16 ZOOLOGY. 



into the large bag-like stomach, in whose walls a gastric 

 juice is secreted. This is an acid fluid (containing a frac- 

 tional percentage of hydrochloric acid), and contains a 

 ferment called pepsin. The effects of the acid are (1) to 

 destroy the ptyalin of the saliva and so stop any further 

 change of starch into sugar ; (2) to dissolve any such salts 

 as calcium carbonate (in bone-eating animals, like the dog, 

 there is a greater proportion of acid) ; (3) to combine with 

 proteids ; and (4) to destroy bacteria introduced with the 

 food. The special use of the ferment pepsin is that it 

 converts proteids into the soluble and diffusible form known 

 as peptone. 



The food remains in the stomach (the entrance to and 

 exit from which are closed by muscular action) some con- 

 siderable time, and undergoes considerable change. It 

 becomes more liquid, both by admixture of gastric juice and 

 solution of peptone, and also by the melting of any fats 

 present at the higher temperature of the stomach. In this 

 condition the food has become chyme. Probably a certain 

 amount of the dissolved material is absorbed, i.e. diffuses 

 into the blood in the walls of the stomach ; but most 

 of the absorption takes place farther along the canal. 



10. The Intestines. Following the stomach comes the 

 simple U-shaped loop called duodenum, and after this a 

 complicated series of small loops forming the ileum, which 

 ends in an expanded bulb, the sacculus rotundus. This 

 portion of the intestine from the end of the stomach to the 

 sacculus rotundus is called the small intestine. The sacculus 

 opens by a valved aperture into the colon, and this passes 

 on into the rectum which extends to the anus. At the 

 junction of the ileum and colon, there is given off a great 

 blind outgrowth, the csecum, which is large in plant-eating 

 mammals and ends in the vermiform appendix. The ileum- 

 colon loop and the csecum together are coiled up in a spiral : 

 colon, csecum, and rectum form the large intestine. 



11. The Bile. The duodenum is separated from the 

 stomach by a ring-like muscular valve, the pylorus ; this 

 valve belongs to the class of muscles called sphincters, which, 



