GIZZARDS OF BIRDS. 



109 



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 consideration. 



The internal machinery for grinding is exem- 

 plified on the largest scale in granivorpus birds; 

 where it forms part of the stomach itself, and is 



termed a Gizzard. It is 

 shown in Fig. 298, repre- 

 senting the interior 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 power- 

 ful muscles (g), of a hemis- 

 pherical shape, with their 

 flat sides applied to each other, and their edges 

 united by a strong tendon, 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 

 CraWy (represented laid open at c), in which the 

 food is collected for the purpose of its being 



