ACTION OF THE GIZZARD. 171 



till John Hunter directed the powers of his mind 

 to the inquiry, and gave the first rational expla- 

 nation of the mechanism by which they are pro- 

 duced. He found that the motion of tlie sides of 

 the gizzard, when actuated by its muscles, is 

 lateral, and at the same time circular; so that 

 the pressure it exerts, though extremely great, is 

 directed nearly in the plane of the grinding sur- 

 faces, and never perpendicularly to them ; and 

 thus the edges and points of sharp instruments 

 are either bent or broken off by the lateral pres- 

 sure, without their having an opportunity of 

 acting directly upon those surfaces. Still, how- 

 ever, it is evident that the effects produced upon 

 sharp metallic points and edges, could not be 

 accomplished by the gizzard without some as- 

 sistance from other sources ; and this assistance 

 is procured 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 particles of gravel, 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 tri- 

 turating 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 swal- 



