420 THE HUMAN BODY. 



sweat-centres in the spinal cord, which may either be di- 

 rectly 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 fundamentally 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 

 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. 



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 

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

 the skin) and impede their activity. Hence the value ta 

 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 is a habit and the skin be well rubbed afterwards 

 until dry. Soap nearly always contains an excess of alkali 

 which in itself injures some skins, and, besides, is apt to com- 

 bine chemically with the sebaceous secretion and carry it 

 too freely away. Persons whose skin will not stand soap 

 can find a good substitute, for washing the hands and face, 

 in a little cornmeal. No doubt many folk go about in- 

 very good health with very little washing ; contact with 

 the clothes and other external objects keeps its excretions 

 from accumulating on the skin to any very great extent. 

 But apart from the duty of personal cleanliness imposed on. 



