536 THE HUMAN BODY 



is acid, and in 1,000 parts contains 990 of water to 10 of solids. 

 Among the latter are found urea (1.5 in 1,000), fatty acids, sodium 

 chlorid, and other salts. In diseased conditions of the kidneys the 

 urea may be greatly increased, the skin supplementing to a certain 

 extent deficiencies of those organs. 



The Nervous and Circulatory Factors in the Sweat Secretion. 

 It used to be believed that an increased flow of blood through the 

 skin would suffice of itself to cause increased perspiration; but 

 against this view are the facts that, in terror for example, there 

 may be profuse sweating with a cold pallid skin ; and that in many 

 febrile states the skin may be hot and its vessels full of blood, and 

 yet there may be no sweating. 



Direct experiment shows that the secretory activity of the 

 sweat-glands is under immediate control of nerve-fibers, and is 

 only indirectly dependent on the blood-supply in their neighbor- 

 hood. Stimuating the sciatic nerve of the freshly amputated 

 leg of a cat will cause the balls of its feet to sweat, although there 

 is no blood flowing through the limb. On the other hand, if the 

 sciatic nerve be cut so as to paralyze it, in a living animal, the 

 skin arteries dilate and the food gets more blood and becomes 

 warmer; but it does not sweat. The sweat-fibers doubtless com- 

 municate with sweat-centers in the medulla, which may either be 

 directly excited by blood of a higher temperature than usual flow- 

 ing through them or, reflexly, by warmth acting on the exterior 

 of the Body and stimulating the sensory nerves there. Both of 

 these agencies commonly also excite the vasodilator nerves of the 

 sweating part, and so the increased blood-supply goes along with 

 the secretion; but the two phenomena are fundamentally inde- 

 pendent. Since the sweat-glands are innervated through the 

 autonomic system they share in the emotional reactions which 

 are characteristic of this system. The effect of embarrassment to 

 cause profuse sweating is too well known to require comment. 



The Sebaceous Secretion. This is oily, semifluid, and of a 

 special odor. It contains about 50 per cent of fats (olein and 

 palmatin). It lubricates the hairs and usually renders them 

 glossy. No doubt, too, it gets spread more or less over the skin 

 and makes the cuticle less permeable by water. Water poured 

 on a healthy skin does not wet it readily but runs off it, as "off 

 a duck's back" though to a less marked degree. 



