PHYSIOLOGICAL ANATOMY OF THE HAIRS 311 



the last phalanges of the fingers and toes, the lips, the upper eyelids, 

 the lining of the prepuce, and the glans penis. Some of the hairs are 

 long, others are short and stiff, and others are fine and downy. These 

 differences have led to a division of the hairs into three varieties : 



The first variety includes the long soft hairs, found on the head, on 

 the face in the male adult, around the genital organs and under the arms 

 in both the male and the female, and sometimes on the breast and over 

 the general surface of the body and extremities, particularly in the male. 



The second variety, the short stiff hairs, is found just within the 

 nostrils, on the edges of the eyelids and on the eyebrows. 



The third variety, the short, soft, downy hairs, is found on parts of 

 the general surface not provided with the long hairs, and in the carun- 

 cula lachrymalis. In early life and ordinarily in the female at all ages, 

 the trunk and extremities are covered with downy hairs ; but in the adult 

 male, these frequently become developed into long soft hairs. 



The hairs usually are set obliquely in the skin and take a definite 

 direction as they lie on the surface. On the head and face, and, indeed, 

 the entire surface of the body, the general course of the hairs may be 

 followed out ; and they present currents or sweeps that have 'nearly 

 always the same directions. 



The diameter and length of the hairs are variable in different per- 

 sons, especially in the long soft hairs of the head and beard. It may 

 be stated in general terms that the long hairs attain the length of twenty 

 inches to three feet (500 to 900 millimeters) in women, and considerably 

 less in men. Like the nails, the hair, when left to itself, attains in three 

 or four years a definite length, but when it is habitually cut it grows 

 constantly. The short stiff hairs are \ to J of an inch (6.4 to 12.7 milli- 

 meters) in length. The soft downy hairs measure ordinarily ^ to | of 

 an inch (2.1 to 12.7 millimeters) in length. 



Of the long hairs, the finest are on the head, where they average 

 about -%^-Q of an inch (64 #) in diameter. The hair ordinarily is coarser 

 in women than in men. Dark hair usually is coarser than light hair; 

 and on the same head the extremes of variation are sometimes observed. 

 The hairs of the beard and the long hairs of the body are coarser than 

 the hairs of the head. The average number of hairs on a square inch 

 of the scalp is about 1000 (155 in a square centimeter) and the number 

 on the head, about 120,000 (Wilson). 



In a normal condition the hairs are elastic and may be stretched to 

 one- fifth or one-third more than their original length. Their strength 

 varies with their thickness, but an ordinary hair from the head will bear 

 a weight of six to seven ounces (170 to 200 grams). A well-known prop- 

 erty of the hair is that of becoming strongly electric by friction ; and this 



