THE ORGANS OF EXCRETION 259 



found in the same nerve trunks. There are subsidi- 

 ary nervous centers for this secretion in different parts 

 of the spinal cord; but the chief center controlling the 

 others is the medulla oblongata, and the nerve fibers for 

 the sweat glands run in the nerve trunks supplying the 

 different parts of the body. For example, the sciatic 

 nerve, supplying the muscles of the leg, carries secretory 

 fibers to the swe'at glands of the leg. 



377. Functions of Perspiration. By means of the sweat 

 glands waste water taken from the blood continually 

 passes into the air. The amount varies greatly, but may 

 be said to average about one and a half pints daily. A 

 very little carbonic acid and solid matter are found in 

 the sweat, along with the fat from the sebaceous glands, 

 and a mere trace of urea. 



Besides the removal of waste matter, another important 

 function belongs to the skin in connection with perspi- 

 ration. It is well known that as water passes from the 

 liquid to the gaseous state a large amount of heat becomes 

 latent, and this heat is supplied by adjacent bodies. 

 Hence it is clear that by the evaporation of the perspi- 

 ration the surface of the body is cooled, and the sweat thus 

 becomes a regulator of the temperature of the body, the 

 amount of evaporation depending upon the state of the 

 body and of the surrounding air. 



The amount of sweat secreted is also affected by nerv- 

 ous impulses from the emotional centers in the brain. 

 Fear, for example, sometimes causes profuse sweating. 

 Strong muscular activity, developing heat and stimulating 

 the circulation, increases perspiration. A low temperature 

 in the surrounding air constricts the blood vessels of the 

 skin, and so diminishes the production of sweat and pre- 

 vents loss of heat from the surface. 



