NO. 12 BODY TEMPERATURE OF BIRDS — VVETMORE 23 



This lack of heat regulation by means of the skin would throw the 

 vital work of temperature control directly upon the respiratory system. 

 In this fact then we have a ready explanation for the presence of the 

 great series of pulmonary air-sacs that are developed throughout the 

 avian class as a whole. Birds in order to maintain a high rate of 

 metabolism, necessary to continued activity without reference to shifts 

 and changes in the temperature of their surrounding media, have been 

 forced to develop an auxiliary to the small amount of heat that may 

 be thrown ofif through the lungs. This has led to the evolution of the 

 air-sacs that, while connected by ostia directly with the lungs, radiate 

 throughout the coelom and penetrate the bones to serve as an agency 

 of temperature control. In other words, safety to the organism 

 demanded that if activity be great and continued, there be some safe 

 release for the excess heat developed during rapid muscular move- 

 ment. 



The proper function of the air-sacs has been a moot point for many 

 years and has given rise to considerable discussion. Some have con- 

 sidered that these sacs acted as reservoirs to replenish air in the lungs, 

 as containers that, balloon like, raised the weight of the bird in flight, 

 or that the presence of these open spaces reduced the relative specific 

 gravity of the body. While the idea of the true use of the air-sacs 

 in birds as organs of temperature control was arrived at independently 

 by the present writer, subsequently an admirable exposition of the 

 same fact has been found in an account by J.-M. Soum."" This author 

 in turn believed that the discovery of this fact originated in an hypoth- 

 esis first advanced by De Vescovi.^ W. P. Pycraft ' also has adopted 

 this view as he states that " the air stored in these reservoirs serves 

 not only for respiratory purposes, but also as regulators of tempera- 

 ture, thereby compensating for the lack of sweat glands." With this 

 comment, however, he goes no further, as he gives no details to support 

 this statement.* M. Soum, however, made an admirable exposition 

 of his hypothesis. He pointed out that all birds possess air-sacs, 

 have a covering of feathers, and lack skin glands, and all have a 

 high temperature. To correlate these facts he believed it necessary 

 to consider the air-sacs as a means of temperature control. The addi- 

 tional facts that I am able to bring forth leave no doubt as to the cor- 

 rectness of this belief. 



* Soc. Linn, de Lyon, Vol. XLII, 1805, pp. 153-157. 



^ Res Zoologicae, Ann. i, No. i, Rome. (This publication I have not seen.) 

 ' History of Birds, London, 1910, p. 17. 



* With regard to statements by other authors consult also Headley, Structure 

 and Life of Birds, pp. 100-103. , 



