24 The Gizzard. 



seen to be larger than on the right. This appears, on 

 reflection, to be of great advantage in producing the neces- 

 sary motion ; for if the two muscles were of equal strength, 

 they must keep a greater degree of exertion than is neces- 

 sary ; while, in the present case, the principal effect is 

 produced by that of the left side, and a smaller force is used 

 by that on the right to bring the parts back again. The 

 two bellies of the muscle, by their alternate action, produce 

 two effects the one a constant friction on the contents of 

 the cavity ; the other, a pressure on them. This last arises 

 from a swelling of the muscle inwards, which readily 

 explains all the instances which have been given by Spallan- 

 zani and others, of the force of the gizzard upon substances 

 introduced into it a force which is found by their experi- 

 ments always to act in an oblique direction. The internal 

 cavity, when opened in this distended state, is found to be 

 of an oval form, the long diameter being in the line of the 

 body ; its capacity nearly equal to the size of a pullet's 

 egg ; and on the sides there are ridges in their horny coat 

 (lining membrane) in the long direction of the oval. When 

 the horny coat is examined in its internal structure, the 

 fibres of which it is formed are not found in a direction per- 

 pendicular to the ligamentous substance behind it ; but in 

 the upper portion of the cavity it is obliquely upwards. 

 From this form of cavity it is evident that no part of the 

 sides is ever intended to be brought in contact, and that the 

 food is triturated by being mixed with hard bodies, and 

 acted on by the powerful muscles which form the gizzard." 

 The experiments of Spallanzani show that the muscular 

 action of the gizzard is equally powerful whether the small 

 stones are present or not ; and that they are not at all 

 necessary to the trituration of the firmest food, or the hardest 

 foreign substances ; but it is also quite clear that when 

 these small stones are put in motion by the muscles of the 

 gizzard they assist in crushing the grain, and at the same 

 time prevent it from consolidating into a thick, heavy, com- 

 pacted mass, which would take a far longer time in under- 

 going the digestive process than when separated and 

 intermingled with the pebbles. 



