522 APPLIED BIOLOGY 



no hairs. Each sweat-gland is a tube extending from the so- 

 called " pore " down into the dermis, where it is much coiled 

 and surrounded by blood-capillaries. 



446. Functions of the Skin. Next to protection, the 

 most important function of the human skin is heat regulation. 



While the soft skin of frogs and other lower animals is im- 

 portant for respiration, the dry hard skin of mammals and 

 man is of little use in this way. It has been shown by putting 

 a man's body in a rubber bag which was tightly fitted around 

 the neck that the lungs give off nearly two hundred times more 

 carbon dioxide than the skin does. Hence, skin respiration 

 is of no practical importance to us. 



Absorption by the skin is of little importance. Oily 

 materials are often rubbed into the skin, and small quantities 

 appear to be absorbed. Probably the massage effect is most 

 important. It is known that some drugs can be absorbed 

 when thus rubbed into the skin. 



Secretion of Sweat. Under ordinary temperature conditions 

 the amount of secretion does not attract attention, because it 

 is evaporated as rapidly as formed. Large quantities of water 

 taken into the stomach are absorbed, raise the blood-pressure, 

 and increase perspiration. Certain drugs bring about dila- 

 tion of blood-vessels and cause profuse sweating; others 

 act in an opposite manner. 



Normal sweat is nearly 99 per cent water. The one per 

 cent of dissolved substances are chiefly mineral excretions 

 similar to those in the urine. If the skin were varnished, the 

 same excretion would be easily eliminated by the kidneys. 

 Sweat, then, is not necessary as an excretion, but as an aid 

 to heat regulation (see next section). 



447. Skin as Heat-Regulator. The fact that the notice- 

 able activities of the sweat-glands are usually associated with 

 great internal heat suggests that the chief function of these 

 glands is not so much to get rid of sweat (which the kidneys 

 could manage) as to discharge heat. 



