124 



THE VITAL FUNCTIONS. 



naria. The flplysia has a considerable number of these gas- 

 tric 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 in their de- 

 scription, 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 exemplified on 

 the largest scale in granivorous birds; where it forms part 

 298 |J!;ii^'j>!l!iU of the stomach itself, and is termed 



a Gizzard. It is shown in Fig. 

 298, representing the interior of 

 the stomach of a Swan. Both the 

 structure and the mode of operation 

 of this organ bear a striking analo- 

 gy to a mill for grinding corn, for 

 it consists of two powerful mus- 

 cles (G,) of a hemispherical shape, 

 with their flat sides applied to each 

 other, and their edges united by a 

 strong tendon, which leaves a va- 

 cant 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 ac- 

 tion, performs an office similar to that of mill-stones. In 

 most birds, there is likewise a sac, .or receptacle, termed 

 the Craw, (represented laid open at c) in which the food 

 is collected for the purpose of its being dropped, in small 

 quantities at a time, into the gizzard, in proportion as the 

 latter gradually becomes emptied.* Thus, the analogy be- 

 tween this natural process and the artificial operation of a 

 corn-mill is preserved even in the minuter details; for while 

 the two flat surfaces of the gizzard act as mill-stones, the 

 craw supplies the place of the hopper, the office of which is 



* The gastric glands, which are spread over the greater part of the inter- 

 nal surface of the craw, and which prepare a secretion for macerating the 

 grain, are also seen in this part of the figure. 



