by a mere trace of fluid. The substance of the pleural membrane is mainly connective tissue; and the interior of the pleural cavity is lined with a single layer of flat epithelial cells exhibiting the stomata already referred to.

The Thorax.—The chest or thorax is formed by the dorsal section of the spinal column behind, with the ribs that spring from it on each side, and the sternum, which lies between the ends of the ribs, in front. The dorsal spine is bowed, so that its concavity looks forward. The ribs, speaking generally, are bowed with their concavity turned towards the interior of the chest; and if we consider the plane of each rib, f.e., that plane in which the arched rib would (approximately) lie flat, we shall find that it declines from the horizontal in a twofold manner—first the rib-plane slopes from behind downwards and forwards, and secondly it slopes on each side from the mesial plane of the body downwards and outwards. The ribs 1-7 are connected with the sternum by means of pieces of cartilage which really form the anterior portion of each rib arch; these ribs are called “true”; the eighth, ninth, and tenth ribs are united by cartilage, not to the sternum, but to the cartilages of the seventh, eighth, and ninth ribs respectively; these ribs are called ‘ ‘ false”; the eleventh and twelfth ribs are called ‘ ‘ floating” because they are unattached anteriorly. Each rib has a head, by which it is joined to the vertebral bodies constituting the spinal column; a tubercle or shoulder at a little distance away from the head, by which in all cases, except those of the tenth and eleventh ribs, it is joined to the transverse process of a vertebra; an angle or rough line a little beyond the tubercle, where the rib, rather suddenly, begins to sweep forwards; and a neck, the part intervening between the head and tubercle. The space between the ribs is filled up by two layers of muscles called intercostal—an outer or superficial layer, whose fibres run from above downwards and forwards, and a deeper or internal layer, whose fibres cross those of the former. The outer layer is not found between the costal cartilages in front, nor the inner layer between the costal necks behind. The upper opening of the thorax is filled by the windpipe blood-vessels and other structures passing into or out of the thorax. The floor of the thorax is formed by

The Diaphragm.—This consists of a thin arched muscular partition, whose fibres spring from the edge of the lower opening of the thorax and converge towards a sheet of tendon in the. centre, which is shaped somewhat like a trefoil leaf. We may group the muscular fibres of the diaphragm according to the quadrant from which they spring :— (1) a vertebral portion, whose fibres stretch down to be attached in two well-marked columns or pillars to the bodies of some of the lumber vertebrae and by tendinous arches to the transverse processes of the first lumbar vertebra and the twelfth rib; (2) a sternal portion, which springsïrom the back of the tip of the sternum, and from the sheath of the rectus abdominis muscles below it; (3) and (4) two lateral or costal portions, which spring from the lower edge of the thorax all round from the tip of the twelfth rib to the junction between the sixth and seventh costal cartilage where the sternal portion begins. The whole diaphragm forms a dome or cu pola projecting so far into the thorax that the lateral vertical portions of the dome lie in close apposition to the walls of the thorax, as is shown in fig. 2. The top of the dome is somewhat flat and the right moiety of the top is on a higher level than the left, the highest point corresponding with the level of the junction of the right fifth rib with the sternum.

Other Muscles of Respiration.—The ribs are movable in the sense that each rib plane, which has been described as declining in two ways from the horizontal plane, may be made to approach the horizontal, and may afterwards return to its original position. To accomplish these movements various muscles are provided, the exact position of which need not be very fully described. Suffice it to say that in general they arise from the vertebral axis, or from some extra-thoracic fixed point, and take hold of the movable parts of the thorax in such a manner that they can pull them up or pull them down. Such muscles are the following (enumerated without reference to their function as inspiratory or expiratory muscles; particular descriptions of them must be sought in the article Anatomy) :—levatores costarum; the three scalene muscles; the sterno-cleidomastoid; serratus posticus superior; parts of the great erector spinae muscle; possibly serratus magnus; pectoralis minor and major in certain positions of the arm; obli-quus externus; obliquus internus; transversalis abdominis; triangularis sterni; serratus posticus inferior; quadratus lumborum. In addition to these muscles many others may lend occasional aid in respiration by fixing points, such as the scapula, otherwise too freely movable to act as a point cl'appui; such are trapezius, latis-simus dorsi, rhomboidei, and levator anguli scapuli.

Dimensions of the Thorax.—The circumference of the chest just below the level of the arms is about 34∣ inches in men, and 32 inches in women. At the level of the tip of the sternum it is 32 and 30⅛ inches respectively. The measurement from clavicle to lower edge of thorax varies very much in different cases. The transverse diameter above the nipple is about 10 to 10∣ inches in men, and about 9⅛ to 9f inches in women. The antero-posterior diameter, measured from the spines of the vertebrae behind to the surface of the chest in front is in the upper part of the chest about 6⅜ inches, and in the lower 7⅜ inches. The right half of the chest is generally somewhat larger than the left, because its muscles are usually better developed. Instruments .for measuring the exact circumference of the chest at a given level are called cyrtometers; the best of these is a strip of lead which can be laid along the side of the chest from the spine round to the sternum, and which is pliable enough to follow the inequalities of the chest wall, yet resistent enough to keep the shape imparted to it.

For other figures illustrating the structure of thorax and lungs, see Anatomy.

The Physiological Actions of the Respiratory Organs.

The organs above described perform during the whole of life certain movements. The commonest are the movements of ordinary quiet respiration, but we must include under the head of physiological actions, as distinguished from those provoked by disease, other movements, viz., forced respiration, such as accompanies strenuous muscular exertion, and those modifications of the respiratory act called coughing, hawking, sneezing, snoring, crying, sighing, laughing, yawning, and hiccoughing.

Normal Respiration.— If the naked body of a person asleep or in perfect inactivity be carefully watched, it will be found that the anterior and lateral walls of the chest move rhythmically up and down, while air passes into and out of the nostrils (and mouth also if this be open) in correspondence with the movement. If we look more closely we shall find that with every uprising of the chest walls the membranous intercostal portions sink slightly as if sucked in, while at the same time the flexible walls of the abdomen bulge as if protruded by some internal force. If respiration be in the slightest degree hurried these motions become so marked as to escape the attention of no one. The elevation of the chest walls is called inspiration, their depression expiration. Inspiration is slightly shorter than expiration, and usually there is a slight pause or momentary inaction of the chest between expiration and the following inspiration. Apparatuses for measuring the excursion of a given point of the chest wall during respiration are called thoracometers or stethometers. Apparatuses for recording the movements of the chest are called stethographs or pneumographs. Frequency of Respiration.— The frequency of respiration during perfect rest of the body is 16 to 24 per minute, the pulse rate being usually four times the rate of respiration; but the respiratory rhythm varies in various conditions of life. The following are the means of many observations made by Quetelet: at the age of one year the number of respirations is 44 per minute; at 5 years, 26; from 15 to 20 years, 20; from 25 to 30, 16; from 30 to 50, 18 , l. Muscular exertion always increases the frequency of respiration. The higher the temperature of the environment the more frequent is the respiration. Bert has shown that with higher atmospheric pressures than the normal the frequency of respiration is diminished while the depth of each inspiration is increased. The frequency of respiration diminishes until dinner time, reaches its maximum within an hour of feeding, and thereafter falls again; if dinner is omitted, no rise of frequency occurs. The respiratory act can be interrupted at any stage, reversed, quickened, slowed, and variously modified at will, so long as respiration is not stopped entirely for more than a short space of time; beyond this limit the will is incapable of suppressing respiration.

