& 
120 NATURAL HISTORY. 
that have teeth; the Bird using its bill only for gather- 
ing its food, and not for masticating it. The arrange- 
ment described does not exist in full in all birds, but only 
in those that live on grains, termed granivorous birds. 
In other birds it varies according to the nature of the 
food. In those that live altogether on flesh, or on fishes, 
there is no real gizzard, but a thin and membranous stom- 
ach, for there is no need in them of any grinding and 
crushing process. 
204. There is one part of the digestive apparatus of 
birds yet to be noticed. Before the food is subjected to 
the gr inding of the gizzard, it is macerated or soaked for 
some time in the crop, as it is called, a 
sac or pouch which opens into the be 
let. When the grains are first swallow- 
ed, they are passed into the crop; and 
when they are sufficiently macerated, 
they are forced out of the crop, down 
the gullet, into the gizzard to be ground. 
The crop, you see, is to the Bird what 
the paunch is to the Ruminant quadru- 
ped (§ 154), a convenient receptacle for 
the food, and a place for its maceration. 
In Fig. 100 you have a representation 
of the parts mentioned, @ being the gul- 
l let, d the crop, ¢ that part of the gullet 
Fig. 100 where the gastric juice is made, and d 
the gizzard. 
205. The incubation, or hatching of eggs, requires dif- 
ferent periods in different species of birds. In the Hum- 
ming-birds it is but twelve days, in the Canaries fifteen 
to eighteen, Fowls twenty-one, Ducks twenty-five, and 
Swans forty to forty-five. The object of sitting on the 
eggs is simply to provide the requisite amount of heat. 
The same degree provided in any other way will answer, 
and eggs have often been hatched by steam. The heat 
of the sun is sufficient to hatch the eggs of some birds 
