906 ANIMAL FORMS 
life. The deeper and more frequent the breathing the 
greater the amount of energy produced. Birds habitually 
breathe deeper breaths than other animals. The air pass- 
ing into the body traverses the entire extent of the lung 
on its way back to the air-sacs, with the result that large 
quantities of oxygen are taken into the body. This is dis- 
tributed by a circulatory system of a more highly developed 
type than in any of the preceding groups of animals. The 
ventricles of the heart no longer communicate with each 
other, and the pure and impure blood never mingle. Fur- 
thermore, the beating of the heart is comparatively rapid, 
rushing the oxygen as fast as it enters the blood to all por- 
tions of the body. The result is that everywhere heat is 
being generated, so necessary to life and activity. 
In the lower animals no special means are employed to 
husband the energy thus produced, but in the birds the 
body is jacketed in a non-conducting coat of feathers which 
prevents its dissipation. For this and other reasons the 
birds, summer and winter, maintain an even and relatively 
high temperature (102°-110°). Like the mammals, birds 
are warm-blooded animals, full of energy, restlessly active 
to an extent realized in few of the cold-blooded animals. 
193. Digestive system.—This life, at high pressure, de- 
mands a relatively large amount of food to make good the 
losses due to oxidation. The appetites of some growing 
birds is only satiated after a daily meal equal to from one 
to three times their own weight, and after reaching adult 
size the amount of daily food required is probably not less 
than one-sixth their weight. The nature of the food is 
exceedingly varied, and the digestive tract and certain ac- 
cessory structures are obviously modified in accordance 
with it. The beak, always devoid of teeth in the living 
form, varies extremely according to the work it must per- 
form. The same is true of the tongue, and many correlated 
modifications exist in the digestive apparatus. In the 
birds of prey and the larger seed-eating species, such as the 
