GIZZARDS OF BIRDS. 173 



may be derived from the experiments of Spal- 

 lanzani himself, who ascertained that grain is 

 not digested in the stomachs of birds, when it is 

 protected from the effects of trituration. 'i 



Thus the gizzard may, as Hunter remarks, be 

 regarded as a pair of jaws, whose teeth are taken 

 in occasionally to assist in this internal mastica- 

 tion. The lower part of the gizzard consists of 

 a thin muscular bag, of which the office is to 

 digest the food that has been thus triturated. 



Considerable differences are met with in the 

 structure of the gizzards of various kinds of 

 birds, corresponding to differences in the texture 

 of their natural food. In the Turkey, the two 

 muscles which compose the gizzard are of un- 

 equal strength, that on the left side being consi- 

 derably larger than that on the right ; so that 

 while the principal effort is made by the former, 

 a smaller force is used by the latter to restore 

 the parts to their situation. These muscles pro- 

 duce, by their alternate action, two effects ; the 

 one a constant trituration, 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 flattened, 

 and its lateral edges are very thin. The surfaces 



