446 EXCRETION 



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 

 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. The average loss by cutaneous and pulmonary exhalation in 

 a minute during a state of rest was eighteen grains the minimum eleven 

 grains, the maximum thirty-two grains. Of the eighteen grains, eleven 

 passed off by the skin and seven by the lungs. 



The quantity of watery vapor lost by transpiration is, of course, influenced 

 by all external circumstances which affect the exhalation from 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 calculation has been exactly made. 



The quantity of carbon dioxide exhaled by the skin on an average is 

 said to be about one two-hundredth of that eliminated by the pulmonary 

 respiration. 



The cutaneous exhalation is most abundant in the lower classes of ani- 

 mals, more particularly the naked amphibia, as frogs and toads, whose skins 

 are thin and moist, and readily permit an interchange of gases between the 

 circulating blood and the surrounding atmosphere. Bischoff found that, 

 after the lungs of frogs had been tied and cut out, from 3 to 4 c.c. of carbon 

 dioxide 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 10 c.c. of carbon dioxide 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 re- 

 moval 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 is the interference with temperature regulation. 



Influence of the Nervous System on Sweat Secretion. The secre- 

 tion of sweat is closely connected with the quantity of blood flowing through 

 the cutaneous vessels. The quantity of sweat increases with vaso-dilatation 

 and diminishes with vaso-constriction. The sweat glands are also under the 

 control of efferent impulses passing to them from the special sweat centers 

 in the brain and spinal cord through special sweat nerves. Thus, if the 

 sciatic nerve be divided in a cat and the peripheral end be stimulated, beads of 

 sweat are seen to appear upon the pad of the corresponding foot. The sweat 

 appears even though at the same time the blood vessels are constricted, or 

 the blood flow entirely stopped by compression of the aorta, whereas if atropin 

 is injected previously to the stimulation, no sweat appears, although dila- 



