ccviii HAIRS. 



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

 and pushed from their original place by others, become flattened 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 free 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. When 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 in the foetus. In the third month of intra-uterine life the part 

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

 commencing curvilinear groove, which limits it posteriorly and laterally. The 

 epidermis on the matrix then begins to assume, in its under part, the characters of a 

 nail, which might, therefore, be said to be at first covered over by 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 continues to grow in length. 

 At birth the free end is long and thin, being manifestly the earlier formed part which 

 has been pushed forward. This breaks or is pared off after birth, and, as the 

 infantile nail continues to grow, its flattened cells, at first easily separable, become 

 harder and more coherent, as in after life. 



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 often more 

 or less flattened ; sometimes it is grooved along one side, and therefore 

 reniform in a cross section : when the hair is entire, it becomes gradually 

 smaller towards the point. The length and thickness vary greatly in 

 different individuals and races of mankind as well as in different regions of 

 the body. Light-coloured hair is usually finer than black. 



Fig. CXIII. 



Fig. CXIII. A, SURFACE OP A WHITE HAIR, MAGNIFIED 160 DIAMETERS. THE WAVED 



LINES MARK THE UPPER OR FREE EDGES OF THE CORTICAL SCALES. B, SEPARATED 



SCALES, MAGNIFIED 350 DIAMETERS (after Kolliker). 



The stem is covered with a coating of finely imbricated scales, the 

 upwardly projecting serrated edges of which give rise to a series of fine 

 waved transverse lines, which may be seen with the microscope on the 

 surface of the hair (fig. cxm. A). Within this scaly covering, by some 

 called the hair-cuticle, is a fibrous substance which in all cases constitutes 

 the chief part and often the whole of the stem ; but in many hairs the axis 

 is occupied by a substance of a different nature, called the medulla or pith, 

 for which reason the surrounding fibrous part is often named " cortical," 



