Page:Encyclopædia Britannica, Ninth Edition, v. 17.djvu/726

668 NUTRITION 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 local ized in the tongue and throat. Although thirst may be artificially produced by drying, as by the passage of a cur rent 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 which ever 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 con stituents. 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 pro cesses which have their seat and conditions outside the body which it is destined to nourish, though unquestion ably 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 ab sorbed 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 sub stances ; 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 irre gular cavity of the mouth, which contains the tongue and the mas ticatory 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 (esophagus, or gullet, until the level of the diaphragm is reached, the muscular partition be tween 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, tz 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 c&cum 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 ttio-cucal 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 connective 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 rich supply of fine blood-vessels,lymph- atic vessels, and doubtless also of delicate nervous filaments. Besides these elements there are found numerous small bundles, or in parts even sheets, of in voluntary muscular fibres, which probably give to the mucous membrane the power of limited self-contraction, of svich 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 cer tain of the glands. To the fairly continuous tract or sheet of in voluntary muscle which lies at the base or deepest part of the mucous membrane or mucosa the term of muscular is mucosee (tunica) is Fio. l. Divisions of the Alimentary Canal. M, mouth ; Ph, pharynx ; (E, oesophagus ; 8, stomach ; D, duo denum ; J, jejunum; I, ileum; AY, appendix vermiformis ; AC, ascend ing colon ; HF, hepatic flexure ; TO, transverse colon ; SF, splenic flexure; DC, descending colon ; Sg, sigmoid flexure ; R, rectum; L, larynx; e, Eus- tachian tube; G, gall-bladder; H, hepatic duct; DC , common bilo- duct ; P, pancreatic duct. (From Turner s Anatomy, tig. 177.) 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 communi cate with the interior of the tube by ducts which open into it, and which pour into it their secre tion ; 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 mem brane 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 mus cular tube by a loose layer of connective tissue containing man} 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 sub ordinate 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