SOURCES OF ENERGY AND MATERIALS 



103 



characteristic structural features and each occupies a certain portion of 

 the intestine. The duodenum receives the secretions of two large diges- 

 tive glands, the liver and the pancreas. In the frog the secretions of 

 these two glands are discharged through the common hile duct into the 

 middle region of the duodenum. A reservoir, the gall bladder, attached 

 to the liver and connected with the bile duct, serves as a storage place 

 for the hile, one of the secretions of the liver. The small intestine is 

 connected at its posterior end with the large intestine which in the frog 

 is subdivided into two portions, namely, the rectum and the cloaca. The 

 term cloaca is used to designate that portion of the large intestine 

 which is used as a common passage for undigested materials from the 



Fig. 90. — Digestive systems of the frog (left) and nlan, somewhat simplified. 



alimentary tract, for urine, and for reproductive cells from the urino- 

 genital system. It occurs in a few mammals and in most other verte- 

 brates. The large intestine opens to the exterior by means of the anus. 



The human digestive system differs little enough from that of the 

 frog that the illustration in Fig. 90, right, should suffice without further 

 description. That figure, however, omits the mouth and its associated 

 salivary glands, and the small intestine is greatly shortened. 



It is worthy of note that the intestine of the frog is relatively short. 

 This condition is found in flesh-eating animals in general. Animals which 

 feed entirely or largely upon vegetable food have long intestinal tracts 

 and frequently have a large caecum (a blind pouch) at the junction of 

 the small and large intestines. The rabbit and muskrat have a large 

 caecum with a vermiform appendix at its end; the chicken and dove have 

 two caeca. In man the caecum is small, rudimentary, with a vermiform 



