206 ANIMAL FORMS 



life. The life of birds is at high pressure, hence their need 

 of much oxygen. They habitually breathe deeper breaths 

 than other animals. The air passing into the body trav- 

 erses 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 distributed 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. Furthermore, the beating of 

 the heart is comparatively rapid, rushing the oxygen as 

 fast as it enters the blood to all portions of the body. The 

 result is that everywhere heat is being generated, so neces- 

 sary 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 



