tractions of that organ roll it about and mix it thoroughly with the gastric juice.

19. It is easy to ascertain the properties of gastric juice experimentally, by putting a small portion of that part of the mucous membrane which contains the peptic glands into acidulated water containing small pieces of meat,

FIG. 42. THE STOMACH LAID OPEN BEHIND.

a, the oesophagus; b, the cardiac dilatation; c, the lesser curvature; d, the pylorus; e, the biliary duct; f, the gall-bladder; g, the pancreatic duct, opening in common with the cystic duct opposite h; h, i, the duodenum.

hard-boiled egg, or other proteids, and keeping the mixture at a temperature of about 100°. After a few hours it will be found that the white of egg, if not in too great quantity, has become dissolved; while all that remains of the meat is a pulp, consisting chiefly of the connective tissue and fatty matters which it contained. This is artificial digestion, and it has been proved by experiment that precisely the same operation takes place when food undergoes natural digestion within the stomach of a living animal.

The proteid solution thus effected is called a peptone, and has pretty much the same characters, whatever the nature of the proteid which has been digested.

Peptone differs from all other proteids in its extreme solubility, and in the readiness with which it passes through animal membranes. Many proteids, as fibrin,

are naturally insoluble in water, and others, such as white of egg, though apparently soluble, are not completely so, and can be rendered quite solid or coagulated by being simply heated, as when an egg is boiled. A solution of peptone however is perfectly fluid, does not become solid, and

is not at all coagulated by boiling. Again, if a quantity of white of egg be tied up in a bladder, and the bladder immersed in water, very little of the proteid will pass through the bladder into the water, provided that there are no holes. If, however, peptone be used instead of albumin, a very large quantity will speedily pass through into the water, and a quantity of water will pass from the outside into the bladder, causing it to swell up. This process is called osmosis, and is evidently of great importance in the economy; and the purpose of the conversion of the various proteids by digestion into peptone seems to be, in part at least, to enable this class of food-stuff to pass readily into the blood through the thin partition formed by the walls of the mucous membrane of the intestine and the coats of the capillaries.

Similarly, starch, even when boiled, and so partially dissolved, will not pass through membranes, whereas sugar does so with the greatest ease. Hence the reason of

the conversion of starch, by digestion, into sugar.

It takes a very long time (some days) for the dilute acid alone to dissolve proteid matters, and hence the solvent power of gastric juice must be chiefly attributed to the pepsin.

As far as we know gastric juice has no direct action on fats; by breaking up, however, the proteid framework in which animal and vegetable fats are imbedded, it sets these free, and so helps their digestion by exposing them to the action of other agents. It appears, too, that gastric juice has no direct action on amyloids; on the contrary, the conversion of the starch into sugar begun in the mouth appears to be wholly or partially arrested by the acidity of the contents of the stomach, ptyalin being active only in an alkaline or neutral mixture.

29. By continual rolling about, with constant additions of gastric juice, the food becomes reduced to the consistence of pea-soup, and is called chyme. In this state it is, in part, allowed to escape through the pylorus and to enter the duodenum; but a great deal of the fluid (consisting of peptone together with any saccharine fluids resulting from the partial conversion of starch, or otherwise) is at once absorbed, making its way, by imbibition, through the walls of the delicate and numerous vessels of

FURTHER

A. Cavity of the thorax, pleural cavity of either side; B. Diaphragm ;

the stomach into the current of the blood, which is rushing though the gastric veins to the vena porta.

21. The intestines form one long tube, with mucous and muscular coats, like the stomach; and, like it, they are enveloped in peritoneum. They are divided into two portions-the small intestines and the large intestines; the latter having a much greater diameter than the former. The small intestines again are subdivided into the duodenum, the jejunum, and the ileum, but there is no natural line of demarcation between these. The duodenum, however, is distinguishable as that part of the small intestine which immediately succeeds the stomach, and is bent upon itself and fastened by the peritoneum against the back wall of the abdomen, in the loop shown in Fig. 42. It is in this loop that the head of the pancreas lies (Fig. 38).

The ileum (Fig. 45, a) is no wider than the jejunum or duodenum, so that the transition from the small intestine to the large (e) is quite sudden. The opening of the small intestine into the large is provided with prominent lips which project into the cavity of the latter, and oppose the passage of matters from it into the small intestine, while they readily allow of a passage the other way. This is the ileo-cæcal valve (Fig. 45, d).

The large intestine forms a blind dilatation beyond the ileo-cæcal valve, which is called the cæcum; and from this an elongated, blind process is given off, which, from its shape, is called the vermiform appendix of the oæcum (Fig. 45, b).

The cæcum lies in the lower part of the right side of the abdominal cavity. The colon, or first part of the large intestine, passes upwards from it as the ascending colon; then making a sudden turn at a right angle, it passes across to the left side of the body, being called the

C, ventricles of the heart; D, auricles; E, pulmonary artery; F, aorta; G, lungs, collapsed, and occupying only back part of chest; H, lateral portions of pleural membranes; I, cartilage at the end of sternum (ensiform cartilage); K, portion of the wall of body left between thorax and abdomen; a, cut ends of the ribs; L, the liver, in this case lying more to the left than the right of the body; M, the stomach, a large part of the greater curvature being shown; N, duodenum; 0, small intestine; P, the cæcum, so largely developed in this and other herbivorous animals; Q, the large intestine.