107 
Fig. 1. Horizontal section of the gizzard in a state of relaxation. 
Fig. 2. The same in a fully contracted state. 
The following explanation of the action of the gizzard as a simple 
crushing-organ seems to me much more in accordance with the known 
principles of animal mechanics. 
As is well known, muscular tissue, when it contracts, does not 
alter in volume, but gains in breadth what it loses in length during 
its action. Consequently when a large mass of short muscular fibres 
contracts it must alter its shape considerably, increasing greatly in 
breadth. 
This fact being borne in mind, the action of the gizzard is easily 
explained. 
The two enormous lateral muscles, with their fibres tending for- 
wards and backwards, when relaxed, have a large cavity between 
them, into which the seeds and stones are thrust by the simultaneous pyz¢ 52s. 
contraction of the superior and inferior muscular bags. Directly 
these have become fully contracted, the lateral muscles act; and by 
approximating the anterior and posterior tendons they become greatly 
expanded laterally. But this expansion can take place in one 
direction only—namely, towards the gizzard-cavity; for the anterior 
and posterior tendons being situated obliquely with regard to one 
another, and the contraction taking place through the whole mass, 
