GRINDING OF FOOD. 



space between their surfaces 

 This mechanism is shown by , fHHf 



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 

 backward and forward, are 

 capable of crushing the hardest! 

 seeds, and of reducing them to' 

 powder. To assist in this oper- 

 ation, many birds swallow small 

 stones, whicli mixing with the 

 grain facilitate the process. 



286. 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 moist- 

 ure 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. 



287. The gizzards of birds have been the subject of nu- 

 merous and elaborate experiments, by various physiolo- 

 gists. 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 appears to have suffered not the least 

 injury. 



288. These results at the time they were made and 

 published, struck all philosophers with wonder and amaze- 

 ment, and calculations were soberly made in order to esti- 

 mate the actual power required in the muscles of the 

 gizzard to perform such feats. 



289. But the celebrated John Hunter having instituted 

 further inquiries, found that the pressure of the two faces, 



4 



