THE SKIN. 



developing nail the superficial layer of the epidermis or epitrichial layer (vide ante, 

 p. 412) is greatly thickened, and below this thickening is a well-marked stratum 

 lucidum and subjacent granular layer. Before the anterior border of the nail be- 

 comes free, it appears to be continuous with the stratum lucidum in front of it. At 

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

 has been pushed forward. As the infantile nail continues to grow, its flattened 

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



The average rate of growth of the nails is about -^ of an inch per week. 



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

 Malpighian layer at the root. These cells acquire eleidin granules, and each 

 successive series being followed and pushed from their original place by others, 

 they become flattened into dry, hard, and inseparably coherent scales. By this 

 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 at the lunula, it grows in 

 thickness ; so that it is thicker at the anterior border of the lunula than nearer the 

 root. It does not appear to increase in thickness while passing over the bed. 

 When a nail is thrown off by suppuration, or pulled away by violence, a new one 

 is produced in its place, provided any of the cells of the deeper layers of the 

 epithelium are left. 



Hairs. A hair consists of the root, which is fixed in the skin, and the shaft or 

 stem. The stem is generally cylindrical, but may be more or less flattened : when 

 the hair is young, 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. In the straight-haired races (e.g. Mongolian), the 

 individual hairs are coarser and thicker and the section more circular than in the 

 crisp-haired races (negro) in which the section is smaller and oval, the hairs being 

 sometimes markedly flattened. The section is largest in the North American Indians, 

 Chinese, and especially in the Japanese. Light-coloured hair is usually finer than 

 black. 



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

 projecting edges of which give rise to a series of fine waved transverse lines, which 



Fig. 480. HUMAK HAIR. (E. A. S.) 



A t The surface of the hair focussed to show the cuticular 

 iles. B, optical section. The medulla looks clear, the air 

 having been expelled from it by Canada balsam. 



may be seen with the microscope on the surface of the 

 hair (fig. 480, A). Within this scaly covering, called 

 the hair-cuticle, is a fibrous or cortical substance which 

 in all cases constitutes the chief part and often the 

 whole of the stem ; but in many hairs the axis is occu- 

 pied by a substance of a different nature, called the 

 medulla or pith. The fibrous substance is translucent, with short longitudinal opaque 

 streaks of darker colour intermixed. It is formed of straight, rigid, longitudinal fibres, 

 .which again may be resolved into flattened cells of a fusiform outline ; they may be 

 marked with ridges and furrows, and united with one another by fibrils, as with 

 the deeper cells of stratified epithelium (Waldeyer). The colour of the fibrous 

 substance is caused by oblong patches of pigment-granules, and generally diffused 

 colouring matter of less intensity. Very slender elongated nuclei are also discovered 

 by means of reagents, whilst specks or marks of another description in the fibrous 

 substance are occasioned by minute irregularly-shaped cavities containing air. 

 These air-lacunules are abundant in white hairs, and are best seen in them, there 

 being no risk of deception from pigment-specks ; indeed they may be altogether 



