18(5 THE VITAL FUNCTIONS. 



304, a gland from the stomach of the American 

 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 which 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. Hence the coats of the stomach are 

 provided with muscular fibres, passing, some in 

 a longitudinal, and others in a transverse, or 

 circular direction ; while a third set have an 

 oblique, or even spiral course. t When the 

 greater number of these muscles act together, 



* 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. 



t See Fig. 51, vol. i. p. 137, and its description, p. 138. 



