THE SO-CALLED INSENSIBLE PERSPIRATION 397 



Various experimental facts favor the view that sweat glands are under the 

 influence of inhibitory nerves, which, like the secretory fibers, traverse sympa- 

 thetic paths. 



Many animals do not sweat at all ; others, like the cat, sweat only in certain 

 places, as the balls of the feet. In man the ability to sweat is very highly 

 developed : in varying degrees it is a function of the entire skin principal 

 places being the brow, palms of the hands and the soles of the feet. 



3. THE SO-CALLED INSENSIBLE PERSPIRATION 



We include under this head the excretion of carbon dioxide through the 

 skin, and the exhalation of water independently of the sweat glands. 



The elimination of carbon dioxide through the skin is very small in com- 

 parison with the elimination through the lungs. Both this and the exhalation 

 of water vapor have repeatedly been studied on limited areas of the skin ; but 

 such investigations, although they may yield valuable results as to the influ- 

 ence of different factors, give no certain criteria for the estimation of the 

 total output of C0 2 and water vapor for the whole surface of the body. In 

 order to make such a determination, an individual is inclosed, all but his 

 head, in a cabinet suitably ventilated, so that the elimination may go on 

 continuously. 



According to Schierbeck and v. Willebrand, the output of C0 2 at a tem- 

 perature of 20-33 C. is fairly constant, and amounts to 0.35 g. per hour 

 i. e., 7.2-8.4 g. per day. If the surrounding temperature be raised above 

 33 C., the output of C0 2 suddenly increases, so that at 33.5-34 C. it reaches 

 the relatively high value of 0.87-1.35 g. per hour (= 20.9-32.4 g. per day). 



This sudden rise is coincident with the appearance of " sensible perspira- 

 tion " ; it is possible, therefore, that it may be due to the increased work of 

 the sweat glands. 



Excretion of water vapor goes on also below this critical temperature. 

 Other conditions being equal, it is greater the higher is the surrounding tem- 

 perature, and from 12-31 C. the output from the naked body, according to 

 v. Willebrand, is proportional to the atmospheric temperature (e. g., at 12, 

 10.5 g. per hour; at 18.2, 18.4 g. ; at 24, 22.7 g. ; and at 28, 27.3 g.), but 

 with the appearance of visible sweat it rises suddenly. 



We can think of two possibilities as to the source of the water given off 

 from the skin before the appearance of sweat: either it is a product of the 

 sweat glands, or it is derived by a purely physical process of diffusion from 

 the gland cells and the epidermis. Considering the proportional increase 

 parallel with the temperature up to the point where water is poured out as 

 visible sweat, the latter possibility seems the more likely. 



