intestine through an opening, guarded by the ileo-cæcal valve; the colon, continuous with the cæcum, which forms the principal part

Fig. 198.-Section of small intestine, showing villi, Lieberkühn's glands and a Peyer's solitary gland. m, m, muscularis mucosa. (Klein and Noble Smith.)

of the large intestine, and is divided into ascending, transverse, and descending portions; and the rectum, which, after dilating at its lower part, again contracts, and immediately afterwards opens

Fig. 199.-Vertical section of a villus of the small intestine of a cat. a, striated basilar border of the epithelium; b, columnar epithelium; c, goblet cells; d, central lymph-vessel; e, smooth muscular fibres; f, adenoid stroma of the villus in which lymph corpuscles lie. (Klein.)

externally through the anus. appendix vermiformis.

Attached to the cæcum is the small

Structure.-Like the small intestine, the large intestine is constructed of four principal coats, viz., the serous, muscular, sub

mucous, and mucous. The serous coat need not be here particularly described. Connected with it are the small processes of peritoneum containing fat, called appendices epiploica. The fibres of the muscular coat, like those of the small intestine, are arranged in two layers-the outer longitudinal, the inner circular. In the cæcum and colon, the longitudinal fibres, besides being, as in the

small intestine, thinly disposed in all parts of the wall of the bowel, are collected, for the most part, into three strong bands, which, being shorter, from end to end, than the other coats of the intestine, hold the canal in folds, bounding intermediate sacculi. On the division of these bands, the intestine can be drawn out to its full length, and it then assumes, of course, an uniformly cylindrical form. In the rectum, the fasciculi of these longitudinal bands spread out and mingle with the other longitudinal fibres, forming with them a thicker layer of fibres than exists on

any other part of the intestinal canal. The circular muscular fibres are spread over the whole surface of the bowel, but are somewhat more marked in the intervals between the sacculi. Towards the lower end of the rectum they become more numerous,

Fig. 201. Diagram of lacteal vessels in small intestine. A, lacteals in villi; r, Peyer's glands; B and D, superficial and deep network of lacteals in submucous tissue; L, Lieberkühn's glands; E, small branch of lacteal vessel on its way to mesenteric gland; и and o, muscular fibres of intestine; s, peritoneum. (Teichmann.)

and at the anus they form a strong band called the internal sphincter muscle.

The mucous membrane of the large, like that of the small intestine, is lined throughout by columnar epithelium, but, unlike it, is quite smooth and destitute of villi, and is not projected in the form of valvule conniventes. Its general microscopic structure resembles that of the small intestine and it is bounded below by the muscularis mucosa.

The general arrangement of ganglia and nerve-fibres in the large intestine resembles that in the small (p. 298).

Glands. The glands with which the large intestine is provided are of two kinds, (1) the tubular and (2) the lymphoid.

(1.) The tubular glands, or glands of Lieberkühn, resemble those of the small intestine, but are somewhat larger and more numerous. They are also more uniformly distributed.

(2.) Follicles of adenoid or lymphoid tissue are most numerous in the cæcum and vermiform appendix. They resemble in shape

Fig. 202.-Horizontal section through a portion of the mucous membrane of the large intestine, shewing Lieberkühn's glands in transverse section. a, lumen of gland-lining of columnar cells with c, goblet cells, b, supporting connective tissue. Highly magnified. (V. D. Harris.)

and structure, almost exactly, the solitary glands of the small intestine. Peyer's patches are not found in the large intestine.

Ileo-cæcal Valve. The ileo-cæcal valve is situate at the place of junction of the small with the large intestine, and guards against any reflex of the contents of the latter into the ileum. It is composed of two semilunar folds of mucous membrane. Each fold is formed by a doubling inwards of the mucous membrane, and is strengthened on the outside by some of the circular muscular fibres of the intestine, which are contained between the outer surfaces of the two layers of which each fold is composed. While the circular muscular fibres, however, of the bowel at the junction of the ileum with the cæcum are contained

between the outer opposed surfaces of the folds of mucous membrane which form the valve, the longitudinal muscular fibres and the peritoneum of the small and large intestine respectively are continuous with each other, without dipping in to follow the circular fibres and the mucous membrane. In this manner, therefore, the folding inwards of these two last-named structures is preserved, while on the other hand, by dividing the longitudinal muscular fibres and the peritoneum, the valve can be made to disappear, just as the constrictions between the sacculi of the large intestine can be made to disappear by performing a similar operation. The inner surface of the folds is smooth; the mucous membrane of the ileum being continuous with that of the cæcum. That surface of each fold which looks towards the small intestine is covered with villi, while that which looks to the cæcum has none. When the cæcum is distended, the margin of the folds are stretched, and thus are brought into firm apposition one with the other.

DIGESTION IN THE INTESTINES.

After the food has been duly acted upon by the stomach, such as has not been absorbed passes into the duodenum, and is there subjected to the action of the secretions of the pancreas and liver which enter that portion of the small intestine. Before considering the changes which the food undergoes in consequence, attention should be directed to the structure and secretion of these glands, and to the secretion (succus entericus) which is poured out into the intestines from the glands lining them.

THE PANCREAS, AND ITS SECRETION.

The Pancreas is situated within the curve formed by the duodenum; and its main duct opens into that part of the small intestine, through a small opening, or through a duct common to it and to the liver, about two and a half inches from the pylorus.

Structure. In structure the pancreas bears some resemblance to the salivary glands. Its capsule and septa, as well as the bloodvessels and lymphatics, are similarly distributed. It is, however, looser and softer, the lobes and lobules being less compactly arranged. The main duct divides into branches (lobar ducts), one for each lobe, and these branches subdivide into intralobular ducts,