Depth of Respiration.— The depth of respiration is measured by the quantity of air inspired or expired in the act; but the deepest expiration possible does not suffice to expel all the air the lungs contain. The following measurements have been ascertained, and are here classified according to the convenient terminology proposed by Hutchinson. (1) Residual air, the volume of air remaining in the chest after the most complete expiratory effort, ranges from 100 to 130 cubic inches. (2) Reserve or supplemental air, the volume of air which can be expelled from the chest after an ordinary quiet expiration, measures about 100 cubic inches. (3) Tidal air, the volume of air taken in and given out at each ordinary respiration may be stated at about 20 cubic inches. (4) Complemental air, the volume of air that can be forcibly inspired over and above what is taken in at a normal inspiration, ranges from about 100 to 130 cubic inches. By vital capacity, which once had an exaggerated importance attached to it, is meant the quantity of air which can be expelled from the lungs by the deepest possible expiration after the deepest possible inspiration; it obviously includes the complemental, tidal, and reserve airs, and measures about 230 cubic inches in the Englishman of average height, i.e., 5 feet 8 inches (Hutchinson). It varies according to the height, body weight, age, sex, position of the body, and condition as to health of the subject of observation.

Vital capacity is estimated by means of a spirometer, a graduated gasometer into which air may be blown from the lungs.

The residual air, which for obvious reasons cannot be actually measured, may be estimated in the following way (Harless, Gréhant). At the end of ordinary expiration, apply the mouth to a mouth-piece communicating with a vessel filled with pure hydrogen, and breathe into and out of this vessel half a dozen times—until, in fact, there is reason to suppose that the air in the lungs at the time of the experiment has become evenly mixed with hydrogen. Then ascertain by analysis the proportion of hydrogen to expired air in the vessel and estimate the amount of the air which the lungs contained by the following formula υ : V +v≈ρ : 100; r(100- p).

P where V=volume of air in the lungs at the time of experiment, υ= volume of the vessel containing hydrogen, p= proportion of air to hydrogen in the vessel at the end of the experiment. V, then, is the volume of air in the lungs after an ordinary expiration; that is, it includes the residual and the reserve air; if we subtract from this the amount of reserve air ascertained by direct measurement, we obtain the 100-130 cubic inches which Hutchinson arrived at by a study of the dead body.

Volume of Respiration.— It is clear that the ventilation of the lungs in ordinary breathing does not merely depend on the quantity of air inspired at each breath, but also on the number of inspirations in a given time. If these two values be multiplied together we get what might be called the volume of respiration (^Athmungsgrösse, Rosenthal) in contradistinction to depth of respiration and frequency of respiration. Various instruments have been devised to measure the volume of respiration, all more or less faulty for the reason that they compel respiration under somewhat abnormal conditions (Rosenthal, Gad, Panum, Hering). From the data obtained we may conclude that the respiratory volume per minute in man is about 366 cubic inches (6000 cubic centim.). In connexion with this subject it may be stated that, after a single ordinary inspiration of hydrogen gas, 6-10 respirations of ordinary air must occur before the expired air ceases to contain some trace of hydrogen.

Types of Respiration.— The visible characters of respiration in man vary considerably according to age and sex. In men, while there is a moderate degree of upheaval of the chest, there is a considerable although not preponderating degree of excursion of the abdominal walls. In women the chest movements are decidedly most marked, the excursion of the abdominal walls being comparatively small. Hence we may distinguish two types of respiration, the costal and the abdominal, according to the preponderance of movement of one or the other part of the body wall. In forced respiration the type is costal in both sexes, and so it is also in sleep. The cause of this difference between men and women has been variously ascribed (a) to constriction of the chest by corsets in women, (δ) to a natural adaptation to the needs of childbearing in women, and (c) to the greater relative flexibility of the ribs in women permitting a wider displacement under the action of he inspiratory muscles.

Certain Concomitants of Normal Respiration.— If the ear be placed against the chest wall during ordinary respiration we can hear with every inspiration a sighing or rustling sound, called “vesicular,” which is probably caused by the expansion of the air vesicles; and with every expiration a sound of a much softer sighing character. In children the inspiratory rustle is sharper and more pronounced than in adults. If a stethoscope be placed over the trachea, bronchi, or larynx, so that the sounds generated there may be separately communicated to the ear, there is heard a harsh to-and-fro sound during inspiration and expiration which has received the name of “bronchial.”

In healthy breathing the mouth should be closed and the ingoing current should all pass through the nose. When this happens the nostrils become slightly expanded with each inspiration, probably by the action of the M.

dilatatores naris. In some people this movement is hardly perceptible unless breathing be heavy or laboured. As the air passes at the back of the throat behind the soft palate it causes the velum to wave very gently in the current; this is a purely passive movement. If we look at the glottis or opening into the larynx during respiration, as we may readily do with the help of a small mirror held at the back of the throat, we may notice that the glottis is wide open during inspiration and that it becomes narrower by the approximation of the vocal chords during expiration. This alteration is produced by the action of the laryngeal muscles. Like the movements of the nostril, those of the larynx are almost imperceptible in some people during ordinary breathing, but are very well marked in all during forced respiration.

The Mechanics of Respiration.

In the description of the anatomy of the thorax it has been shown that the thorax is practically a closed box entirely filled by the lungs, heart, and other structures contained within it. If we were to freeze a dead body until all its tissues were rigid, and then were to remove a portion of the chest wall, we should observe that every corner of the thorax is accurately filled by some portion or other of its contents. If we were to perform the same operation of removing a part of the chest wall in a body not first frozen we should find, on the other hand, that the contents of the thorax are not by any means in such circumstances bulky enough to fill up the space provided for them. If we were to measure the organs carefully we should find that those which are hollow and whose cavities communicate with the regions outside the thorax are all larger in the frozen corpse than in that which was not frozen. In other words, the organs in the thorax are distended somewhat in order that they may completely fill the chest cavity; and the nature of this curious and important condition may best be illustrated by the simple diagrams, figs. 3 and 4 (from Hermann’s Physiologie des Menschen),— where t is the trachea, l the lung, υ the auricle of the heart, k the ventricle, i an intercostal space with its flexible membranous covering. When the interior of the vessel is rendered vacuous by exhaustion through the tube o, the walls of the lungs and heart are expanded until the limits of the containing vessel are accurately filled, while all flexible portions of the walls of the vessel (corresponding to the intercostal membranes and the diaphragm of the thorax) are sucked inwards.

From this description it follows that the lungs, even when the thorax is most contracted, are constantly overdistended, and that, when the cause of this over-distension is removed, the lungs, being elastic, collapse. It further follows that if the thorax is dilated, the flexible hollow organs it contains must perforce be still more distended, —a distension which in the case of the lungs is followed by an indrawing of air through the trachea in all cases where the trachea is open. Thus, as the act of respiration is primarily a dilatation of the thorax, the part played by the lungs is, as Galen knew, a purely passive one.

