372 ORGANSYSTEMSOFMAN 



modern man. This loss of body covering tected skin, as most people know. Continual 



carries with it the obvious disadvantage of exposure to bright sunlight produces a 



rapid heat loss in a cold climate and man dark, tough skin, even in white people — 



has thus been forced to clothe himself with a far cry from the complexion recommended 



an artificial covering. The adornment fea- by beauty experts. Yet would-be beauties 



ture of clothing cannot be overlooked, for will often expose their bodies too long and 



even in hot climates aboriginals frequently too often so that the damaging rays of the 



cover themselves, particularly if the cloth sun produce the type of skin that is consid- 



is highly colored. ered undesirable. 



Hairs are arranged in definite patterns Glands. Near the base of each hair is a 



in the various regions of the body of man as tiny sebaceous gland which secretes an oily 



well as other mammals. They are not per- substance designed to keep both the hair 



pendicular to the skin surface but slant, and the skin in a soft, pliable condition, 



usually in a specific direction. For example. These glands, like other skin glands, come 



in a dog the hair slants away from the mid- from the epidermis, although they are 



dorsal line, usually in the direction of the buried deep in the dermis (Fig. 14-1). The 



pull of gravity, and this probably helps in tiny, much coiled, sweat glands are likewise 



shedding water. In man the direction of the found deep in the dermis where they func- 



hair slant in the back region is the same as tion in extracting water from the blood and 



in dogs. On the arms and legs it also follows tissues and spreading it over the surface 



a common pattern but on the top of the head of the skin for the purpose of cooling the 



it sometimes forms whorls or "cowlicks," body (Fig. 14-1). The resulting evapora- 



which, strangely enough, are specifically tion reduces the temperature of the skin 



inherited from generation to generation. and thereby aids in the regulation of the 



Attached to the base of each hair are tiny temperature of the entire body. This is very 

 muscle fibers which, when contracted, cause important to a mammal, although in many 

 the hair to stand on end, producing "goose species these glands are localized in small 

 flesh" (Fig. 14-1). These muscles are under areas, which are quite different in different 

 the influence of the autonomic nervous sys- mammals. The cow, for instance, has them 

 tem and are therefore beyond voluntary confined to its nose, whereas others such as 

 control. When one is frightened or some- the horse and man have them distributed 

 times under other emotional stress, the hair rather generally over the body. In man they 

 can be seen to stand on end, particularly are concentrated in tlie palms of the hands, 

 along the spine. The common statement, soles of the feet, and under the arms. 

 "Chills run up my back," has a physiological Another very interesting skin gland 

 foundation. A similar reaction can be ob- found only among mammals and fully de- 

 served in a dog and cat when in the presence veloped only in females is the milk or rmim- 

 of potential enemies. marij gland, a name that is linked with the 



Pigment granules lying in the lower epi- group. These are modified sebaceous glands 



dermal layers of the human skin give it and are confined to areas most convenient 



color ranging from no color at all, as in the for suckling the young mammal, which 



abnormal albinos, to the dense pigmenta- they supply with a complete food during its 



tion in black people. Sunlight has a decided early post-embryonic life, a sort of continu- 



effect, not only on increasing the amount of ation of umbilical feeding, 

 pigment (tanning), but also on increasing 

 the thickness of the epidermis itself. The 



purpose of such a response is protection This layer of the skin is much thicker 



because ultraviolet light damages unpro- than the epidermis and is composed of 



