HAIRS. 



219 



wards with water ; and then it is seen that they still retain their 

 nuclei. In chemical composition the nails resemble epidermis ; but, 

 according to Mulder, they contain a somewhat larger proportion of 

 carbon and sulphur. 



The growth of the nail is effected by a constant generation of cells at 

 the root and under surface. Each successive series of these cells being 

 followed and pushed from their original place by others, they become flat- 

 tened into dry, hard, and inseparably coherent scales. By the addition 

 of new cells at the posterior edge the nail is made to advance, and by 

 the apposition of similar particles to its under surface it grows in 

 thickness; so that it is thicker at the fi-ee border than at the root. 

 The nail being thus merely an exuberant part of the epidermis, the 

 question at one time raised, whether that membrane is continued, 

 underneath it, loses its significance. AVhen a nail is thrown off by 

 suppuration, or pulled away by violence, a new one is produced in its 

 place, provided the matrix remains. 



Development of tlie nails. — In the tliiixl month of intra-uterine life the paii; 

 of the embryonic cerium which becomes the matrix of the nail is marked off by 

 the commencing cm'vilinear gi-oove. which limits it posteriorly and laterally. 

 The epideiTuis on the matrix then begins to assume, in its under part, the cha- 

 racters of a nail, wliich might, therefore, be said to be at first covered over hy 

 the embryonic cuticle. After the end of the fifth month it becomes free at the 

 anterior border, and in the seventh month decidedly begins and thenceforth con- 

 tinues to gi-ow in length. At bu-th the free end is long and thin, being mani- 

 festly the earlier fonned part which has been pushed fom^ard. This breaks or 

 is pared off after birth, and, as the infantUe nail continues to grow, its flattened 

 cells, at fii'st easily separable, become harder and more coherent, as in after-life. 



Kate of growth. — The average rate of growth of the naUs is about ^^ of 

 an inch per week (Benham). Berthold found that the nails grow rather faster 

 in summer than in winter, and faster in the right hand than in the left. He 

 also observed a difference in the nails of different fingers : thus it was fastest in 

 the middle finger and slowest in the thumb. A careful series of experiments by 

 Mr. H. J. Benham, hitherto unpublished, confirm generally the observations of 

 Berthold, but no clear difference could be observed between the two hands, and 

 tlie gi-owth apijeared to be slowest in the little finger. In some individuals these 

 differences were not observed. 



Hairs. — A hair consists of the root, which is fixed in the skin, the 

 shaft or stem, and the point. The stem is generally cylindrical, but 



FiiT. 146. 



Fig. 116. — A, Surface op a White Hair, magnified 160 Ciameters. The Waved 

 Lines mark the upper or free edges up the Cortical Scales. B, Separated 

 Scales, magnified 350 Diameters (after KoUiker). 



cften more or less flattened : when the hair is entire, it becomes gra- 