How is dilatation of the thorax effected? It has been pointed out that the rib-planes decline from the horizontal in two directions, viz., from behind forwards, and from the antero-posterior mesial plane outwards; a glance at fig. 5 will make this double sloping clear to the reader. It has, moreover, been explained that the diaphragm arches upwards into the thorax in such a manner that the lateral parts of the arch are vertical and in contact with the inner face of the tho racic -walls (see fig. 2). This being the structure of the thorax, the enlargement of its cavity is brought about

(1) by raising the rib-

planes until they ap proach the horizontal, and (2) by depressing the diaphragm and making its rounded dome more cone-like in outline. A moment’s consideration will show how these actions en large the boundaries of the thorax. (ct)

When the postero-an-

terior slope of the rib-

planes is diminished by the raising of the anterior ends of the ribs, the whole ster num is thrust upwards and forwards, and the anteroposterior diameter of the thorax is increased, (Z>) When the lateral slope of the rib-planes is diminished by the ribs being moved upwards about an axis passing through their sternal and vertebral extremities, it is evident that the lateral diameter of the thorax must be increased, (c) When the muscular portion of the diaphragm contracts, the curves of its dome-like shape are straightened, the whole diaphragm comes to look more conical on section, and the apposition of its lateral parts to the inner surface of the thorax is destroyed; the two apposed surfaces are drawn apart much as the leaves of a book might be, and a space is formed between them, into which some portion of the lung slips, (c?) When the diaphragm descends it draws with it the whole contents of the thorax; inasmuch as the contents as a whole are conical in shape with the apex upward and are fitted into the conical space of the thoracic cavity, it is clear that the descent of the contents will tend to create a space between them and the thoracic walls; for each stratum of lung, &c., which is adapted to fit a certain level of thorax, will thereby be brought into a lower and (as the thorax is conical) a more spacious level. Hence the descent of the diaphragm causes a much greater enlargement of the thorax than is measured by the mere elongation of the vertical diameter. In this manner the thorax is distended and air is drawn into the lungs. The contraction of the thorax in expiration is brought about by the return of the ribs and diaphragm to their original position of rest.

We must now explain how the respiratory muscles effect these movements.

How the Inspiratory Movements are Produced.—The Rib Movements.— These are caused by the contraction of muscles which are fixed either to the central axis of the body (including under that term the head and vertebral column) or to some point rendered sufficiently stable for the purpose by the action of other adjuvant muscles. Thus the M. levatores costarum arise from the transverse processes of the 7th cervical and eleven upper dorsal vertebrae, and are attached to the ribs below in series; the M. scaleni spring from the cervical vertebrae, and are attached to the anterior parts of the first and second ribs; he M. sterno-cleido-mastoidei arise from the side and back of the skull, and are inserted into the upper part of the sternum and the clavicle; the M. pectoralis minor arises from the coracoid process of the scapula, and is inserted into the anterior ends of some of the ribs; the M. serratus posticus superior arises from certain of the cervical and dorsal vertebrae, and is inserted into the posterior part of certain of the ribs; the M. cervicalis ascendens (part of the M. erector spinae) arises from certain of the cervical vertebrae, and is inserted into the posterior part of certain ribs. The M. serratus magnus and the M. pectoralis major, which are affixed on the one hand to the upper arm and to the scapula respectively and on the other to the ribs and to the sternum respectively, may in certain elevated positions of the arm and shoulder act as in spiratory muscles. When all these muscles contract, the ribs are raised in the twofold way already de scribed, some pulling up the anterior ends of the ribs, and others causing the arched ribs to rotate about an axis passing through their vertebral and sternal joints.

In addition to the muscles just enumerated the M. intercostales externi are undoubtedly inspiratory muscles. Every external intercostal muscular fibre between a pair of ribs must, when it contracts, of necessity raise both ribs, as is clearly shown by the'accompanying diagram (fig. 6). Here a'd must be shorter than ab, for if BAα=sc, then

αδ 2 =AB 2 + (Bδ - Aα) 2 + 2AB (Bδ - ∆α) cos x; hence ab will be larger the smaller the angle x, for the cosine increases as the angle diminishes.

By a similar geometrical treatment of the question it may be shown that the in-tefnal intercostal muscles when they contract must of necessity depress both the ribs to which they are attached. If the angle BAc'=x (fig. 7), then c'<Z' 2 =AB 2 + (Ac'- B<Z') 2 - 2AB(Ac ’ -B<Z') cos x; hence e'd' will be larger the larger the angle x.

The case, however, is not so clear with reference to the anterior portions of the internal intercostals which lie between the cartilages; for it is evident that these fibres have the same direction with regard to the sternum as an axis as the external intercostals have with regard to the vertebral column as an axis; that is to say, the geometrical diagram in fig. 6 applies to the intercartilaginous internal intercostals as perfectly as it does to the interosseous parts of the external intercostals, the inference being that the intercartilaginous internal intercostals tend to elevate the pair of ribs between which they stretch. The geometrical argument is, however, overborne by physiological experiment: Martin and Hartwell have observed in the dog and the cat that the internal intercostals throughout their whole extent contract (not synchronously) but alternately with the diaphragm; hence we must conclude that their function throughout is not inspiratory like that of the diaphragm, but expiratory.

The Movements of the Diaphragm.— The muscular fibres of the diaphragm are arranged in a radial manner, or more strictly speaking in a manner like the lines of longitude on a terrestrial globe. The central tendon of the diaphragm corresponds to the pole of such a globe. The contraction of the fibres is expended on straightening the longitudinal curves rather than on pulling down the central tendon to a lower level; in fact the central tendon moves very little in ordinary respiration.

How the Expiratory Movements are Produced.— The action of inspiration disturbs many organs from the position of rest into which gravity and their own physical properties have thrown them. The ribs and sternum are raised from the position of lowest level; the elastic costal cartilages are twisted; the elastic lungs are put upon the stretch; the abdominal organs, themselves elastic, are compressed and thrust against the elastic walls of the belly, causing these to bulge outwards. In short the very act of inspiration stores up, as it were, in sundry ways the forces which make for expiration. As soon as the inspiratory muscles cease to act these forces come into play, and the position of rest or equilibrium is regained. It is very doubtful whether any special expiratory muscles are called into action during ordinary respiration. The internal intercostals may in man be exercised in ordinary expiration (although they are certainly not so exercised in the dog and the cat); but in laboured expiration many muscles assist in the expulsive effort. The muscles forming the belly walls contract and force the abdominal contents against the relaxed diaphragm in such a manner as to drive it farther and farther into the thorax. At the same time by their attachment to the lower edge of the thorax these same muscles pull down the ribs and sternum. The M. triangularis sterni, which arises from the back or thoracic aspect of the sternum and lower costal cartilages and is inserted into the costal cartilages higher up, can obviously depress the ribs. So also can the M. serratus posticus inferior, which arises from the thick fascia of the loins and is inserted into the last four ribs. So also can the M. quadratus lumborum, which springs from the pelvis and is attached to the last rib. Indeed there is hardly a muscle of the body but may be called into play during extremely laboured respiration, either because it acts on the chest, or because it serves to steady some part and give a better purchase for the action of direct respiratory muscles.

Certain Abnormal Forms of Respiration. Coughing.— There is first a deep inspiration followed by closure of the glottis. Then follows a violent expiratory effort which bursts open the glottis and drives the air out of thé lungs in a blast which carries away any light irritating matter it may meet with. The act is commonly involuntary, but may be imitated exactly by a voluntary efibrt.

Hawking, or Clearing the Throat.— In this act a current of air is driven from the lungs and forced through the narrow space betλveen the root of the tongue and the depressed soft palate. This action can only be caused voluntarily.

Sneezing.— There is first an inspiration which is often unusually rapid; then follows a sudden expiration and the blast is directed through the nose. The glottis remains open all the time. The act is generally involuntary, but may be more or less successfully imitated by a voluntary effort.

Snoring is caused by unusually steady and prolonged inspirations and expirations through the open mouth,—the soft palate and uvula being set vibrating by the currents of air.

Crying consists of short deep inspirations and prolonged expirations with the glottis partially closed. Long-continued crying leads to sobbing, in which sudden spasmodic contractions of the diaphragm cause sudden inspirations and inspiratory sounds generated in larynx and pharynx.

Sighing is a sudden and prolpnged inspiration following an unusually long pause after the last expiration.

Laughing is caused by a series of short expiratory blasts which provoke a clear sound from the vocal chords kept tense for the purpose, and at the same time other inarticulate but very characteristic sounds from the vibrating structures of the larynx and pharynx. The face has a characteristic expression. This act is essentially involuntary, and often is beyond control; it can only be imitated very imperfectly.

