SALIVARY APPARATUS. 127 



ferences In the texture of their natural food. In the Tin^keyy 

 the two muscles which compose the gizzard are of unequal 

 strength, that on the left side being considerably larger than 

 that on the right; so that while the principal cifort is made 

 by the former, a smaller force 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 tritura- 

 tion, by a rotatory 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 contact, and the interposed materials 

 are triturated by their being intermixed with hard bodies. 

 In the Goose and Swan, on the contrary, the cavity is flat- 

 tened, and its lateral edges are very thin. The surfaces ap- 

 plied 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 require the aid of any 

 intervening hard substances of a large size. This motion 

 bears a great resemblance 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 tritu- 

 ration, for being digested.'^ 



^ 6. Beglulition, 



The great object of the apparatus which is to prepare the 

 food for digestion, is to reduce it into a soft pulpy state, so 

 as to facilitate the chemical action of the stomach upon it: 

 for this purpose, solid food must not only be subjected to 

 mechanical trituration, but it must also be mixed with a cer- 



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



