146 THE VITAL FUNCTIONS. 



tails necessary for fully illustrating this proposition, I must 

 content myself with indicatino; a few of the most general re- 

 suits of the inquiry.* 



As tiie food of birds varies, in difierent 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 granivorous birds, which occupies the other 

 extremity of the series. This gradation is established by 

 the muscular fibres, which surround the former, acquiring, 

 in difTcrent tribes, greater extent, and forming stronger mus- 

 cles, adapted to the corresponding variations in the food, 

 more especially as it partakes of the animal or vegetable 

 character. 



In all the cold-blooded vcrtebrata, where digestion is not 

 assisted by any internal heat, that operation proceeds more 

 slowly, though in the end not less effectually, than in ani- 

 mals where the contents of the stomach are constantly main- 

 tained at a high temperature. They almost all rank as car- 

 nivorous animals, and have accordingly stomachs, which, 

 however they may vary in their form, are alike simply 

 membranous in their structure, and act by means of the sol- 

 vent power of their secretions. Among reptiles, only a few 

 exceptions occur to this rule. The common sea-turtle that 

 is brought to our tables, is one of these; for it is found to 

 feed exclusively on vegetable diet, and chiefly on the sea- 

 weed called zostira maritima, and the structure of its sto- 

 mach corresponds exactly to the gizzard of birds. Some 

 tortoises, also, which eat grass, make an approach to the 

 same structure. 



In fishes, indeed, although the membranous structure of 



* The comparative anatomy of the stomach has been investig-atcd with 

 great diligence by the late Sir E. Home, and the results recorded in the pa- 

 pers he communicated, from time to time, to the I{oyul Society, and which 

 have been republished in his splendid work, entitled *' Lectures on Compara- 

 tive Anatomy," to which it will be seen that I have been largely indebted for 

 the ficts and observations relating to this subject, detailed in the text. 



