312 EXHALATION FKOM THE SKIN. 



and this amount is greatly increased under particular circum- 

 stances. A continual evaporation takes place from the surface 

 of the skin, wherever it is not protected by hard scales or 

 plates ; and the amount of it will depend upon the warmth, 

 dryness, and motion of the surrounding air, exactly as if the 

 fluid were being evaporated from a damp cloth. Every one 

 knows that the drying of a cloth is much more rapidly effected 

 in a warm dry atmosphere, than in a cold moist one ; more 

 quickly, too, in a draught of air, than in a situation where 

 there is no current, and where the air is consequently soon 

 charged with moisture. That all these influences affect the 

 evaporation from the bodies of Animals, there is ample evi- 

 dence derived from experiment. 



371. But besides this continual evaporation, a special 

 exhalation of fluid takes place from the vast number of 

 minute perspiratory glands imbedded in the fatty layer just 

 beneath the Skin ( 37). Every one of these glandule, when 

 straightened out, forms a tubule about a quarter of an inch in 

 length ; and as it has been estimated that in a square inch of 

 surface on the palm of the hand there are no fewer than 3528 

 of these glandule, the length of their tubing must be 882 

 inches or 73 J feet. The average number in other parts of the 

 body may be estimated at about 2800 per square inch ; and 

 as the number of square inches of surface on a man of ordinary 

 stature is about 2500, the total number of perspiratory glan- 

 dulse must be not less than seven millions, and the length of 

 their tubing nearly twenty-eight miles. The fluid which these 

 perspiratory glands ordinarily exhale, is dissolved by the atmo- 

 sphere, and carried off in the state of vapour, so as to pass 

 away insensibly ; but they are stimulated to increased action 

 by the exposure of the body to heat, which causes them to 

 pour forth their secretion in greater abundance than the air 

 can carry off, and this consequently accumulates in drops upon 

 the surface of the skin. The amount of perspiration may be 

 considerably increased, without its becoming sensible, if the air 

 be warm and dry, so as to carry off, in the form of vapour, 

 the fluid which is poured out on the skin ; but, on the other 

 hand, a very slight increase in the ordinary amount immedi- 

 ately becomes sensible on a damp day, the air being already 

 too much loaded with moisture to carry off this additional 

 quantity. The distinction between insensible and sensible 