Yawning is a long deep inspiration followed by a shorter expiration, the mouth, fauces, and glottis being kept open in a characteristic fashion. It is involuntary, but may be imitated.

Hiccough is really an inspiration suddenly checked by closure of the glottis; the inspiration is due to a spasmodic contraction of the diaphragm. The closure of the glottis generally leads to a characteristic so(ind.

Innervation of the Respiratory Movements.

The respiratory actions are seen from the above description to be very complicated: their regular performance depends upon the coordination of a great number of factors; and, inasmuch as the respiratory movements do not happen invariably after one stereotyped pattern, but admit of various modifications, such as sighing, hiccoughing, &c., we must infer that the coordination is such as to admit of corresponding variations. The coordination of the inspiratory and expiratory movements is brought about through the nervous system, the connexion of which with the organs of respiration already described must now be stated.

Speaking very generally, there is a nervous centre in the cerebro-spinal axis from which certain nerve fibres proceed to the muscles of respiration; these are called the efferent or centrifugal fibres. There are other fibres which run from the peripheral parts of the body towards the same centre and exert a control over it; these are called afferent or centripetal. The impulses to movement start from the centre and travel down the efferent fibres to the muscles, while impressions arising in the periphery of the body are carried along the afferent fibres to the centre and modify its action.

The Respiratory Centre.— There is a portion of the medulla oblongata destruction of which causes immediate and permanent cessation of all respiratory movements. This spot has been found to lie in the grey substance near the tip or nib of the calamus scriptorius; it extends on both sides of the middle line, and has received the name of “le noeud vital” (Flourens). If the spinal cord be severed immediately below the spot, all respiratory movements in the parts of the body below the level of section cease at once, while movements of respiration in parts situated above the level {e.g., in the face) continue momentarily. If the severance be made above the vital knot, the facial movements of respiration cease, while those of the trunk continue. The nervous centre is bilateral, each half serving its proper side of the body; if the medulla oblongata be split longitudinally in the middle line, through the noeud vital, respiration goes on unchecked; but if one lateral half of the vital area be destroyed, respiration at once ceases on the same side of the body.

While the above hypothesis of a respiratory centre in the medulla presiding over the movements of respiration is in all probability substantially correct, it must be stated that, in certain circumstances of experiment {e.g., when the animal is young and has been poisoned with strychnia, or is kept in a warm chamber), respiratory movements do occur even when the medulla oblongata with the noeud vital has been removed. That is to say, there are centres in the cord of a lower order than the medullary respiratory centre which are under ordinary circumstances dependent on the main centre but which may act independently.

The respiratory centre must be regarded as the seat of origin of the impulses which cause the muscular movements of inspiration and expiration. During the whole of intra-uterine life the centre is inactive, but almost immediately after the child is born, or the placental circulation is interrupted by compression of the umbilical blood-vessels, the centre becomes quickened, and fails not to yield the appropriate stimulus at short intervals during the whole afterperiod of life. It is impossible to resist the conclusion that the immediate cause of the activity of the respiratory centre is the impure and impoverished state of the blood, which it is the function of respiration to remedy. So long as the placenta performs the function of purifying the blood of the foetus and supplying it with oxygen the respiratory centre is quiescent; the moment the placenta becomes incapable of purifying the foetal blood, as, for example, when the mother is suffocated or asphyxiated, the respiratory centre prepares to act and respiratory movements follow.

If it is true that the presence of impure or venous blood in the respiratory centre is the cause of the first respiratory act of an animal, it is also true that the presence of highly arterialized blood is sufficient to render the centre absolutely inactive again. If air or oxygen gas be driven through the lungs of an animal and allowed to escape through holes made in the chest walls, the blood becomes so rapidly and perfectly purified that the whole vascular system is filled with the pure blood which is commonly found in the arteries only. In this case the movements of respiration cease until such time as the blood has again lost its purity, when respiration begins, as one might say, da capo. There is no doubt that the essential cause of the inactivity of the respiratory centre in this experiment is the presence in excess of oxygen gas, an∣l that the absence of a certain proportion of oxygen in the blood circulating in the body endows the blood with a power of stimulating directly or indirectly the respiratory centre. To this subject we shall return.

The Efferent Fibres.— The fibres proceeding from the respiratory centre run down the spinal chord and emerge for the most part in the cervical and dorsal region. Some fibres are collected into a nerve trunk which takes its rise in the third, fourth, and fifth cervical nerves; this is the phrenic nerve, which supplies that half of the diaphragm lying on the corresponding side of the body. Section of one phrenic nerve causes paralysis of the corresponding half of the diaphragm. Other fibres run in the intercostal nerves for the supply of the intercostal muscles, levatores costarum, &c.; others again run in the cervical nerves for the supply of the sterno-mastoid, scaleni, &c. Section of these nerves leads to paralysis of the muscles supplied by them. The facial muscles of respiration are supplied by efferent fibres from the seventh cranial nerve, and the laryngeal muscles by the laryngeal branches of the vagus.

In addition to the nerves which supply the respiratory muscles, there are efferent fibres which run in the course of the vagus to the muscles supplying the larynx and the bronchial tubes. The vagi (also called the pneumogastric nerves) are two important trunks which arise from the medulla oblongata, and, after receiving communications for neighbouring nerves, run down by the side of the windpipe to reach the thorax and abdomen. By means of appropriate apparatus, of which several varieties have been devised, contraction of the small bronchial tubes may be readily made sensible and even graphically recorded. A study of the experimental results obtained after electrical stimulation of the pneumogastric nerves leaves no doubt that these nerves contain motor fibres for the bronchial tubes. The exact distribution of the superior and inferior laryngeal branches of the vagus to the muscles of the larynx is described under Anatomy.

The Afferent Fibres.— Many nerve fibres have been ascertained by physiological experiment to have communication with the respiratory centre. For example, the nerves of the skin of the chest, t when stimulated by the application of cold water, cause a gasping inspiration; the nerves of the skin of the sides of the body and of the soles of the feet, when stimulated by tickling, cause that peculiar series of spasms of the diaphragm which constitute laughing; and so on. But the vagus nerve is that which contains the afferent fibres of chief importance to the respiratory movements. The vagus fibres proceeding from larynx, trachea, lung, and stomach certainly, and possibly also those proceeding from other organs of the abdomen, are all capable of influencing the activity of the main respiratory centre. If the trunk of the vagus nerve on each side of the neck be laid bare and divided in an animal such as a rabbit, the respiratory movements become much less frequent but at the same time deeper. If the end of the nerve above the point of section be carefully stimulated by a weak electric current, respiration again becomes quicker and proportionately shallow until in fact the natural type is restored. If the strength of stimulus be still further increased, the rapidity and the shallowness of breathing become still more remarkable, until a degree of stimulation is reached when there is no longer any rhythmical character in respiration, which is reduced to a feeble inspiratory spasm. These facts are conveniently grouped and explained by the following hypothetical statement. The vagus nerve contains fibres running up to the respiratory centre which have the function of modifying the native tendencies of the centre in such a manner as to accelerate its explosions of activity and at the same time render them less formidable. These fibres are constantly in action during healthy life; hence, on dividing the vagus trunk including these fibres, the respiratory centre reverts to its natural type of slow and deep action. On stimulating these fibres artificially they may be brought to spur on the centre once more, or even carry the accelerating process to the point of producing continuous inspiration.

