178 THE VITAL FUNCTIONS. 



are, Avith certain modifications, formed very much 

 upon the models of those already described ; ac- 

 cording as the food consists of animal or of vegetable 

 materials, or presents more or less resistance from 

 the cohesion of its texture. As it would be impos- 

 sible in this place to enter into all the details neces- 

 sary for fully illustrating this proposition, I must 

 content myself with indicating a few of the most 

 general results of the inquiry.* 



As the food of Birds varies, in different species, 

 from the softest animal matter to the hardest grain, 

 so we observe every gradation in their stomachs, 

 from the membranous sac of the carnivorous tribes, 

 which is one extreme, to the true gizzard of gra- 

 nivorous birds, which occupies the other extremity 

 of the series. This gradation is established by the 

 muscular fibres, which surround the former, ac- 

 quiring, in different tribes, greater extent, and 

 forming stronger muscles, adapted to the corres- 

 ponding variations in the food ; more especially in 

 as far as it partakes of the animal or the vegetable 

 character. 



In all the cold-blooded vertebrata, where diges- 

 tion is not assisted by any internal heat, that ope- 

 ration proceeds more slowly, though in the end not 

 less effectually, than in animals where the con- 

 tents of the stomach are constantly maintained at 

 a high temperature. They almost all rank as car- 



* The comparative anatomy of the stomach lias been investigated 

 ■with great diligence by the late Sir E. Home, and the results re- 

 corded in the papers he communicated from time to time to the 

 Royal Society, and which have been republished in his splendid 

 work, entitled " Lectures on Comparative Anatomy," to which it 

 will be seen that I have been largely indebted for the facts and 

 observations relating to this subject, detailed in the text. 



