ACTION OF THE GIZZARD. 153 



tremely 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 opportu- 

 nity of acting directly upon those surfaces. Still, 

 however, it is evident that the effects produced 

 upon sharp metallic points and edges, could not be 

 accomplished by the gizzard without some assist- 

 ance 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 constantly 

 found to contain a certain quantity of small parti- 

 cles 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 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 explana- 

 tion, and has contended that the pebbles found in 

 the gizzard are swallowed merely by accident, or 

 in consequence of the stupidity of the bird, which 

 mistakes them for grain. But this opinion has 

 been fully and satisfactorily refuted, both by For- 

 dyce and by Hunter, whose observations concur in 

 establishing the truth of the common opinion, that 

 in all birds possessing gizzards, the presence of 

 these stones is essential to perfect digestion. A 

 greater or less number of them is contained in 

 every gizzard, when the bird has been able to meet 

 Mdth the requisite supply ; and they are never 

 swallowed but in order to assist digestion. Several 



