General Introductory Considerations.— There is no conception which we can form in reference to a living being, however rudimentary its structure, which is so general as the following a living animal, so long as it manifests those attributes which characterize it as living, is the seat of continual transformations of potential into kinetic energy.

Such transformations are connected with oxidation of organic matters which, primarily derived from the vegetable kingdom, have been assimilated, i.e., have been converted into the substance of the animal, and are effected through the agency of oxygen gas introduced into the body in the process of respiration. The act of living is an act of combustion in which the animal body actually burns, and the energy at the disposal of the body, and which is employed in raising its temperature or effecting the movements which are essential to its continued life, is energy which was potential in the organic constituents burned. The main products of the combustion of the body are carbonic acid and water, besides certain other less completely oxidized substances, which are the analogues of the sooty and tarry products of combustion in a furnace. Of these products some, as carbonic acid and a part of the water formed, are removed from the body almost as soon as they are formed, while others are thrown off at intervals.

It follows from what has been stated that the act of living necessarily implies not only transformation of energy but actual waste of the matter of the body, and that if an animal is continuously to manifest the phenomena of life it must be supplied with oxidizable organic matter to take the place of that which has been oxidized. Accordingly, animal life is impossible unless the creature, besides receiving continual supplies of oxygen gas, receives at intervals supplies of food. The food of an animal consists (1) of oxidizable organic matters which, although they may have been derived in part or wholly from some other animal body, have primarily been built up through the instrumentality of vegetable organisms,—these organic matters belong to a few well-defined groups: (2) of mineral matters, including large quantities of water, which form an important part of the substance of the body, and the presence in and passage of which through the organism is essential to the physical processes which have their seat in it.

In brief, the animal body is the seat of processes of disintegration associated with the manifestation of kinetic energy, and of processes of integration in which oxidizable matters take the place of the oxidized constituents. If life is to continue, in a sense, indefinitely it is essential that the processes of integration and disintegration should balance, i.e., that the receipts of the body in assimilable oxidizable matter should balance the expenditure of the body both in matter and energy. There are, it will be observed, very close analogies between an animal and such a mechanism as a steam-engine, the energy at the disposal of both being primarily derived from the oxidation of combustible matters. Some of the most salient points of difference must, however, not be lost sight of. (1) The waste of the essential parts of such a machine as a steam-engine is insignificant, and bears no definite relation to the work done. The kinetic energy of the machine is primarily due to oxidation processes taking place in the furnace, and in no respect to changes in the substance of the machine. The animal, on the other hand, wastes continuously in all its parts and organs, and much of its energy is derived immediately from material which has become part and parcel of the various mechanisms. (2) Any substance capable of being rapidly oxidized (burned), and thus of generating heat, may be used as fuel for a steam-engine, provided its combustion admits of being conducted with safety in its furnace, whilst the substances which can form the food of animals belong to a limited number of groups, which include but a comparatively small number of bodies. The constituents of food have not only to supply energy to the body, but they must further be capable of prior conversion into the very substance of the animal body, into its very “flesh and blood.”

Moreover, the constituents of food must be free from all traces of the peculiar substances which we term “poisons,” and which by their presence have the power of impairing and arresting the action of various organs of the body.

I.— Function of Digestion. Hunger and Thirst.— These terms are used to express peculiar sensations which are produced by and give expression to general wants of the system, satisfied respectively by the ingestion of organic solids containing substances capable of acting as food, and by water or liquids and solids containing water.

Hunger is a peculiarly indefinite sensation of craving or want which is referred to the stomach, but with which is often combined, always indeed in its most pronounced stages, a general feeling of weakness or faintness. The earliest stages are unattended with suffering, and, leading the animal to wish and seek for food, are characterized as “appetite for food.” Hunger is normally appeased by the introduction of solid or semi-solid nutriment into the stomach, and it is probable that the almost immediate alleviation of the sensation under these circumstances is in part due to a local influence, perhaps connected with a free secretion of gastric juice. Essentially, however, the sensation of hunger is a mere local expression of a general want, and this local expression ceases when the want is satisfied, even though only liquid and no solid food is introduced into the stomach, or even though no food be introduced into the stomach, the needs of the economy being satisfied by the introduction of food through other channels, as, for example, when food which admits of being readily absorbed is injected into the large intestine.

Thirst is a peculiar sensation of dryness and heat localized in the tongue and throat. Although thirst may be artificially produced by drying, as by the passage of a current of air over the mucous membrane of the above parts, normally it depends upon an impoverishment of the system in water. And, when this impoverishment ceases, in whichever way this be effected, the sensation likewise ceases. The injection of water into the blood, the stomach, or the large intestine appeases thirst, though no fluid is brought in contact with the part to which the sensation is referred.

