102 GENERAL ZOOLOGY 



actually much longer than that of the adult frog, which is 

 mainly carnivorous. The length of the alimentary canal of a 

 ruminant mammal is from 20 to 28 times the length of the body 

 while that of a meat-eating form is only five to six times the 

 body length. This may be understood if it is remembered that 

 plant eaters require a larger volume of food than carnivorous 

 forms, because the bulkier vegetable food such as grass, hay, 

 clover, etc., contains less nutriment and more of difficultly 

 digestible material than an equal volume of meat. 



The large intestine of the frog is simply the enlarged continua- 

 tion of the small intestine. In mammals the small intestine 

 joins the large intestine at right angles in such a way as to leave 

 a blind end of the large intestine at one side of the union (Fig. 66). 

 At this junction is a circular valve, the ileocaecal valve. The 

 blind end of the large intestine, called the caecum, is highly varia- 

 ble in form and size, being best developed in herbivorous forms 

 and least in carnivorous ones. Thus in the horse it measures 

 about four feet in length and has a capacity of seven or eight 

 gallons. In the cat it is practically absent. In man the caecum 

 measures about 2% in. in length and Z% in. in width. The 

 vermiform appendix is a diverticulum of the caecum about 3 in. 

 long and about % in. wide. The human large intestine is known 

 as the colon. From its point of union with the small intestine 

 in the lower right abdominal (pelvic) region, the colon passes 

 anteriorly as the ascending colon, continues to the left beneath 

 the anterior wall of the abdomen as the transverse colon, and then 

 turns posteriorly as the descending colon, which takes an S-shaped 

 turn, the sigmoid flexure, before joining the rectum which termi- 

 nates at the anus (Fig. 66). The longitudinal layer of the tunica 

 muscularis of the colon is arranged in three equidistant longi- 

 tudinal flat bundles or bands between which the longitudinal 

 layer is reduced to a thin sheet. These bands, known as taeniae, 

 are shorter than the internal layers of the colon, and as a result, 

 the colon is divided into pockets, called haustra, by transverse 

 folds or plicae semilunares. 



Liver. — The liver is a large gland whose duct, the bile duct, 

 empties into the duodenum. The tributaries to the bile duct are 

 the bile capillaries, which course between the liver cells and send 

 lateral branches into the cells. Bile is a fluid secreted by the 

 liver cells into the bile capillaries, which unite to form larger 