Experiment has, however, shown that the influence of the vagus nerve is not so simple as is here supposed. There are other fibres in the vagus which seem to arise in the larynx and run in the trunk of the two laryngeal (but chiefly in the superior laryngeal) branches of the nerve; these have powers exactly opposed to those ascribed to the main-trunk fibres. If the superior laryngeal nerve be divided and the higher end at the point of division be stimulated, respiration becomes less frequent and more powerful. These fibres of opposite tendencies run side by side in the vagus trunk, and must be alike excited when an electrical stimulus is thrown into the nerve. The fact that the total result of stimulation is in favour of the first-described accelerating nerves may be hypothetically explained by supposing the accelerating fibres to be more numerous, or more potent with the given stimulus, than the slowing fibres.

Nature of the Activity of the Respiratory Centre.— Various interest ing questions arise as to the exact mode of action of the respiratory centre. Is there an expiratory centre apart from the inspiratory? In ordinary quiet breathing expiration is not a muscular act in the sense that inspiration is; but in laboured breathing many muscles are coordinated to help in the act; we may therefore assume that there is an expiratory coordinating mechanism as well as an inspiratory. Another inquiry touches the question of the stimulation of the respiratory centre. We have arrived at the conclusion that a certain impoverished or impure state of the blood is the cause of the activity of the respiratory centres. Is it by a direct action of the blood on the nerve centre, or is it by an action of the blood upon the end-organs of nerves in the peripheral parts of the body which indirectly induce the respkatory centre to act? In other words, λvould the respiratory centre act under the stimulus of a certain degree of impurity of its blood if it were cut off absolutely from its afferent fibres? This is not a question which admits of an absolutely decisive experimental answer; but this may be said in reference to it, that, if the cerebrum be extirpated, the pneumo-gastric nerves divided, and the spinal chord severed at a point below that from which the main respiratory efferent nerves emerge, notwithstanding that the respiratory centre is thus separated from by far the greater number of the sensory nerves of the body, respiration still goes on. Further, if the spinal axis is divided at a level immediately above that of the respiratory centre, respiration goes on in the lower parts of the body but ceases in the upper—that is, the facial movements cease; while, if the division be carried through a point immediately below the level of the respiratory centre, although chest movement ceases and respiration may in strictness be said to be permanently stopped, yet the facial movements, which are just as essential to the complete idea of respiration, and which are innervated from the common respiratory centre, continue to be performed so long as any life remains in the mutilated animal. There is therefore a very strong presumption that the respiratory centre is automatic, that is, that it has within itself the spring of its own activity, and that the impurity of the blood which sets the respiratory mechanism agoing does so by acting upon the respiratory nervous centre directly. However started, the inspiratory stimulus is discharged rhythmically down the efferent respiratory nerve during healthy life. The rhythm is capable of wide variation, both by the exercise of the will and by the operation of external conditions, such as heat and muscular exercise of the body, which accelerate respiration independently of the will. The question why the discharge is rhythmical is one of the deeper problems of physiology, and touches the essential nature of nervous actions in general. It is customary in physiological treatises to illustrate it by supposing a mechanism which offers a resistance to the discharges of its own energy; when the energy reaches a certain degree of tension resistance is overcome and the discharge takes place, but a certain time must elapse before energy can again accumulate to the extent of overcoming resistance. It is further supposed that resistance is capable of being varied by external agencies; when it is increased the discharges occur at longer intervals, because more energy must be accumulated before resistance can be overborne, but at the same time they are more violent; when resistance is diminished, the discharges are more rapid but less powerful, until, when resistance becomes nil, the discharge is continuous. Such illustrations are, however, of very doubtful value; they impart a sense of clearness to our views of nervous action by turning attention from the problem we wish elucidated to another and altogether different problem. There is in fact no probability whatever, so far as facts yet go, that nervous rhythmical action is brought about in any such way.

Chemistry of Respiration.

The mechanism which has just been described has for its object an interchange of substances between the body and the external medium. Certain substances pass out of the body at the lungs, and others are taken into the body. The discussion of this interchange pertains to what is called the Chemistry of Respiration.

Comparison of Inspired and Expired Air.— No inconsiderable knowledge of this exchange of matters may be obtained at once by a simple comparison of inspired and expired air. The air we breathe varies somewhat according to the various circumstances of season, time of day, height above sea level, prevailing wind, &c.; but all samples of pure atmospheric air have substantially the following composition :—

1. Gases :—

Oxygen...........................20 ’ 84 to 20 , 92 vols. p.c.

Nitrogen..........................79 Ό0 to 79 Ό5

Carbon dioxide............................. 0Ό4

2. Aqueous vapour.—The absolute amount of this ingredient varies very much, and must be distinguished carefully from the relative or sensible moisture, which depends more upon the temperature of the air than upon the absolute'quantity of watery vapour it contains.

Air which is expired from the lungs has approximately the following composition :—

1. Gases :—

Oxygen.............. . ......................16‘03 vols. p.c.

Nitrogen............:.....................79 - 02 ,,

Carbon dioxide.....................3'3 to 5 5 ,,

Probably there is an exceedingly small excess of nitrogen in expired air as compared with inspired air.

2. Aqueous vapour.—The absolute amount is always such as to saturate the air at the temperature which it has on expiration, i.e., 36° ∙3 C.

3. Organic matter of uncertain composition but hurtful when reinhaled.

On an average (subject, however, to considerable variations) it appears that the body takes up in respiration oxygen gas to the amount of 4 - 78 vols, per cent, of the air inhaled, and it gives off carbon dioxide to the average amount of 4 ’ 38 vols, per cent., besides traces of nitrogen, ammonia, hydrogen, and light carburetted hydrogen. Leaving out of calculation the minute traces of the latter bodies, it will be observed that the volume of oxygen taken into the body is larger than the volume of carbon dioxide given off; and, since, under like conditions of pressure and temperature, equal volumes of oxygen gas and carbon dioxide gas contain the same quantity of oxygen, it is clear that more oxygen enters the body at the lungs than escapes from it; therefore the whole of the oxygen taken into the body does not leave it as carbonic acid or expired air. The ratio of CO., expired to O absorbed is called the “respiratory quotient”—

∞= i^∙9O 4782

Composition of Air in the Air Cells.— The whole of the air of the lungs is not expelled at each breath, some remaining in the depths as residual air lodged in the alveoli or air cells. Hence we cannot assume, from an examination of expired air, that we know the constitution of the air in the recesses of the lungs where it comes most intimately into contact with the blood. To attain such knowledge it is necessary to examine the deeper air directly, and the air is obtained for such purposes by means of a lung catheter. In this way it has been ascertained that the alveolar air of a dog’s lung contained about 3 - 8 per cent, of CO., at a time when the expired air contained about 2 - 8 per cent. As to the amount of O we may safely assume that the alveolar air never in ordinary circumstances contains less than 10 per cent, when the expired air contains 16 per cent.

Daily Quantity of Substances Exchanged.— During 24 hours an average person would take in about 10,000 grains of oxygen in respiration, and give out about 12,000 grains of carbon dioxide, corresponding to 3300 grains of carbon; at the same time about 9 oz. of water would be exhaled. These quantities vary, however, within wide limits according to the conditions of age, sex, atmospheric pressure, and the like. Thus, for example, in young persons the O absorbed is relatively greater than the CO.given off, and a child gives off twice as much COin relation to its body weight as an adult. Again, males after the first few years of life give off more CO 2 than females. When the external temperature is so low as to depress the body temperature, less CO 2 is given off; if it is so high as to raise the body temperature, the CO 2 is increased. If, however, the surrounding medium is cooler than the body but not cold enough to lower the body temperature, more O is taken in, and more CO 2 is given out; and vice versa. Muscular exercise also increases considerably the CO 2 given off; and more CO 2 is given off a short time after a meal than during fasting, especially when the meal includes substances rich in carbon. Speaking generally, alcohols, ethers, tea, <fcc., diminish the CO 2 ; but the results are not constant. Again, while the number and depth of the respirations do not influence the formation of CO 2 in the body, they affect the removal of that which is already formed. Increased rate of respiration and increased depth of respiration both cause an absolute increase in the quantity of CO 2 expired, although with reference to the total amount of air which passes into and out of the lungs during such laboured breathing the CO., is relatively diminished. Lastly, when the atmospheric pressure is diminished, as in ballooning, respiration becomes difficult, CO 2 is imperfectly removed from the body, and the blood contains less O. When pressure is increased, respiration is easy and slow (2-4 per minute), the capacity of the lungs increases, the activities of the tissues are marked, and as a result of this more O is absorbed and more CO s is excreted.

