34:8 EXCRETION BY THE SKIN AND KIDNEYS. 



growth. The Malpighian layer is stationary, but the horny layer is con- 

 stantly growing, if the nails be cut, from the root and bed. It is evident that 

 the nails grow from the bed, as their thickness progressively increases in the 

 body from the root to near the free border ; but their longitudinal growth 13 

 by far the more rapid. Indeed, the nails are constantly pushing forward, 

 increasing in thickness as they advance. Near the end of the body of the 

 nail, as the horny layer becomes thinner, the growth from below is dimin- 

 ished. 



Physiological Anatomy of the Hairs. Hairs, varying greatly in size, 

 cover nearly every portion of the cutaneous surface. The only parts in 

 which they are not found are the palms of the hands and soles of the feet, the 

 palmar surfaces of the fingers and toes, the dorsal surfaces of 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, which are found on the 

 head, on the face in the adult male, around the genital organs and under the 

 arms in both the male and the female, and sometimes upon 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, 

 upon the edges of the eyelids and upon the eyebrows. 



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

 general surface not occupied by the long hairs, and in the caruncula lachry- 

 malis. 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 fre- 

 quently become developed into long, soft hairs. 



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

 tion as they lie upon the surface. Upon 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 in different persons. 



The diameter and length of the hairs are variable in different persons, es- 

 pecially 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 mm.) 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 \ of an inch (6*4 to 12'7 mm.) in length. The 

 soft, downy hairs measure ordinarily T ^- to of an inch (2-1 to 12'7 mm.) in 

 length. 



Of the long hairs, the finest are upon the head, where they average about 

 5-^-5- of an inch (64 /*) in diameter. The hair ordinarily is coarser in women 

 than in men. Dark hair is generally coarser than light hair ; and upon 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 



