92 



COMPARATIVE ZOOLOGY. 



Butterfly; but solid substances, to facilitate their solu- 

 tion, are ground or torn into pieces by teeth, as in Man; 

 by jaws, as in the Lobster; or by a gizzard, as in the 

 Turkey. 



The chemical preparation of food is indispensable. 47 It 

 is accomplished by one or more solvent fluids secreted in 

 the alimentary canal. The most important, and one al- 

 ways present, is the gastric juice, the secretion of which 

 is restricted to the stomach, when that cavity exists. In 

 the higher animals, numerous glands pour additional flu- 

 ids into the digestive tube, as saliva into the upper part 

 or mouth, and bile and pancreatic juice into the upper 

 part of the intestine. In fact, the mucous 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 further in the more highly de- 

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

 by some secretion from the walls of the body-cavity, the 

 exact nature of which is unknown. In the Star-fish and 

 Sea-urchin, we find two solvents — a gastric juice, and 

 another resembling bile; but the two appear to mingle in 

 the stomach. Mollusks and Arthropods show a clear 

 distinction between the stomach and intestine, and the 

 contents of the liver are poured into the latter. There 

 are, therefore, two stages in the digestive act: first, the 

 food is dissolved by the gastric juice in the stomach, form- 

 ing chyme ; secondly, the chyme, upon entering the intes- 

 tine, is changed into chyle by the action of the bile, and 

 is then ready to be absorbed into the system. 



In Vertebrates, a third solvent is added, the pancreatic 

 juice, which aids the bile 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 



