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 protoplasm of the body, the 
exact nature of which is unknown. In the Star-fish 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 intestine, and the 
contents of the pancreas 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, forming 
chyme; secondly, the chyme, upon entering the intestine, 
is changed into chyle by the action of the pancreatic secre¬ 
tion, 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 
