ACTION OF THE GIZZARD. 125 



to allow the grain to pass put in small quantities into the 

 aperture of the upper mill-stone, which brings it within the 

 sphere of their action. 



Innumerable are the experiments which have been made, 

 particularly by Reaumur and Spallanzani, with a view to 

 ascertain the force of compression exerted by the gizzard 

 on its contents. Balls of glass, which the bird was made to 

 swallow with its food, were soon ground to powder: tin 

 tubes, introduce^ into the stomach, were flattened, and then 

 bent into a variety of shapes; and it was even found that 

 the points of needles and of lancets fixed in a ball of lead, 

 were blunted and broken off by the power of the gizzard, 

 while its internal coat did not appear to be in the slightest 

 degree injured. These results were long the subject of ad- 

 miration to physiologists; and being echoed from mouth to 

 mouth, were received with a sort of passive astonishment, 

 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 pressure 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 sur- 

 faces. Still, however, it is evident that the effects we ob- 

 serve produced upon sharp metallic points and edges, could 

 not be accomplished by the gizzard without some assistance 

 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 s'mall pebbles, \vhich must 

 have been swallowed by the animal. The most natural rea- 

 son 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 ope- 



