22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 72 



It has been estimated by Helmholtz ^ that in the human body heat 

 lost through transpired air amounts to 5.2^, through the water of 

 respiration to I4.7;» and through the skin to 77.5^. The remaining 

 amount disseminated is given off in egesta or is consumed in warming 

 ingesta. The part played by the skin glands in regulating temperature 

 in the mammal is readily seen. The distribution of skin glands varies 

 in different groups, though such glands are known in all save 

 Cetaceans, elephants. Echidna, and some others. It is claimed, for 

 example, that in the dog skin glands are present only in the legs or feet 

 but in this case the open mouth and protruding tongue act as organs for 

 reducing excess body heat. The presence of skin glands in the horse 

 is readily observed in an animal that is hard driven in warm weather. 



Amphibians agree with mammals in the presence of many integu- 

 mentary glands, though in this group the use of these structures is 

 in some ways different in purpose. In reptiles and in birds, the two 

 classes joined in the supergroup Sauropsida, skin-glands are practi- 

 cally wanting and no case is known in which glands similar to those 

 in mammals are found. In birds the development of feathers with 

 their filamentous barbs and barbules, as a body covering, would not 

 have been possible had sudoriferous glands been present in the skin. 

 Excretion of fluid through such glands inevitably would have soiled 

 such delicate structures as feathers and ultimately have destroyed 

 them. The diffusion of heat through the skin in birds is confined to 

 the amount, notably small in quantity, that is, given off by direct 

 radiation. It is a fact easy of verification that the skin in birds is defi- 

 cient in blood supply when compared with mammals. Only a compara- 

 tively small amount of blood, therefore, can be cooled to any extensive 

 degree through the agency of the skin. 



As a matter of fact the feathers that form a loose covering over 

 the bodies of birds are not adapted to the radiation of heat but on the 

 contrary tend to conserve it and hold it within. Though the contour 

 feathers lie smoothly one upon the other yet they are permeated and 

 separated by innumerable air-spaces varying in size from the tiny 

 interstices between barbs, barbules, and barbicels in individual 

 feathers, to the broader areas separating one feather from another. 

 These all go to make up series of more or less closed air cells that act 

 efficiently as non-conductors and serve to retain the bodily heat 

 within. The use of so-called " dead " air-spaces between walls as 

 a protection against conduction of heat and cold is too well known to 

 make further explanation of this factor necessary. 



' Smith, R. M., Physiology of Domestic Animals, 1889, p. 696. 



