170 THE VITAL FUNCTIONS. 



dropped, in small quantities at a time, into the 

 gizzard, in proportion as the latter gradually 

 becomes emptied.* Thus the analogy between 

 this natural process and the artificial operation 

 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 Spal- 

 lanzani, with a view to ascertain the force of 

 compression exerted by the gizzard on its con- 

 tents. 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 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 admiration to physio- 

 logists ; and being echoed from mouth to mouth, 

 were received with a sort of passive astonishment, 



* 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. 



