152 THE VITAL FUNCTIONS. 



gradually becomes emptied.* Thus the analogy 

 between this natural process and the artiticial ope- 

 ration of a corn mill is preserved even in the minuter 

 details ; for while the two flat surfaces of the gizzard 

 act as mill-stones, the craw supplies the place of 

 the hopper, the office of which is to allow the grain 

 to pass out 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 Spallan- 

 zani, with a view to ascertain the force of com- 

 pression 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, introduced 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 lan- 

 cets, 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 admiration to physiologists ; and being echoed 

 from mouth to mouth, were received with a sort of 

 passive astonishment, till John 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 mo- 

 tion of the sides of the gizzard, when actuated by 

 its muscles, is lateral, and at the same time cir- 

 cular ; so that the pressure it exerts, though ex- 



* The gastric glands, which are spread over the greater part of 

 the internal surface of the craw, and which prepare a secretion for 

 macerating the grain, are also seen in this part of the figure. 



