GRINDING OF FOOD. 



105 



Fig. 73. space between their surfaces. 



This mechanism is shown by 

 Fig. 73, which represents the 

 gizzard of a swan laid open so 

 as to display the two grinding 

 faces g. These surfaces are 

 covered with a dense horny 

 substance, which, when brought 

 together and made to move 

 backwards and forwards, are 

 capable of crushing the hardest 

 seeds, and of reducing them to 

 powder. To assist in this ope- 

 j ration, many birds swallow 

 small stones which mixing with 

 the grain, facilitate the process. 

 In most birds with gizzards, there is a part called the 

 crop ^represented and laid open and empty at c, in which 

 the food is collected and softened by heat and moisture, 

 before it enters the gizzard. This part therefore, acts as 

 the hopper to the mill, and from it only a few grains are 

 admitted at a time, as they are ground and pass on to 

 the digestive organ, or proper stomach. 



The gizzards of birds have been the subjects of nu- 

 merous, and elaborate experiments, by various physi- 

 ologists. Those of Spallanzani were the best conducted, 

 and are the most celebrated. He introduced balls of 

 glass into the gizzard of a turkey, and found that they 

 were ground to powder. Tin tubes were also flattened 

 and bent into various shapes by the powerful action of 

 its muscles ; and even the points of needles and lancets, 

 set in balls of lead were worn, or broken off, while the 

 grinding part itself, appeared to have suffered not the 

 least injury. 



These results at the time they were made and pub- 

 lished, struck all philosophers with wonder and amaze- 

 ment, and calculations were soberly made in order to 

 estimate the actual power required in the muscles of 

 the gizzard to perform such feats. 



But the celebrated John Hunter having instituted 

 further inquiries, found that the pressure of the two 

 faces, instead of being perpendicular, as was supposed, 



