118 FACULTIES OF BIRDS. 



obstruct the view, the belly of the muscle on the left 

 side is seen to be larger than on the right, as may be 

 observed in the annexed engraving. 



44 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 up 

 a greater degree 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 of 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 the swelling 

 of the muscle inwards ; which readily explains all 

 the instances which have been given by Spallanzani 

 and others, of the force of the gizzard upon sub- 

 stances introduced 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 an oval form, the long diame- 

 ter being in the line of the body : its capacity nearly 

 equal to the size of the pullet's egg ; and on the 

 sides there are ridges in the horny coat, 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 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 sides are ever intended to be brought in 

 contact ; and that the food is triturated by being 

 mixed with hard bodies and acted on by the power- 

 ful muscles which form the gizzard*." 



Grew had long before most correctly investigated 

 the action of the gizzard, so far as it is mechanical, 

 * Comp. Anat. i. 316. 