The Blood in Respiration.

Having ascertained the nature and quantity of the materials exchanged in the lungs, we may now ask concerning the method of the exchange. There is no difficulty in understanding how the cold air introduced into the lungs is warmed and saturated with moisture by contact with the moist walls of the air passages and bronchi. As to the gaseous substances which appear in traces in the expired air nothing definite is known; they may in part arise in the decomposition of the solid organic impurities of expired air, and may in part escape from the blood itself. The origin of the solid poisonous organic substances, which are of such vital interest from a sanitary point of view, is also a matter of great obscurity. Some portions of it doubtless consist of effete particles of tissue from the walls of the bronchial tubes and cavity of the mouth and nose; for example, epithelial scales may be discovered in the condensed moisture of the air expired into an ice-cold globe, and so also may the organized bacilli of tubercle when the subject of experiment happens to be phthisical. Other portions may spring directly from the blood. The chief inquiry, however, centres about the origin of the carbon dioxide and the absorption of the oxygen, for the understanding of which a knowledge of the blood is necessary.

The Blood.— This, as it circulates in the vessels of man and vertebrates generally, is a viscous and to the naked eye homogeneous liquid of red colour,—the blood of the pulmonary veins, of the left side of the heart, and of the systemic arteries being normally of a bright scarlet hue, and the blood of the right side of the heart, of the systemic veins, and of the pulmonary artery being of a brownish-red colour. In other words the blood enters the capillaries of the lungs of a brownish-red colour, and leaves them bright scarlet in hue; it enters the general capillaries of the body as a bright scarlet fiuid and leaves them as a brownish-red fluid. Although homogeneous to the naked eye, the blood is found on microscopic examination to consist of a colourless fluid, called liquor sanguinis or plasma, holding in suspension large numbers of solid bodies, the corpuscles of the blood; the more numerous of these are red, the others are colourless and are commonly spoken of as white. The specific gravity of blood probably varies between 1045 and 1075. Fresh blood is feebly alkaline in reaction.

When blood is shed it remains for a minute or two as fiuid as it is in the blood-vessels; but in 2-6 minutes it begins to pass into the state of a soft red jelly, which gradually acquires greater consistence, and by the contraction of one of its constituents at length expresses a fluid; the contracted jelly is called the clot, or crassameηtum ; the expressed fluid is the serum ; and the whole process is denominated coagulation. Contraction of the clot may go on for 10-48 hours. This process of coagulation is due to the separation from the plasma of a body called fibrin, which entangles in its meshes the corpuscles of the blood. When coagulation is delayed for several minutes (as it always is in horse’s blood, and as it usually is in the blood of men suffering from inflammatory diseases), the blood corpuscles, being specifically heavier than the plasma, have time to subside a little way before coagulation commences. Hence the uppermost layers of such blood become nearly free from coloured corpuscles; and subsequently, when the blood coagulates, the clot exhibits the phenomenon of the bujky coat,— that is, the upper part of the clot is of a yellowish colour. If, instead of allowing blood to coagulate undisturbed, it be stirred or whipped with twigs, the fibrin does not entangle the corpuscles but separates as a stringy mass which adheres to the twigs; the corpuscles remain in the serum and constitute defibriηated blood. Coagulation is promoted (a) by exposure to a temperature slightly higher than that of the living body; (δ) by contact with foreign matter; (c) by the addition of minute quantities of common salt or other neutral salts. It is delayed or suspended (a) by exposure to an ice-cold temperature; (6) by contact with the living blood-vessels; (c) by the addition of a sufficient quantity of sodium chloride, sodium sulphate, or some other neutral salts.

Reviewing all the facts which have been ascertained respecting coagulation, it would appear that the process is dependent upon the presence in the liquor sanguinis of a proteid body called fibrinogen, which under favourable circumstances undergoes conversion, or perhaps decomposition, into fibrin. This conversion, when it occurs outside the body, appears to be connected with the action of a ferment produced in the colourless corpuscles, and probably only set free when they break down.

The red corpuscles of man and the Mammalia generally, except the Camelidae, are biconcave disks, possessing neither skin nor nucleus or interior body. In birds, reptiles, and most fishes they are nucleated, elliptical, and biconvex. In the camel the red corpuscles are oval. The average diameter of the disk in man is ∙ 5 - i ⅛ σ inch and the thickness about i n θh∙

The substance of which the red disks is composed is elastic; the disks may therefore be squeezed through fine chinks smaller than their own diameter and may afterwards regain their original shape. When in the bloodvessels the red corpuscles are hurried along indiscrimately in the blood current, but when the blood stands the red corpuscles cling together in rows like piles of coin, which are technically called rouleaux·, this is a physical,phenomenon entirely due to the shape of the corpuscles. The white or colourless corpuscles are globular masses of granular protoplasm, provided with one or more interior bodies called nuclei, but destitute of a skin. They have the power of independent movement, which the red cells have not, resembling in this respect the amoeba. They can send out processes, change their outline, and move from place to place by laying hold on resistent objects by means of a projected process and dragging their bodies along. They have a diameter of inch when at rest and globular. They are far less numerous than the red corpuscles; the proportion of white and red corpuscles in various conditions is shown in the following table :— [table]

In addition to typical white and red corpuscles there are others, provisionally called intermediate corpuscles, red granular corpuscles, or haematoblasts, which are granular and nucleated. Probably these corpuscles are involved in a special manner in the development of the fibrin-ferment. It has been calculated that one cubic millimetre (f.c., a cube whose side measures about ∙∕ κ inch) contains five million corpuscles of all kinds.

The chief constituents of the blood may now be discussed seriatim :—

Constituents of the Liquor Sanguinis. Fibrin.— When fresh this is an elastic substance belonging to the group of proteid bodies; it is insoluble in pure water, but is soluble in solutions of common salt and in dilute hydrochloric acid at the temperature of the body. Human venous blood yields from 2 ’ 2 to 2'8 parts per thousand of fibrin. So far as we know, fibrin does not exist as such in the plasma before it is coagu lated; but there does exist a body, fibrinogen, which is the precursor of fibrin.

It may be separated from plasma by diluting the plasma freely with water at an ice-cold tempera ture, and passing for a long time a stream of CO 2 through it; or by adding common salt to plasma until the plasma contains

12 to 16 per cent, of salt the fibrinogen is precipitated in a flaky form. Besides fibrinogen, liquor sanguinis contains another proteid body intimately concerned in the formation of fibrin; this is serum-globulin or para-globulin. It also may be precipitated from diluted plasma by a stream of CO,; but it may be much more perfectly eliminated by adding magnesium sulphate until the plasma is saturated. It is a body the presence of which greatly facilitates or promotes the development of fibrin from fibrinogen; indeed some observers have supposed that fibrin arises in the union of the two bodies fibrinogen and para-globulin.

Serum-albumin.— This is a proteid body which coagulates on heating.

Certain extractive matters.— These include neutral fats, lecithin, and cholesterin; sugar; urea, uric acid, creatine, creatinine, &c.; and a yellow pigment.

Inorganic Salts.— These are the chlorides, phosphates, and sulphates of sodium, potassium, calcium, and magnesium; sodium and chlorides are the main constituents.

