448 THE HUMAN BODY. 



Direct experiment shows that the secretory activity of 

 the sweat-glands is under immediate control of nerve-fibres, 

 and is only indirectly dependent on the blood-supply in their 

 neighborhood. Stimulating 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 para- 

 lyze it, in a living animal, the skin arteries dilate and the 

 foot gets more blood and becomes warmer; but it does not 

 sweat. The siveat-fibres originate in certain sweat-centres in 

 the spinal cord, which may either be directly excited by 

 blood of a higher temperature than usual flowing 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 vaso-dilator nerves 

 of the sweating part, and so the increased blood-supply goes 

 along with the secretion; but the two phenomena are funda- 

 mentally independent. 



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, even in persons who use none of the various 

 compounds sold as " hair-oil." No doubt, too, it gets spread 

 more or less over the skin and makes the cuticle less permea- 

 ble 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. 



Hygiene of the Skin. The sebaceous secretion, and the 

 solid residue left by evaporating sweat, constantly form a 

 solid film over the skin, which must tend to choke the 

 mouths of the sweat-glands (the so-called " pores " of the 

 skin) and impede their activity. Hence the value to health 

 of keeping the skin clean: a daily bath should be taken by 

 every one. Women cannot well wash their hair daily as it 

 takes so long to dry, but a man should immerse his head 

 when he takes his bath. As a general rule, soap should only 

 be used occasionally; it is quite unnecessary for cleanliness, 

 except on exposed parts of the Body, if frequent bathing be a 

 habit and the skin be well rubbed afterwards until dry, 

 Soap nearly always contains an excess of alkali which in itselJ 

 injures some skins, 'and, besides, is apt to combine chemically 

 with the sebaceous secretion and carry it too freely away 



