126 THE VITAL FUNCTIONS. 



ration. Spallanzani, however, has called in question the 

 souiuhicss of this explanation, and has contended that the 

 pebbles found in the gizzard are swallowed merely by acci- 

 dent, or in consequence of the stupidity of the bird, which 

 mistakes them for grain. But this opinion has been fully 

 and satisfactorily refuted both by Fordyce and by Hunter, 

 whose observations concur in establishing the truth of the 

 common opinion, that in all birds possessing gizzards, the 

 presence of these stones is essential to perfect digestion. A 

 greater or less number of them is contained in every giz- 

 zard, when the bird has been able to meet with the requi- 

 site supply, and they are never swallowed but along with 

 the food. Several hundred w^ere found in the gizzard of a 

 turkey; and two thousand in that of a goose: so great an 

 accumulation could never have been the result of mere ac- 

 cident. If the alleged mistake could ever occur, w^e should 

 expect it to take place to the greatest extent in those birds 

 which are starving for want of food; but this is far from be- 

 • ino- the case. It is found that even chickens, which have 

 been hatched by artificial heat, and which could never have 

 been instructed by the parent, are yet guided by a natural 

 instinct in the choice of the proper materials for food, and 

 for assistinsi its digestion: and if a mixture of a large quan- 

 tity of stones with a small proportion of grain be set before 

 them, they will at once pick out the grain, and swallow 

 alono- with it only the proper proportion of stones. The 

 best proof of the utility of these substances may be derived 

 from the experiments of Spallanzani himself, who ascer- 

 tained that grain is not digested in the stomachs of birds, 

 when it is protected from the effects of trituration. 



Thus, the gizzard may, as Hunter remarks, be regarded 

 as a pair of jaws, whose teeth are taken in occasionally to 

 assist in tliis internal mastication. The lower part of the 

 gizzard consists of a thin muscular bag, of which the office 

 is to digest the food which has been thus triturated. 



Considerable differences are met with in the structure of 

 the gizzards of various kinds of birds, corresponding to dif- 



