GIZZAHDS OF BIRDS. 1-55 



made by the former, a smaller fbrce is used by the 

 latter to restore the parts to their situation. These 

 muscles produce, by their alternate action, two 

 effects ; the one a constant trituration, by a rota- 

 tory motion ; the other a continued, but oblique 

 pressure of the contents of the cavity. As this 

 cavity is of an oval form, and the muscle swells 

 inwards, the opposite sides never come into con- 

 tact ; and the interposed materials are triturated 

 by their being intermixed with hard bodies. In 

 the Goose and Swan, on the contrary, the cavity is 

 flattened, and its lateral edges are very thin. The 

 surfaces applied to each other are mutually adapted 

 in their curvatures ; a concave surface being every 

 where applied to one which is convex : on the left 

 side, the concavity is above ; but on the right side, 

 it is below. The horny covering is much stronger, 

 and more rough, than in the turkey ; so that the 

 food is ground by a sliding, instead of a rotatory 

 motion of the parts opposed ; and they do not re- 

 quire the aid of any intervening hard substances 

 of a large size. This motion bears a great resem- 

 blance to that of the grinding teeth of ruminating 

 animals, in which the teeth of the under jaw slide 

 upwards, within those of the upper, pressing the 

 food between them, and fitting it, by this peculiar 

 kind of trituration, for being digested. 



* Home, Phil. Trans, for 1810, p. 188. 



