ACTION OF THE GIZZARD. 171 



till Hunter directed the powers of his mind to the 

 inquiry, and gave the first rational explanation 

 of the mechanism by which they are produced. 

 He found that the motion of the sides of the 

 gizzard, when actuated by its muscles, is lateral, 

 and at the same time circular ; so that the pres- 

 sure it exerts, though extremely great, is directed 

 nearly in the plane of the grinding surfaces, and 

 never perpendicularly to them ; and thus the 

 edges and points of sharp instruments are either 

 bent or broken off by the lateral pressure, without 

 their having an opportunity of acting directly 

 upon those surfaces. Still, however, it is evident 

 that the effects we observe produced upon sharp 

 metallic points and edges, could not be accom- 

 plished by the gizzard without some assistance 

 from other sources ; and this assistance is pro- 

 cured in a very singular, and, at the same time, 

 very effectual manner. 



On opening the gizzard of a bird, it is con- 

 stantly found to contain a certain quantity of 

 small pebbles, which must have been swallowed 

 by the animal. The most natural reason that 

 can be assigned for the presence of these stones, 

 is, that they aid the gizzard in triturating the 

 contained food, and that they, in fact, supply 

 the office of teeth in that operation. Spallanzani, 

 however, has called in question the soundness of 

 this explanation, and has contended that the 

 pebbles found in the gizzard are swallowed 



