180 BIOLOGY. [Book ii. 



of the aliments witli the mucous membrane, that saliva is 

 secreted abundantly, and assumes all its characteristics. Mitscher- 

 lich observed that the parotidian saliva of man was acid in the 

 state of comparative repose of the gland, but became alkaline 

 during mastication. The accidental acidity of the saliva is 

 probably due to a secondary modification of the starch which 

 becomes first of all sugar under the influence of the salivary 

 diastasis, then evolves into lactic acid. In its turn this acid is 

 carried with the aliments into the stomach, where it contributes 

 its share to the gastric digestion. 



The alimentary mass, more or less impregnated with saliva, 

 passes from the mouth into the first portion of the digestive 

 canal, into the pharynx when there is one, lined in certain 

 reptiles with a vibratile epithelium destined to aid in the transfer 

 of the alimentary particles, but clothed in man alone with a 

 simple epidermoidal epithelium called pavimenious. From the 

 oesophagus the alimentary mass passes into the stomach, where 

 the most important digestive acts are accom^plished. 



B. Gastric Digestion. — The gastric digestion is the principal 

 act of the digestive function, since, as we are about to see, the 

 stomach is the grand laboratory where is principally operated the 

 transformation of the albuminoidal alimentary matters into 

 absorbable peptones. The state, still so imperfect, of comparative 

 physiology does not permit us to draw here a complete picture 

 of albuminoidal digestion in the whole of the animal kingdom. 

 Nevertheless we know that in the inferior organisms, the in- 

 vertebrates, for example, are for the most part carnivorous. It 

 is manifest that, spite of the imperfection of their digestive 

 system, they must succeed in effecting this transformation of 

 albuminoids into peptones, which is achieved among the superior 

 vertebrates only in a differentiated stomach provided with special 

 secretory apparatus. In the transparent invertebrates we can 

 demonstrate the transformation undergone by the aliments, 

 Dugfes has thus seen, through the tissues, in the planaries and 

 the clepsines the blood swallowed losing by degrees its red 



