42 THE SKIN, ITS APPENDAGES AND ITS FUNCTION. 



The sweat glands, on the other hand, are more fre- 

 quent on the palms and soles and though sometimes 

 found in the derma are usually situated lower down in 

 the subcutaneous cellular tissue. They are least nu- 

 merous on the back and neck. Coiled up in the lower 

 layers of the skin, they discharge the sweat through a 

 spiral excretory duct upon its free surface. 



The sweat is a clear, colorless, watery fluid with a 

 salty taste, an alkaline reaction, and a characteristic 

 odor that varies with the individual. If very scanty, 

 it may be acid in reaction. Besides water it contains 

 a small percentage of solids, as inorganic salts, especi- 

 ally sodium choloride, fatty acids, neutral fats, and 

 at times, especially in some diseases of the kidneys, 

 urea, that is, the end-products of the metabolism of 

 starches and fats chiefly. There is usually also some 

 carbon dioxide, whence the expression cutaneous respi- 

 ration. 



The sweat serves to keep the skin moist and in good 

 condition, to remove outworn and poisonous or ir- 

 ritating matters, and to regulate the temperature. As 

 a rule it evaporates upon reaching the surface, in which 

 case it is known as invisible or insensible perspiration, 

 but if conditions of the atmosphere are not favorable 

 to prompt evaporation, as when the air is damp, the 

 skin becomes damp and there is visible perspiration. 



Though an abundant supply of blood increases the 

 action of the sweat glands, they are regulated by defi- 

 nite secretory nerves rather than by the vasomotor 

 nerves. In a cold sweat the action is probably due 

 to some disturbance of the nerve supply without in- 

 crease of the blood supply. Ordinarily perspiring is 

 a reflex act due to the stimulation of the afferent cu- 

 taneous nerves, as by the application of heat, but some- 

 times, as in cases of strong emotions, involuntary im- 

 pulses are sent from the brain to the spinal centers and 

 so arouse the action of the glands. Atropin has the 

 power of preventing the secretion of sweat by paralyz- 



