348 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



upon each other and interlock with those of the hail 

 cuticle in such a manner that if a hair is pulled out 

 the root sheath comes away with it, the break taking 

 place along Huxley's and Henle's layers. 



The hair papilla indents the lower end of the root 

 and is of connective-tissue origin. It has a rich 

 blood supply which contributes nourishment to the 

 adjacent epithelial cells of the root which are con- 

 stantly dividing. It is this cell division that brings 

 about the growth of a hair. If the papilla is de- 

 stroyed the hair dies. When a hair is pulled out 

 with its root the papilla and some adjacent epithelial 

 cells usually remain uninjured. The epithelial cells 

 in due time reproduce a new hair. 



The arrector pili muscle consists of bundles of 

 smooth muscle fibers that pass obliquely downward 

 from the upper surface of the dermis to be inserted 

 in the connective-tissue tunic of the hair follicle near 

 its lower extremity. The insertion of these fibers is 

 always on the side toward which the hair inclines, so 

 that when the fibers contract the root is drawn to a 

 vertical position and the hair becomes erect. 



The hair on the scalp grows approximately at the 

 rate of twelve inches a year, or one inch a month. 

 The average duration of a hair is about four years. 

 Many vertebrates, as horses and cattle, shed their 

 hair annually, every spring, a phenomenon called 

 moulting. In mankind the hair of the scalp is con- 

 stantly dropping out and being replaced by growths 

 of new shafts. Occasionally hair grows where it 

 normally does not belong and for cosmetic effect 

 requires removal. This is done by electrolysis, 



