CHAP, in.] CUTANEOUS SECRETION. 385 



portion of the carbonic acid, and a considerable quantity of water, 

 leave the body by the lungs in respiration ; while all (or nearly all) 

 the urea, the greater portion of the salts, and a large amount of 

 water, with an insignificant quantity of carbonic acid, pass away 

 by the kidneys. The work therefore of the remaining excretory 

 tissue, the skin, is confined to the elimination of a comparatively 

 small quantity of salts, a little carbonic acid, and a variable but 

 on the whole large quantity of water in the form of perspiration. 

 The actual excretion by the bowel, that is to say, that portion of 

 the faeces which is not simply undigested matter, we have seen to 

 be very small. 



The Nature and Amount of Perspiration. 



The quantity of matter which leaves the human body by way 

 of the skin is very considerable. Thus it has been estimated that, 

 while 7 grains pass away through the lungs per minute, as much 

 as 11 grains escape through the skin. The amount however varies 

 extremely ; it has been calculated, from data gained by enclosing the 

 arm in a caoutchouc bag, that the total amount of perspiration 

 from the whole body in 24 hours might range from 2 to 20 kilos ; 

 but such a mode of calculation is obviously open to many sources 

 of error. 



Of the whole amount thus discharged, part passes away at 

 once as watery vapour mixed with volatile matters, while part may 

 remain for a time as a fluid on the skin ; the former is frequently 

 spoken of as insensible, the latter as sensible perspiration. The 

 proportion of the insensible to the sensible perspiration will depend 

 on the rapidity of the secretion in reference to the dryness, tem- 

 perature, and amount of movement, of the surrounding atmosphere. 

 Thus, supposing the rate of secretion to remain constant, the drier 

 and hotter the air, and the more rapidly the strata of air in contact 

 with the body are renewed, the greater is the amount of sensible 

 perspiration which is by evaporation converted into the insensible 

 condition; and conversely when the air is cool, moist, and stagnant, 

 a large amount of the total perspiration may remain on the skin 

 as sensible sweat. Since, as the name implies, we are ourselves 

 aware of the sensible perspiration only, it may and frequently does 

 happen that we seem to ourselves to be perspiring largely, when in 

 reality it is not so much the total perspiration which is being in- 

 creased as the relative proportion of the sensible perspiration. 

 The rate of secretion may however be so much increased, that no 

 amount of dryness, or heat, or movement of the atmosphere, is 

 sufficient to carry out the necessary evaporation, and thus the 

 sensible perspiration may become abundant in a hot dry air. And 

 practically this is the usual occurrence, since certainly a high 



25 



