368 BXCBETION- 



a mirror, we can condense the perspiration, and thus 

 demonstrate that even when invisible to the eye the pro- 

 cess is always going on. 



Sebaceous glands. — These are small, saclike ducts em- 

 bedded in the dermis, and opening usually by a smaU duct 

 near the mouth of the hair foUicles. They are lined with 

 epithelial cells whose substance becomes gradually charged 

 with oil drops. Finally the protoplasm disappears, the 

 cells break down and their oil is dischai-ged through the 

 duct, new cells taking the place of the old ones and con- 

 tinuing the process indefinitely. While these glands are 

 always found associated with hair follicles, they are also to 

 be found in parts of the skin free of hair. In these cases 

 the oil discharged spreads over the skin and moistens and 

 softens it. ^Vhen such ducts become stopped by dht the 

 contents become hardened and the ducts become distended 

 and form black spots on the skin. 



Hairs. — A vertical section of a hair shows the following 

 structure: The external hair is a long, hollow shaft made 

 of an extension of the epidermis and embedded at its base 

 in a pocket of the dermis called the hair follicle. This 

 pocket or folhcle extends downward into the subcutaneous 

 tissue, and at its base is a special papilla which forms the 

 hair and which is abundantly supplied with nerves and 

 blood vessels. The follicle is lined with a layer of epider- 

 mal cells, and the upper part of the hair shaft is composed 

 almost entirely of the horny layers of the epidermis, and 

 hence this part is not sensitive to cutting. The nerves at 

 its base in the papilla render the hair sensitive to touch, and 

 give the sharp sensation of pain when the hair is pulled. 

 The relation of the various parts of the hair to the normal 

 skin layers is shown in Figure 165. The color of the hair 



