498 HANDBOOK OF PHYSIOLOGY. 



skin is on an average between 1 and 2 Ib. daily (about 1 kilo). This 

 subject has been very carefully investigated by Lavoisier and Sequin, 

 The latter chemist enclosed his body in an air-tight bag, with a mouth- 

 piece. The bag being closed by a strong band above, and the mouth- 

 piece adjusted and gummed to the skin around the mouth, he was 

 weighed, and then remained quiet for several hours, after which time 

 he was again weighed. The difference in the two weights indicated the 

 amount of loss by pulmonary exhalation. Having taken off the air- 

 tight dress, he was immediately weighed again, and a fourth time after 

 a certain interval. The difference between the two weights last ascer- 

 tained gave the amount of the cutaneous and pulmonary exhalation to- 

 gether; 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 influ- 

 enced 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. 



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

 on an average is about T -J-g- to ^-^ of that furnished by the pulmonary 

 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 atmosphere. 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. 



The secretion of sweat is closely connected with the quantity of blood 

 flowing through the cutaneous vessels. The quantity of sweat in- 

 creases with vaso-d ilatation and diminishes with vaso-constriction. It 



