274 THE HUMAN BODY. 



The latter are lined by secreting cells. The mouths of the 

 ducts of a pair of sebaceous glands are seen on the sides 

 of the hair follicle in Fig. 77. 



The Sebaceous Secretion is oily and semi-fluid. In healthy 

 persons it lubricates the hairs and renders them glossy even 

 when no " hair-oil " is used. It is also spread more or less 

 over all the surface of the skin, and makes the cuticle less 

 permeable by water, which in consequence does not readily 

 wet the healthy skin, but runs off it, as "off a duck's back," 

 though to a less marked extent. 



The Skin as a Sense Organ. Besides its functions as a 

 protective covering and an excretory organ, the skin is of 

 extreme importance, as being the seat of one of our most 

 important senses the sense of touch. In this relationship 

 it will be considered later (Chap. XXI). 



Hygiene of the Skin. The sebaceous secretion and the 

 solid residue left by evaporating sweat form a solid film 

 over the skin, which tends to choke the mouths of the sweat 

 glands (the so-called "pores" of the skin) and impede their 

 action. Yet these glands, minute though each is, have for 

 their function to separate daily from the body a great amount 

 of water* and some little urea and salines. Hence the 

 importance of personal cleanliness. The whole skin, except 

 that of the scalp, should be washed daily. Women cannot 



Describe the structure of a sebaceous gland. 



Describe the sebaceous secretion. Why is the hair of a healthy 

 person, using no hair-oil, glossy? Why does water run off the skin? 



Enumerate the chief functions of the skin. 



How are the mouths of the sweat glands apt to be choked up? 

 Point out functions of these glands? 



How much of the skin should be washed daily? 



* The sweat glands not merely carry off some water from the body, but serve 

 also to regulate its temperature. When water evaporates from the surface of 

 any object it abstracts heat from that object; and when perspiration evapo- 

 rates from the skin it carries off heat and cools the body. In health, the sweat 



