Page 529. 
108 THE MECHANISM OF THE GIZZARD IN BIRDS. 
expansion can only occur towards the base of the triangle. The 
motion in this direction is furthered by the arrangement of some of 
the muscular fibres, as can be seen on close inspection of the section of 
the relaxed gizzard; for the dense horny pads above referred to are 
cupped on their attached surfaces, and the fibres run from one margin 
of this cup to the other, in an arched manner, as seen in the section. 
Those fibres just above the cup are arched also in the same way, and 
the epithelial margins of the cup are more yielding than elsewhere. 
Consequently, when the contraction occurs, the fibres straightening 
reduce the antero-posterior diameter of the cup and make the pad 
more convex towards the intermediate cavity, and push each towards 
its fellow, this action, combined with that of the other more marginal 
fibres, producing a most powerful compression of the contents. 
The great force exercised laterally by the contraction of a muscle 
can be well shown by tying a piece of tape round the middle of the 
arm proper, and then causing the biceps to contract forcibly, whereby 
the tape is broken. 
As remarked by most writers on the subject, every intermediate 
condition of muscularity of stomach may be found in birds, from the 
simple non-tendinous one of the Raptores and others to the most 
muscular of the Anserine birds. The degree of muscularity depends 
on the nature of the food which the bird obtains, as shown by Hunter’s 
experiment, in which he, by giving animal food to a duck (I believe),* 
caused a great diminution in the muscularity of its gizzard. 
The state of the bird as to health also influences the development 
of the muscular fibres, the heart and gizzard being very similarly 
affected by impaired nutrition. 
In the Gallinaceous and Passerine birds there is seldom a callous 
pad formed over the lateral muscles, the epithelium being generally 
plicated at right angles to the direction of the muscular fibres; and in 
them the organ seems to be a more simple squeezing-organ, though 
when rigor mortis occurs in a contracted gizzard it is seen that the 
muscular masses are convex on their opposed faces. 
From these remarks and what has been previously observed on 
the subject, the following summary statement may be made :— 
The gizzard is an organ which crushes, and so renders assimilable 
the harder portions of the food of birds. This food, having been 
previously macerated in the proventriculus or crop, is thrust between 
the lateral muscles (where it gets mixed with the small sharp stones 
it meets there) by the contraction of the superior and inferior gizzard- 
sacs—upon which these lateral muscles contract simultaneously ; and 
* [The bird experimented on was, in reality, a sea-gull. Cf. Everard Home, 
“ Lectures on Comparative Anatomy.” I. p. 271. 1814, Ep.] 
