THEIR STKUCTUKB. 



digestive organs — and, in fact, forms a wonderful grinding- 

 null. The gizzard is composed of numerous powerful muscles, 

 and is lined with a membrane of a tough, leathery nature. 



The grinding is performed by two large and thick hemisphe- 

 rical muscles, opposed to each other, and working in the same 

 nanner as two millstones ; these reduce the grain to a certain 

 consistency, after which the gastric juices take up the dissolving 

 process and finish the digestion of the food. To assist in the 

 grinding, the birds swallow numerons small stones or pebbles ; 

 these are, of course, absolutely essential to the existence of 

 fowls. Sir Everard Home, in his " Comparative Anatomy," 

 makes the following observations on the gizzard of the turkey : — 



" When the external form of this organ is first attentively 

 examined, viewing that side which is anterior in the Hviug 

 bird, and on which the two belhes of the muscle and middle 

 are more distinct, there being no other part to obstruct the 

 view, the belly of the muscle on the left side is seen to be larger 

 than on the right. This appears, on reflection, to be of great 

 advantage in producing the necessary motion; for if the two 

 muscles were of equal strength, they must keep a greater de- 

 gree of exertion than is necessary; 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 mnsdle, by their alternate action, 

 produce two effects, — the one, a constant friction on the con- 

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

 arises from a swelling of the muscle inwards, which readily ex- 

 plains all the instances which have been given by Spallanzani 

 and others, of the force of the gizzard upon substances intro- 

 duced into it — a force which is found by their experiments 

 always to act in an oblique direction. The internal cavity, 

 when opened in this distended state, is found to be of mv oval 

 form, the long diameter being in the line of the body ; itt capa- 

 city nearly equal to the size of a pullet's egg ; and on the sides 

 there are ridges in their homy coat (lining membrane) in the 

 long direction of the oval. 



" When the homy coat is examined in its internal structure, 

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

 perpendicular to the ligamentous substance behind it ; but in 

 the upper portion of the cavity they have a direction obliquely 

 upwards. 



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



