THE DIGESTIVE SYSTEM 



59 





Brunner's glands, the pancreas, and the liver ; the position of the 

 ducts of the two latter organs being indicated in Fig. 20. 



Intestinal Canal. The intestinal canal varies greatly in relative 

 length and capacity in different animals, and it also offers manifold 

 peculiarities of form, being sometimes a simple cylindrical tube of 

 nearly uniform calibre throughout, but more often subject to altera- 

 tions of form and capacity in different portions of its course, the 

 most characteristic and constant being the division into an upper 

 and narrower, and lower and wider portion, called respectively the 

 small and the large intestine, the former being divided quite arbi- 

 trarily and artificially into duodenum, jejun- 

 um, and ileum, and the latter into colon and 

 rectum. One of the most striking peculiari- 

 ties of this part of the alimentary canal is 

 the frequent presence of a diverticulum or 

 blind pouch, the caput ccecum coli, as it was 

 first called, a name generally abbreviated into 

 " caecum," situated at the junction of the 

 large and the small intestine, a structure pre- 

 senting an immense variety of development, 

 from the smallest bulging of a portion of the 

 side wall of the tube to a huge and complex 

 sac, greatly exceeding in capacity the whole 

 of the remainder of the alimentary canal. It 

 is only in herbivorous animals that the caecum 

 is developed to this great extent, and among 

 these there is a curious complementary re- 

 lationship between the size and complexity 

 of this organ and that of the stomach. 

 Where the latter is simple the caecum is 

 generally the largest, and vice versd. Both the 

 caecum and colon are often sacculated, a dis- 

 position caused by the arrangement of the 

 longitudinal bands of muscular tissue in their 

 walls ; but the small intestine is always smooth and simple-walled 

 externally, though its lining membrane often exhibits various 

 contrivances for increasing the absorbing surface without adding to 

 the general bulk of the organ, such as the numerous small villi by 

 which it is everywhere beset, and the more obvious transverse, 

 longitudinal, or reticulating folds projecting into the interior, met 

 with in many animals, of which the " valvulae conniventes " of Man 

 form well-known examples. 



Besides the crypts of Lieberkuhn found throughout the in- 

 testinal canal, and the glands of Brunner confined to the duodenum, 

 there are other structures in the mucous membrane, about the 

 nature of which there is still much uncertainty, called " solitary " and 



FIG. 21. Diagrammatic 

 plan of the general arrange- 

 ment of the alimentary canal 

 in a typical Mammal, o, 

 Oesophagus ; st, stomach ; p, 

 pylorus ; s, s, small intestine 

 (abbreviated) ; c, caecum ; I, I, 

 large intestine or colon, end- 

 ing in r, the rectum. 



