THE STRUCTURE AND FUNCTIONS OF THE SKIN. 351 



mained quiet for several hours, after which time he was again weighed. 

 The difference in the two weights indicated the amount of loss by pul- 

 monary exhalation. Haying taken off the air-tight dress, he was imme- 

 diately weighed again, and a fourth time after a certain interval. The 

 difference between the two weights, last ascertained gave the amount of 

 the cutaneous and pulmonary exhalation together ; by subtracting from 

 this the loss by pulmonary exhalation alone, while he was in the air-tight 

 dress, he ascertained the amount of cutaneous transpiration. During a 

 state of rest, the average loss by cutaneous and pulmonary exhalation in 

 a minute is eighteen grains the minimum eleven grains, the maximum 

 thirty-two grains ; and of the eighteen grains, eleven pass off by the skin, 

 and seven by the lungs. 



The quantity of watery vapor lost by transpiration is of course in- 

 fluenced by all external circumstances which affect the exhalation from 

 other evaporating surfaces, such as the temperature, the hygrometric 

 state, and the stillness of the atmosphere. But, of the variations to 

 which it is subject under the influence of these conditions, no calcula- 

 tion has been exactly made. 



b. Carbonic Acid. The quantity of carbonic acid exhaled by the 

 skin on an average is about y-^ to ^- of that furnished by the pulmo- 

 nary respiration. 



The cutaneous exhalation is most abundant in the lower classes of 

 animals, more particularly the naked Amphibia, as frogs and toads, 

 whose skin is thin and moist, and readily permits an interchange of 

 gases between the blood circulating in it, and the surrounding atmo- 

 sphere Bischoff found that, after the lungs of frogs had been tied and 

 cut out, about a quarter of a cubic inch of carbonic acid gas was exhaled 

 by the skin in eight hours. And this quantity is very large, when it is 

 remembered that a full-sized frog will generate only about half a cubic 

 inch of carbonic acid by his lungs and skin together in six hours. 



The importance of the respiratory function of the skin, which was 

 once thought to be proved by the speedy death of animals whose skins, 

 after removal of the hair, were covered with an impermeable varnish, 

 has been shown by further observations to have no foundation in fact; 

 the immediate cause of death in such cases being the loss of temperature. 

 A varnished animal is said to have suffered no harm when surrounded 

 by cotton wadding, and to have died when the wadding was removed. 



Influence of the Nervous System on Sweat-Excretion. Any 



increase in the amount of sweat secreted is usually accompanied by dila- 

 tation of the cutaneous vessels. It is, however, probable that the secre- 

 tion is like the other secretions, e. g., the saliva, under the direct action 

 of a special nervous apparatus, in that various nerves contain fibres 

 which act directly upon the cells of the sweat-glands in the same way 

 that the chorda tympani contains fibres which act directly upon the sali- 

 vary cells. The local apparatus is under control of the central nervous 

 system sweat centres probably existing both in the medulla and spinal 



