PROTECTIVE AND EXCRETORY FUNCTIONS OF THE SKIN. 415 
Ear-wax and the Meibomian secretion are the work of modi- 
fied sebaceous glands; as also the oil-glands so highly developed 
in birds, especially aquatic forms, of which these creatures 
inake great use in preserving their feathers from wetting. 
THE EXCRETORY FUNCTION OF THE SKIN. 
Sweating in man has been studied by inclosing the greater 
part of the body or a single limb in a caoutchouc or other form 
of impermeable covering and exposing the subject to various 
degrees of heat; but, apart from errors in collecting, weighing, 
etc., such sweating must be regarded as somewhat abnormal. 
It is clear, however, that the quantity of matter discharged 
through the‘skin is large—greater than by the lungs (about as 
% to 11), though the amount is very variable, depending on 
the degree of activity of other related excreting organs, as the 
lungs and kidneys, and largely upon the temperature as a 
physical condition. 
When the watery vapor is carried off, before it can condense, 
the perspiration is said to be insensible ; when small droplets 
become visible, sensible. As to whether the one or the other 
is predominant will, of course, depend on the rapidity of re- 
newal of the air, its humidity, and its temperature. Apart 
from the temperature, the amount of sweat is influenced by the 
quality and quantity of food and, especially, of drink taken, 
the amount of exercise, and psychic conditions; not to speak 
of the effect of drugs, poisons, or disease. 
Perspiration in man is a clear fluid, mostly colorless, with 
a characteristic odor, devoid of morphological elements (except 
epidermal scales), and alkaline in reaction. It may be acid 
from the admixture of the secretion of the sebaceous glands. 
Its solids (less than 2 per cent) consist of sodium salts, 
mostly chlorides, cholesterin, neutral fats, and traces of urea. 
The acids of the sweat belong to the fatty series (acetic, buty- 
ric, formic, propionic, caprylic, caproic, etc.). 
Pathological—_The sweat may contain blood, proteids, abun- 
dance of urea (in cholera), uric acid, oxalates, sugar, lactic acid, 
bile, indigo and other pigments. Many medicines are elimi- 
nated in part through the skin. : 
Respiration by the Skin.—Comparative.—In reptiles and batra- 
chians, with smooth, moist skin, the respiratory functions of 
this organ are of great importance; hence these animals can 
live long under water. 
