EXCRETION AND THE EXCRETORY ORGANS 481 



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 

 sympathetic 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 their^. 

 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. 



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 ac- 

 tivity. Hence the value to health of keeping the skin clean: a 

 daily bath should be taken by every one. 



Bathing. The general subject of bathing may be considered 

 here. One object of it is that above mentioned to cleanse the 

 skin; but it is also useful to strengthen and invigorate the whole 

 frame. For strong healthy persons a cold bath is the best, except 

 in extremely severe weather, when the temperature of the water 

 should be raised to 15 C. (about 60 F.), at which it still feels 

 quite cold to the surface. The first effect of a cold bath is to con- 

 tract all the skin-vessels and make the surface pallid. This is soon 

 followed by a reaction, in which the skin becomes red and con- 

 gested, and a glow of warmth is felt in it. The proper time to 

 come out is while this reaction lasts, and after emersion it should 

 be promoted by a good rub. If the stay in the cold water be too 

 prolonged the state of reaction passes off, the skin becomes cold 