The sensations the causes of which we have briefly attempted to trace lead us, or when urgent compel us, to take food and drink into the mouth. Once in the mouth, the entrance to the alimentary canal, the food begins to undergo a series of processes, the object of which is to extract from it as much as possible of its nutritive constituents. It cannot be sufficiently emphasized that food in the alimentary canal is, strictly speaking, outside the confines of the body; as much so as the fly grasped in the leaves of the insectivorous Dionea is outside of the plant itself. The mechanical and chemical processes to which the food is subjected in the stomach and intestines are processes which have their seat and conditions outside the body which it is destined to nourish, though unquestionably the body is no passive agent, and innumerable glands have to come into action in order to supply the chemical agents which shall dissolve and render assimilable those constituents of the food which are capable of being absorbed into the organism, and of forming part and parcel of its substance.

Structure of the Organs of Digestion in Relation to their Function.— The processes to which the food is subjected, though manifold, are divisible into two great groups: (1) the food must be subjected to the action of certain juices which dissolve insoluble alimentary matters, and modify these no less than certain of the soluble alimentary substances; and (2) it must be mechanically mixed with those juices, and propelled more or less slowly from beginning to end of the alimentary canal. In accordance with this twofold function the alimentary canal is divisible into two distinct but intercalated anatomical tubes,—(1) an internal tube of mucous membrane, and (2), investing this closely, an external muscular tube.

The musculo-membranous double tube thus formed is not, indeed, regularly tubular throughout. At the beginning it forms the irregular cavity of the mouth, which contains the tongue and the masticatory teeth. Thence it passes through the fauces and beneath the pendulous uvula and soft palate into the pharynx. Afterwards it proceeds as a regular tube, the oesophagus, or gullet, until the level of the diaphragm is reached,—the muscular partition between the thorax and the abdomen. Having passed through this structure, the narrow tube of the oesophagus suddenly dilates at the cardiac orifice into the bag called the stomach, at the further or pyloric orifice of which the tube resumes its narrow uniform calibre, and forms successively the duodenum, the jejunum, and the ileum,— parts of the small intestine, of which, in man, the duodenum takes up twelve fingers’ breadth, the ileum the lower three-fifths, and the jejunum the remainder of the total length of 20 feet. The small intestine diminishes somewhat in calibre from duodenum to ileum, and at the lower end of the latter opens suddenly into the much wider large intestine, which it joins, not at the extreme end, which is a cul-de-sac, the caput caecum coli, but at a point a little lower down. The margins of the aperture by which the small opens into the large intestine point or project into the latter in such a manner that, while they readily permit the passage of matters from small to large intestine, any backward movement of the contents of the large intestine would have the effect of compressing the lips of the opening and closing it; this arrangement constitutes the so-called itio-caecal valve. Connected with the caput caecum coli is a small diverticulum like a narrow glove finger, called the vermiform appendage. The first and greater part of the large intestine is known as the colon, the last as the rectum. The total length is from 5 to 6 feet. Its lower orifice is called the anus.

Both muscular and membranous (mucous) tubes are continuous from mouth to anus, and at these, the superior and inferior orifices, the mucous membrane which constitutes what we have hitherto termed the membranous tube is continuous with the skin which covers the general surface of the body. This mucous membrane is covered throughout at its free surface by an epithelium, and gives lodgment to glands, whose characters differ in different parts of the tube in accordance with the function of the part. Below the epithelium is a connectée tissue analogous to the cutis vera of the skin, which in parts has the character of ordinary fibro-areolar tissue, but in all parts from the stomach downwards has the character of so-called adenoid connective tissue, as it is found in the follicles of lymphatic glands. Its meshes support a τich supply of fi ne blood-vessels,lymphatic vessels, and doubtless also of delicate nervous filaments.

Besides these elements there are found numerous small bundles, or in parts even sheets, of involuntary muscular fibres, which probably give to the mucous membrane the power of limited self-contraction, of such a nature as to further the flux and reflux of fluids in the myriad lymphatic vessels of the part, and perhaps to influence in no small degree the outpouring of the secretion of certain of the glands. To the fairly continuous tract or sheet of involuntarymuscle which lies at the base or deepest part of the mucous membrane or mucosa the term of muscularis mucosae {tunica) is applied.

In addition to the glands which lie embedded in and open upon the surface of the mucous membrane of the alimentary canal, others of larger size and not in immediate relation with its walls communicate with the interior of the tube by ducts which open into it, and which pour into it their secretion; such glands are the salivary glands, the pancreas, and the liver.

The muscular tube in the greater part of its extent consists of two layers of involuntary, non-striated, pale, muscular fibres,—an inner layer whose fibres encircle the tube, and an outer one whose fibres run parallel to the long axis of the tube. But this is not the arrangement of every part. There is an apparent rather than a real exception in the stomach, where some layers of fibres of the circular coat course over the dilated walls of the alimentary tube in an oblique direction, giving rise to an oblique layer. In the oesophagus or gullet, besides the typical circular and longitudinal layers, there is at the upper part a second longitudinal layer which takes up a position internal to—that is, nearer the mucous membrane than—the circular layers. In the upper part of the gullet also the muscular fibres are not unstriped, but, although certainly involuntary, are striated like voluntary muscles.

