416 McAteE, Shedding of Stomach Lining by Birds. ee 
which they must of course swallow whole and grind up in their 
gizzards. Such species as the Scoters and Greater Scaup thus 
easily dispose of oysters. 
It is evident that a mass of fragments of such shellfish, or even 
some softer food, interspersed with often sharply angular gravel, 
and subjected to grinding by the tremendous muscular power 
indicated, must have a powerful abrading effect upon the lining 
of the gizzard. Nothing is more natural to suppose, therefore, 
than that the lining wears out and must be replaced. 
The stomach lining when fresh covers the whole interior of the 
organ, being thin about the openings into the stomach and on 
those parts of the wall between the great muscle masses. On the 
inner face of each main muscle-body, however, the lining is greatly 
thickened. This part of the lining is hereafter referred to by the 
term, pad. Each pad has a somewhat crescent-shaped, very firm 
grinding ridge which is opposed to the ridgeless end of its fellow 
pad. | 
The movements of the gizzard during trituration are such as to 
give the food mass a rotary motion. This we know from the 
arrangement of the longer particles in food taken from bird stomachs 
and from the appearance of the caterpillar hairs which remain 
sticking in the lining of cuckoo stomachs. The arrangement of 
these resembles that of the fibers on the top of a well-brushed silk 
hat. 
The rotary movement of the gizzard contents, together with the 
presentation of the hard thick grinding ridge of one pad against 
the thinner, less durable portion of the pad on the other side, 
results in wear first becoming apparent on these thinner ends of 
the pads (Fig. 1). A little such wear reveals the fact that the pads 
have a stratified structure and the gradual approach of the ends 
of the strata toward the middle of the pad indicates the progress 
of the wear (Figs. 2 and 10). It may be that such wear is compen- 
sated for, at least in part, by addition to the pads from below, but 
eventually they become unserviceable and must be shed. 
Often the wear takes the form of a rolling up of the ends of the 
pads, which are thus subjected to much greater stress (Figs. 8 and 
9). Furthermore, in such case, food and grinding material getting 
under the pad tend to force it away from the mucous coat. 