Gases.— Especially CO 2 ; but see below.

Constituents of the Red Corpuscles. Oxyhaemoglobin.— This interesting substance has the general chemical composition of an albuminous body with the addition of the element iron:—C, 54Ό0; H, 7'25; N, 16 - 25; Γe, 0'42; S, 0∙63; 0, 21∙45 = 100O0.

It is the cause of the red colour of the corpuscles. It may be obtained in the form of beautiful crystals by a variety of methods, which all have the following steps in common :—(1) to effect the solution of the oxy haemoglobin of the red corpuscles in the serum or in water, and (2) to effect the crystallization of it from solution either by adding alcohol, or by cold, or by both combined. Theforms of the crystals differ in different animals; the chief forms are given in fig. 9. Oxy haemoglobin has the property of giving up a portion of its oxygen on very slight provocation, e.g., when treated with easily oxidizable sub stances, or when sub mitted to moderate heat in a vacuum.

The resulting sub stance is called re duced haemoglobin or, shortly, haemo globin; it is capable of reproducing the original oxyhaemo globin on simple ex posure to oxygen or to air. This property is of the highest im portance in the func tion of respiration, as will be explained in the section on the gases of the blood.

Solutions of oxyhaemoglobin of moderate strength have the fine scarlet hue of arterial blood. When interposed between the sun and a spectroscope they cause the absorption of definite portions of the spectrum, and give a characteristic appearance to it. After the solution has been reduced (as it may be very readily by means of an alkaline solution of a ferrous salt, in which precipitation of ferrous hydrate is prevented by the presence of tartaric or citric acid), it assumes a brownish colour and the absorption spectrum becomes changed. The spectra of oxyhaemoglobin and of reduced haemoglobin when the solutions are of moderate strength are given in fig. 10.

It has been determined that 1 gramme of haemoglobin can link to itself 1 -671 cc. of oxygen gas (at 0° C. and 760 mm. pressure) in the above-described loose manner,—that is, in such a manner that when admitted to a vacuous space at a moderate temperature the two become again dissociated.

When blood or a solution of oxyhaemoglobin is shaken up with carbon monoxide, the “dissociable” or “respiratory” oxygen is displaced, and a new compound of CO and haemoglobin is formed which has a spectrum very like that of oxyhaemoglobin, but is incapable of reduction by the means which are sufficient to reduce oxyhaemoglobin. In like manner (with certain precautions) a compound of haemoglobin with NO may be formed.

Various products of the destructive decomposition of haemoglobin are known, possessing characteristic spectra and properties; these need not be further described.

Proteid Substances.— Besides oxyhaemoglobin, the red corpuscles contain other proteid substances, probably for the most part paraglobulin.

Lecithin. Cholesterin. Inorganic Salts.— Potassium and phosphates are the main constituent; sodium, calcium, and magnesium are also found as chlorides and sulphates.

Gases.—, See below.

Constituents of the White Corpuscles. Proteid Substances.— Several varieties have been separated.

Lecithin.

Certain extractives, including glycogen. Inorganic Salts.— Especially potassium and phosphates.

The Gases of the Blood.

The blood when admitted into a vacuous space readily gives up more than half its volume of mixed gases, consisting of oxygen, carbon dioxide, and nitrogen. The oxygen is present in much larger quantities than could be held in simple solution by the water of the blood; it is in fact mainly held in feeble combination by the haemoglobin of the coloured corpuscles; only a trace of it is, under ordinary circumstances, held in true solution. The carbon dioxide, whilst not existing in larger quantity in blood than would be possible if it were simply dissolved by the water of that fluid, is nevertheless to a small extent in a state of loose chemical combination in which both plasma and corpuscles share. The nitrogen is held in a state of simple solution in the liquor sanguinis. These gases are yielded in different proportions by arterial and venous blood, as is shown in the following table :— [table]

The method of extracting the gases of the blood which has been found to be most convenient is one in which the blood is introduced into the Torricellian vacuum existing above the column of mercury in a barometer. Blood is collected over mercury in such a way as to avoid all access of air; it is then introduced by an appropriate mechanism into the space above the barometric mercury. Λt once the gases froth up and fill the space; the aqueous vapour which at the same time arises is absorbed by a special drying apparatus communicating with the vacuum. Means exist of warming the vacuous space up to about 45° C., which is a temperature sufficient to cause the escape of nearly all the gases of the blood; the last portions of carbonic acid are, however, more rapidly evolved by allowing a small volume of a thoroughly boiled-out solution of phosphoric acid to enter the blood receptacle near the close of the operation.

How the Gaseous Exchanges are Effected in Respiration.

For the purposes of description, and in reference exclusively to its respiratory function, the blood may be looked upon as a watery solution of certain substances having respectively a slight chemical attraction for oxygen and for carbon dioxide. The chief of these substances is haemoglobin, which is concerned solely with the oxygen; others, less perfectly known, are concerned with the carbon dioxide, and include certain saline constituents of the plasma (NaHCO) and perhaps also certain constituents of the corpuscles. The affinity of these substances for oxygen and carbon dioxide respectively is of so slight a nature that the mere exposure of the substances to a vacuum at a certain moderate temperature is sufficient to overcome the affinity, and dissociate the captive gases. Granted such a solution in the blood-vessels of the lungs, separated from the air of the air cells by the thin moist membranes of the alveolar walls, how are the gaseous exchanges of respiration brought about? In the first place we must premise certain physical relationships which exist between gases and fluids in contact with one another. If a definite quantity of pure water be exposed to the air it will at a given temperature dissolve a definite volume of air. If the air be compressed to one-half (or one-quarter) of its original bulk the water will still absorb the same volume of it, albeit that volume now contains twice (or four times) the former quantity of gas. If the pressure be diminished again the air will escape from the water exactly in proportion to the diminished pressure. This is a concrete example of Dalton’s law that the quantity of a gas dissolved by a liquid varies directly as the pressure of the gas at the surface of the liquid. What is true of the air is true of its constituents. In the above example the quantity of the constituent oxygen absorbed after the compression of the air is twice (or four times) the quantity absorbed before compression. But the quantity which is absorbed of any constituent of a gaseous mixture depends, not upon the total pressure exerted on the liquid by all the gases of the mixture in contact with its surface, but by the partial pressure, or fraction of the total pressure, exerted by the constituent in question. To take an example, let us suppose that a solution of oxygen in water made at a certain definite pressure is exposed to an atmosphere of pure nitrogen exerting as great a pressure, or even a greater, on the surface of the liquid; notwithstanding this pressure the oxygen would escape from solution as readily as if the solution had been placed in a perfectly vacuous space. If water be exposed in a certain space of air (which is, roughly, a mixture of nitrogen and oxygen), the oxygen which it is capable of absorbing would be equally well absorbed if all the nitrogen were removed from the space and the oxygen alone left to fill it. These remarks are equally true of a liquid separated from a gas by moist membranes. In short, the blood of the pulmonary vessels, regarded as a simple liquid capable of absorbing O and CO 2 , and apart from the remarkable chemical bodies contained in it, obeys Dalton’s law like any other liquid.

The blood is, however, not a simple fluid, but contains substances having a peculiar affinity for certain of the gases of the atmosphere; especially does it contain haemo-glôbin. It has been stated that the “dissociable” or “respiratory” oxygen of oxyhaemoglobin is all yielded to a vacuum at the temperature of the body. This yield differs from the yield of a gas simply dissolved in a liquid (which also is perfect in a vacuum) in not proceeding ∕>αri passu as the pressure approaches nil. On the contrary in the case of solutions of haemoglobin, as the oxygen pressure is diminished at the surface of the solution, no changes occur in the oxyhaemoglobin until the oxygen pressure reaches 25 mm. (about 1 inch) of mercury; then the oxyhaemoglobin yields up its respiratory oxygen. If the steps are retraced, and oxygen is gradually readmitted to exercise pressure upon the haemoglobin solution, the latter takes up oxygen once more as soon as the oxygen pressure reaches or exceeds 25 mm. of mercury.

