186 THE VITAL FUNCTIONS. 



ostrich; and Fig. 305, a section of a gastric 

 gland in the beaver, showing the branching of 

 the ducts, which form three internal openings. 

 In birds that live on vegetable food, the structure 

 of the gastric glands is evidently different from 

 that of the corresponding glands in predaceous 

 birds ; but as these anatomical details have not 

 as yet tended to elucidate in any degree the pur- 

 poses to w hich they are subservient in the pro- 

 cess of digestion, I pass them over as being 

 foreign to the object of our present inquiry.* 



It is essential to the perfect performance of 

 digestion, that every part of the food received into 

 the stomach should be acted upon by the gastric 

 juice ; for which purpose provision is made that 

 each portion shall, in its turn, be placed in 

 contact with the inner surface of that organ. 

 This is the more necessary, as many facts ren- 

 der it probable, as will be noticed more parti- 

 cularly hereafter, that, besides the chemical 

 action of the gastric juice, an influence, derived 

 from the nerves, essentially contributes to the 

 accomplishment of the chemical changes which 

 the food undergoes in the stomach. For this 

 reason it is that the coats of the stomach are 

 provided with muscular fibres, passing, some 



* These structures have been examined with great care and 

 minuteness by Sir Everard Home, who has given the results of 

 his inquiries in a series of papers, read from time to time to the 

 Royal Society, and published in their Transactions. 



