THE MECHANISM OF THE GIZZARD IN BIRDS. *109 
their arrangement is such that all the force of their contraction is con- 
verted into a compressing force at right angles to their direction. 
This force, by tending forcibly to obliterate their included cavity, 
comminutes the more yielding of their contents and squeezes from 
between them the resulting chyme, which finds no difficulty in enter- 
ing the small orifice to the duodenum.* 
* Since writing the above, I find there is a peculiarity in the gizzard-pads in the 
swan and goose, which causes a slight up and down movement of the lateral mus- 
cular masses when in action. The lower end of one pad and the upper end of the 
other are much more strongly developed and are thicker than the rest ; this causes 
them to present a surface of contact one with the other, which is somewhat oblique 
with regard to the axis of the lateral muscles. Consequently, when these muscles 
come into play, the oblique surfaces tending to come into contact, the material to 
be crushed intervening, they, being opposed inclined planes, slide slightly on one 
another, the one mass rising while the other descends. During the diastole of the 
gizzard it resumes its former relations, and a reverse sliding occurs. 
