AVES 263 



wing muscles, especially the pectorals, are in flying birds extremely 

 massive, which means that a great excess of energy is always avail- 

 able; the nervous system is highly efiicient; and the supply of oxygen 

 is ensured by the extraordinary development and unique structure of 

 the lungs and air passages, as well as by the adequate blood supply 

 and its circulation. The lungs proper are not unduly large, but their 

 capacity is greatly increased by the addition of large air-sacs, that 

 branch off from the lungs. These air-sacs fill all of the ccelomic 

 spaces and even send fine branches into the hollows of the bones. By 

 this scheme two functions are subserved: that of sending oxygen 

 directly to many tissues, and that of lessening the weight of the body. 

 The lungs moreover differ from those of reptiles or mammals in that 

 a through draft of air is made possible through a system of excurrent 

 bronchi, passages that carry used air out of the lung alveoli without in- 

 terfering with the fresh air that enters through the incurrent bronchi. 

 Thus the bird 's oxygen supply is much better provided for than that 

 of any other vertebrate, and in some respects approximates that pos- 

 sessed by the flying insects. Adequate oxidation is further provided 

 for by the large heart (Fig. 142) and by voluminous blood vessels, 

 both of which are proportionately more generous in their blood- 

 carrying capacity than those of other vertebrates. 



The high temperature of the bird is another important element 

 in its power plant. Obviously, the higher the temperature, the more 

 rapid the combustion. The bird 's temperature is considerably higher 

 than that of mammals, as anyone knows who has felt the skin of a live 

 fowl. In the best fliers it runs up to 110° or 112° F., even when the 

 birds are at rest. Two elements are concerned in maintaining the 

 characteristic avian temperature: a vaso-motor system, similar to 

 that of mammals, and an unusually effective non-conductive coat of 

 feathers, which prevents surface loss of heat; and no known material 

 does this more effectively than the feather coat of a bird, especially 

 when the feathers are arranged as they are in nature. With this 

 equipment the bird is able to endure the intense cold of the upper 

 atmospheric strata without undue loss of heat and without the least 

 danger of freezing. 



The alimentary system is also proportionately effective. It 

 must be, for it is the fuel refinery. Crude power materials are taken 

 into the crop or storage tank, are gradually fed into the grinding mill 

 (gizzard) and passed into the stomach proper, and subsequently into 



