230 ADVANCED PHYSIOLOGY 



and gradually become flattened by pressure. The skin is 

 always wearing away at its surface because of the constant 

 friction it receives. Debris resulting from this wear is not 

 ordinarily noticed, but when it accumulates in the hair, 

 along with the dried secretion of certain skin glands, it is 

 called dandruff. After scarlet fever, measles, etc., the epider- 

 mis may come off in sizable flakes. 



Some of the deeper cells of the epidermis contain pigment 

 matter which gives the skin its color. This pigment differs 

 in the skins of different races; in the negro it is abundant and 

 black; in the yellow races it is of a brownish yellow color; in 

 the skin of the white races there is very little, but here, too, it 

 varies slightly in amount, producing differences in complexion, 

 e.g. blonde and brunette. Freckles are due to the unequal 

 distribution of this pigment which may be especially dense 

 in spots; they are frequently produced in children by much 

 exposure to sunshine or cold winds. 



Thickened Parts of the Epidermis. Wherever there is 

 more than the usual amount of wear, the epidermis grows 

 more rapidly and becomes thicker than elsewhere; examples 

 of this growth are seen in the callouses on the hands and feet. 

 If the rubbing is not severe but long continued, the epidermis 

 grows into prominences called corns or bunions. Blisters 

 result from a sudden rubbing of the skin, the capillaries being 

 stretched and lymph collecting between the dermis and epi- 

 dermis in consequence. 



Skin Grafting. The only way in which epidermis grows 

 is by multiplication of the cells in its deeper layers. When 

 these growing cells are destroyed over a large area, e.g. after 

 severe burns, it may be difficult for the epidermis to be 

 reproduced. If a bit of epidermis is taken from a healthy 

 part of the body, or from some other person, and firmly 

 placed upon the raw surface of such a wound, it will generally 

 grow there, extending rapidly and eventually covering the 

 surface which would not otherwise have healed. Such a 



