BIRDS. 201 
Nutrition. The processes of nutrition are carried 
on in much the same way as in other vertebrates. In 
most birds the food, when swallowed, passes down the 
cesophagus to an enlargement called the crop, where it 
remains for a short time. It then goes to the stomach, 
which frequently consists of two 
parts, the proventriculus, where 
gastric fluids are poured on the 
food, and the gizzard, where it 
is ground. This gizzard is most 
highly developed in seed-eating 
birds, and commonly contains 
stones which the bird has swal- 
lowed and which grind up the 
food that has been softened in 
the crop and mixed with gastric 
juice in the proventriculus. Fic. 157.—Diagram of the 
Respiration. Breathing Heart of a Bird. 
occurs by lungs, the air enter- 
ing and leaving because of the increase and decrease 
of the size of the body-cavity produced by movements 
of the bones and muscles surrounding it. The expan- 
sion of the lungs in breathing is very slight. The lungs 
connect with air-sacs which extend among the other 
internal organs, and even into the bones. These serve 
to increase the bulk of the bird without adding to its 
weight, thus forming a sort of hot-air balloon. 
Reproduction. Eggs are produced in the ovaries, 
and pass from these organs through the left oviduct, 
the right ovary being suppressed in birds, to the cloaca 
and thence out of the body. The cloaca is the 
common cavity into which the alimentary canal, the 
kidneys, and the oviduct empty. 
In its passage through the oviduct the egg receives 
from the walls of this tube its layer of white albumen and 
its calcareous shell. The eggs hatch outside of the body 
and are cared for by the parent birds with much solici- 
tude. Except with a few of the most intelligent insects, 
