GIZZARD OF BIRDS. 



151 



naria. The Aplysia has a considerable number of 

 these gastric teeth. An apparatus of a still more 

 complicated kind is provided in most of the insects 

 belonging to the order of Orthoptera ; but I shall 

 not enter at present into a description of them, 

 as it will be more convenient to include them in 

 the general account of the alimentary canal of 

 insects, which will be the subject of future con- 

 sideration. 



The internal machinery for grinding is exempli- 

 fied on the largest scale in granivorous birds ; where 

 it forms part of the stomach itself, and is termed 



a Gizzard. It is shown in 

 Fig. 298, representing the in- 

 terior of the stomach of a 

 Swan. Both the structure and 

 the mode of operation of this 

 organ bear a striking analogy 

 to a mill for grinding corn; for 

 it consists of two powerful 

 muscles (o) of a hemispherical 

 shape, with their flat sides ap- 

 plied to each other, and their 

 edges united by a strong ten- 

 don, which leaves a vacant space of an oval or 

 quadrangular form, between their two surfaces. 

 These surfaces are covered by a thick and dense 

 horny substance, which, when the gizzard is in 

 action, performs an office similar to that of mill- 

 stones. In most birds, there is likewise a sac, or 

 receptacle, termed the Craiv, (represented laid 

 open at c), in which the food is collected, for the 

 purpose of its being dropped, in small quanties at a 

 time, into the gizzard, in proportion as the latter 



