CUTANEOUS FUNCTIONS. 



657 



skin is, however, of considerable density, and from its structure, as 

 compared with that of the pulmonary mucous membrane, must offer 

 great resistance to the diffusion of gases. As a consequence, the 

 amount of such interchange which occurs through the skin must be 

 slight. In animals, however, in which the skin is not only bare, but 

 thin and moist, gaseous interchange through the skin may be of much 

 greater importance ; thus, for example, it has been found that if the 

 entrance of air ? nto the lungs of the frog be entirely prevented by sur- 

 rounding the head with a rubber membrane, life may be preserved in 

 contact with the air for several da3~s, and b}~ examining the composition 

 of the atmosphere, if the frog be placed in a confined space, it will be 

 found that the frog has absorbed oxygen and set free carbonic acid. In 

 other words, the frog is able to breathe without lungs, the respiration 

 carried on through its skin being sufficient for its needs. 



In cold-blooded animals this degree of cutaneous respiration is much 

 more extensive than in warm-blooded animals, while the function is 

 least developed in warm-blooded animals whose skin is covered with fur, 

 hair, or feathers. In fact, it is probable that whatever gaseous inter- 

 change does occur takes place from the capillary net-work surrounding 

 the sweat-glands. The exhalation of carbon dioxide may be readily 

 demonstrated by placing the arm in a vessel containing distilled water 

 and which is closed from the external atmosphere ; after an hour's im- 

 mersion only in the water the addition of lime-water, by the character- 

 istic precipitate of carbonate of lime, will demonstrate its transudation 

 through the skin. 



The amount of carbon dioxide eliminated by the skin may be readily 

 determined by collecting the total amount of carbon dioxide liberated 

 both by the skin and lungs and then deducting the latter amount from 

 the total. Or it may be directly measured by closing the bod} 7 " in an air- 

 tight chamber and preventing entrance of the expelled air by breathing 

 through a tube. It has been found in this way that the amount elimi- 

 nated by the lungs is, in man, about one hundred and thirty times as 

 much as passes through the skin. The following experiments by Reg- 

 nault and Reiset indicate this amount in the rabbit and dog: 