In the mouth the muscular tube is most irregular and most defective, for the mucosa is in parts directly applied to the bony boundaries, as over the hard palate and gums; in another part it invests the muscular prominence of the tongue; whilst in other regions it lies upon the constrictors of the pharynx and the inner aspect of certain other muscles, as those of the cheeks, lips, and floor of the mouth. The membranous tube is united to the muscular tube by a loose layer of connective tissue containing many blood-vessels and lymphatic vessels and nerves for the supply of the mucosa; it is often called the submucosa.

The mucous membrane is the seat of various secreting glands, which lie embedded in its substance and open upon its surface,— simple or branched tubular recesses running through the depth of the layer, lined by epithelium, continuous with, though not always resembling, that of the surface, and opening at the surface by minute pores. In the mouth, pharynx, and oesophagus those form the acinous or racemose glands, which, according to certain subordinate features which they present, and also according to certain of the characters of the fluids which they secrete, are separated into mucous and serous glands. In the stomach they are represented by the simple or branched tubular gastric glands. In the duodenum we find them as simple tubular deep sockets, the glands or crypts of Lieberkühn, and also as the compound acinous glands of Brunner, which dip below the level of the mucosa and lie in the submucosa. In the jejunum, ileum, and large intestine they appear again as the crypts or follicles of Lieberkühn.

The membranous tube is not everywhere exactly concentric and conterminous with the muscular tube. In the mouth and pharynx it is almost entirely so, but at the gullet the muscular tube so tightly encloses the membranous that the latter is forced into longitudinal plications to find room for itself. In the stomach the membranous layer is raised into ridges or rugse, which intersect and give the surface a honeycombdike aspect. In the upper part of the small intestine the membranous tube is raised into deep annular or, more correctly, crescentic folds, running across the direction of the gut like incomplete diaphragms, or a series of membranous ledges; these are the valvulae conniventes. In the colon the two tubes are so disposed as to form a regular series of saccules or pouches, adding largely to the capacity of the gut. All these foldings greatly increase the superficies of the membranous tube. A further enlargement is effected in the small intestine in an exceedingly interesting fashion; the surface of the mucosa is thickly studded with innumerable fine short projections resembling the pile of velvet. These are invested by surface epithelium, and amongst them, at their feet, open the before-mentioned crypts of Lieberkuhn. They are the so-called villi. Each contains a lymphatic vessel, blood-vessels, and involuntary muscular fibres, all supported by adenoid connective tissue like that of the mucosa below; the lymphatic is in the axis of the villus, the muscles form the next layer, and the blood-vessels lie immediately beneath the epithelium. When the muscular layer of the villus contracts it must of necessity compress the lymph-vessel, whilst causing no impediment to the flow of blood.

We have described the mucosa of the stomach and intestines as containing a framework of adenoid reticular tissue like the tissue of lymphatic follicles. It is, indeed, identical with this,—a network of branched cells with oval nuclei, the meshes of which are crowded with lymph-corpuscles with round nuclei At certain points in the intestines the adenoid tissue of the mucosa presents local nodular enlargements; the mucosa at these points becomes so much thicker that it swells up at the free surface beneath the epithelium into rounded eminences about as large as millet-seeds or the heads of small pins; and at the under surface of the mucosa it dips into the submucous tissue in a similar manner. At the base of this nodule of adenoid tissue in the submucosa there is usually a network of wide, thin-walled, lymphatic vessels. Many of these rounded masses are scattered irregularly over the small and large intestines as the solitary follicles or glands, but at the lower end of the ileum they form little colonies, often covering an area an inch or more in length, situated at that part of the intestine which is remote from the attachment of the mesentery. They then constitute the so-called Peyer s patches.

Nodular adenoid masses are, however, not limited to the adenoid mucosa of the intestines. They are occasionally, though rarely, found in the stomach; they exist beneath the mucous membrane of the tongue, and a colony of them forms the mass of the tonsil on each side,—the almond-shaped body situated between the posterior and anterior pillars of the fauces.

The intrinsic nerves of the alimentary tube consist of two systems of nervous networks with ganglion-cells lying at the nodes; one is found in the submucous layer {plexus of Meissner'), the other lies between the longitudinal and circular muscles {plexus of Auerbach, or plexus myentericus).

The whole of the intestines, and the stomach as well, are sustained in the abdominal cavity by sheets of delicate membrane, formed by folds of peritoneum, and called, in the case of the intestinal portion of the tube, the mesentery. Between the layers of the mesentery run the vessels and nerves for the supply of the bowel. In addition to blood-vessels there are numerous thin-walled lymphatic vessels called lacteals, which are fed by the rich network of lymphatic vessels of the mucosa and submucosa, and which run in the mesentery to the back of the abdominal cavity. Here they are collected into a large lymphatic reservoir, the receptaculum chyli, from which a duct, the thoracic duct, proceeds along the side of the vertebral column to open into the venous system at the junction of the subclavian and jugular veins on the left side of the neck. The lacteal and lymphatic vessels, whose course has been briefly sketched, are interrupted at many points by the presence of lymphatic glands. These may be simply regarded as labyrinthine systems of vessels into which the simple afferent lymphatic or lacteal vessels open, and each of which...