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...