We may now consider the actual physical conditions of the blood in the lungs. Venous blood is hurried into the capillaries surrounding the air cells; much of its haemoglobin has been “reduced” (or deprived of its dissociable oxygen), and further it is rich in carbon dioxide, which it has obtained from the active tissues in distant organs. In the alveolar walls it comes into relationship with the air of the alveoli; probably these are filled with air which never contains less than 10 per cent, of oxygen, and which (in the dog, be it remembered) contains 3 - 8 per cent, of CO., at a time when the expired air contains 2'8 per cent. Are these conditions such that, owing to the physical laws described, oxygen must pass into the blood, and carbon dioxide out of it? First, as regards the oxygen. Au atmosphere containing 10 per cent, of oxygen implies a partial O-pressure of 76 mm. of mercury (10 per cent, of 760 mm.); as this is far above the dissociation point (25 mm.) of oxyhaemoglobin, it is clear that any reduced haemoglobin present would greedily absorb oxygen from such an atmosphere. When the air breathed is much rarefied, the case is different; the partial O-pressure in the alveoli may be so far reduced that the absorption of O by the blood becomes most difficult or impossible. As regards the carbon dioxide the matter is not so clear; but, inasmuch as air drawn from the depth of the lung by means of a catheter contains (in a dog) 3 - 8 per cent, of CO 2 , while at the same time the venous blood of the right side of the heart possesses a CO 2 tension of approximately the same percentage, we may assume that CO 2 escapes from the pulmonary capillaries into the alveoli until equilibrium ensues. It is, however, conceivable that the epithelium of the air cells may assist the elimination of CO 2 from the blood by a process of true excretion independently of the above merely physical considerations.

So-called Internal Respiration.

Venous blood yields up its carbon dioxide and takes up oxygen in the capillaries of the lungs; arterial blood passes to the general capillaries of the tissues and there yields up its oxygen and receives carbon dioxide. To this act, which in its issues is complementary to the interchanges of pulmonary respiration, the term internal respiration, or respiration of the tissues, has been applied. With as much propriety we might speak of internal urination in reference to the nitrogenous effete matters which the tissues cast into the blood. We are in fact not in a position to form any clear picture of the interchanges which occur between the blood and the tissues other than that which is sketched under Nutrition (vol. xvii. pp. 678-682). Muscle is the tissue whose metabolic processes are most clearly understood. Living muscle yields no oxygen to a vacuum, although it gives up carbon dioxide freely; therefore it is presumably itself in a condition to take up oxygen from arterial blood and to give up carbon dioxide to it. But the absorption of oxygen is not immediately necessary to the escape, or the formation, of the carbon dioxide, however much it is so in the last instance. In other words, the oxygen passes into the tissue and is at once combined into some intermediate compound which only at a later stage decomposes and yields carbon dioxide. Whenever an organ is active its blood-vessels dilate and permit a more copious flow of blood through it than when it is at rest. In the salivary glands the blood may even pulsate in the veins and look like true arterial blood in colour. After passing through active muscles the blood contains both more CO 2 and less O than after passing through resting muscle.

Dyspnoea and Asphyxia.

When the entrance of air to the lungs is entirely prevented the phenomena of dyspnoea and asphyxia begin to appear. At first respiration is deeper and more frequent than usual (dyspnoea), the extraordinary muscles being called into play in both inspiration and expiration; the heart beats more quickly at first, but afterwards more slowly; this is the first stage. It is succeeded by the second stage, in which the violence of respiration is less marked, although the coordination of the act is more irregular; indeed towards the end of the stage respiratory movements merge in general convulsions of the whole body. Throughout this stage expiration is more marked than inspiration, and the pressure of the blood in the bloodvessels is very great. The third stage is one of exhaustion, which supervenes suddenly, and is marked by loss of consciousness, dilated pupils, and absence of the powers of reflex action. The animal seems dead, except that at long intervals feeble inspiratory gasps occur. Finally there comes one great inspiratory effort: the mouth is fixed wide open, the head thrown back, the body arched backwards, the nostrils dilated, and the pulse after a second or two is indistinguishable (asphyxia). The whole series of events lasts from three to five minutes if the interruption to the entrance of air has been absolute. After death the right side of the heart, with the vessels immediately opening into it, viz., the venae cavae and veins of the neck and the pulmonary artery, are engorged with black blood containing little or no oxyhaemoglobin. The left side of the heart and the systemic arteries are contracted and empty. All these phenomena are best explained by the known power of venous blood to stimulate the nervous centres. As the blood becomes more and more venous it stimulates more powerfully the great nerve-centres of the medulla oblongata. The respiratory centre is stimulated, especially its expiratory portion; and, finally, the whole muscular centres of the spinal system are excited, causing general convulsions. The vaso-motor centre is stimulated, causing the rise of blood pressure in the early stage. The slowing of the heart during the close of the first and second stages is due to stimulation of the vagus cardio-inhibitory centre in the medulla. Finally the centres become exhausted from the impurity of the blood bathing them, and their activity fails altogether.

Stimulation of the Respiratory Centre.

It remains to ask what property of venous blood confers upon it the power of stimulating nerve centres. Venous blood differs from arterial blood chiefly in the relative amounts of O and CO 2 it contains. Which of these substances is it whose deviation from the arterial standard causes stimulation 1 It so happens that we can experimentally separate the two factors. If an animal be placed in a chamber of some inert gas, such as nitrogen, the escape of CO., from the lungs is unimpeded and no accumulation of CO 2 in the blood is brought about; nevertheless dyspnoea and asphyxia follow as if the entrance of air to the windpipe had been interrupted. Here there is no accumulation of CO 2 in the blood, but total deficiency of O. On the other hand, if an animal be placed in a chamber containing a large excess of CO 2 beyond the standard of expired air, but at the same time a superabundant supply of O, no dyspnoea supervenes, and no asphyxia threatens. The animal may suffer from drowsiness (for CO 2 is narcotic in its effects), but true dyspnoea never occurs so long as the supply of O is sufficient. There is little doubt therefore that the absence of O from venous blood is the essential condition of its stimulating property.

Diversity of Modes in which the Function is Carried cη. Respiration, if we assume the essential fact of that process to be the absorption of O and the elimination of CO 2 by the animal body, is coextensive with the whole animal kingdom. It is not, however, in every case served by special air-containing organs or lungs. Indeed the essential interchange of respiration goes on wherever the blood comes into sufficiently close contact with oxygen. For example, the air which happens to be swallowed with our food is so closely in contact with the blood of the intestinal vessels that an interchange of gases occurs, constituting a true intestinal respiration. The Mammalia all possess true lungs. In birds also there are lungs, but the mechanism of respiration is unlike that of man, since the diaphragm is wanting. There are, in birds, besides lungs, “air sacs” lying among the viscera and communicating with cavities in the bones, these sacs being supplied with air from the lungs. Reptiles and some amphibians breathe by lungs, other amphibians breathe by gills, as also do the young of some of the air-breathing Amphibia (frogs). Fishes breathe by gills, using the O dissolved in water.

Many invertebrates respire in air, which is carried into the midst of their tissues in tracheae or air-tubes branched like a tree; other invertebrates breathe by gills in water; others again have lungs. In some the oxygenized water is carried into the body along a series of tubes—the water-vascular system; and in still simpler animals the general surface of the body seems to serve the respiratory function. (a. g.*)