312 EXHALATION FROM 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 glandulse, 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 glandulse, the length of their tubing must be 882 
inches or 7 3J 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 
