HOW ANIMALS DIGEST 295 



membrane, which lines the alimentary canal throughout, 

 abounds with secreting glands or cells. 



The Digestive Process is substantially the same in- all 

 animals, but it is carried farther in the more highly de- 

 veloped forms. In the Infusoria, the food is acted upon 

 by some secretion from the protoplasm of the body, the 

 exact nature of which is unknown. In the starfish and 

 sea urchin, we find two solvents a gastric juice, and an- 

 other resembling pancreatic juice; but the two appear 

 to mingle in the stomach. Mollusks and arthropods 

 show a clear distinction between the stomach and intes- 

 tine, and the contents of the pancreas are poured into 

 the latter. There are, therefore, two stages in the digest- 

 ive act : first, the food is dissolved by the gastric juice in 

 the stomach, forming chyme ; secondly, the chyme, upon 

 entering the intestine, is changed into chyle by the action 

 of the pancreatic secretion, and is then ready to be 

 absorbed into the system. 



In vertebrates, a third solvent is added, the bile, which 

 aids the pancreatic juice in completing digestion. But 

 mammals and insects have a still more perfect and elab- 

 orate process ; for in them the saliva of the mouth acts 

 chemically upon the food ; while the saliva in many other 

 animals has no other office, so far as we know, than to 

 moisten the food for swallowing. 



Taking man as an example, let us note the main facts 

 in the process. During mastication, by which the rela- 

 tive surface is increased, the food is mixed with saliva, 

 which moistens it, 102 and turns a small part of the 

 s ( tarch into grape sugar. Passed into the stomach, the 

 food meets the gastric juice. This is acid, and, first, 

 stops the action of the saliva ; secondly, by means of 

 the pepsin which it contains, and the acid, it dissolves 

 the albumen, fibrin, and other such constituents of the 

 food. This solution of albuminoids is called -a peptone, 



