170 THE VITAL FUNCTIONS. 



being dropped, in small quantities at a time, 

 into the gizzard, in proportion as the latter gra- 

 dually 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 sup- 

 plies the place of the hopper, the office of which 

 is to allow the grain to pass out in small quan- 

 tities 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 ofl" 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. 



