DIGESTION IN BIRDS. 201 



sible in this place to enter into all the details 

 necessary 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 spe- 

 cies, 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 granivorous birds, which occu- 

 pies the other extremity of the series. This 

 gradation is established by the muscular fibres, 

 which surround the former, acquiring, in dif- 

 ferent tribes, greater extent, and forming stronger 

 muscles, adapted to the corresponding variations 

 in the food, more especially as it partakes of the 

 animal or vegetable character. 



In all the cold-blooded vertebrata, where di- 

 gestion is not assisted by any internal heat, that 

 operation proceeds more slowly, though in the 

 end not less effectually, than in animals where 

 the contents of the stomach are constantly main- 

 tained at a high temperature. They almost all 



* The comparative anatomy of the stomach has been investi- 

 gated with great diligence by the late Sir E. Home, and the 

 results recorded 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. 



