220 THE SKIN. 



dually smaller towards the point. The length and thickness vary 

 greatly in diflferent individuals and races of mankind as 'well as in 

 different regions of the body. 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 may be seen with the microscope on the surface 

 of the hair (fig. 146, a). Within this- scaly covering, by some called 

 the Jiair-cuficle, is a fihrous 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 7?///?, for which reason the surrounding fibrous part is often named 

 " cortical," although this term is more properly applied to the super- 

 ficial coating of scales above mentioned. The fihrous substance is 

 translucent, with short longitudinal opaque streaks of darker colour 

 intermixed. It may be broken up into straight, rigid, longitudinal 

 fibres, which, Avhen separated, are found to be flattened, broad in the 

 middle and pointed at each end, with dark and rough edges. The 

 fibres may be resolved into flattened cells of a fusiform outline ; these 

 are mostly transparent, or marked with only a few dark specks. The 

 colour of the fibrous sul3stance 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 shnped cavitio^ containing air. 

 These air-lacunules are abundant in wdiite hairs, and in very dark hairs 

 may be altogether wanting ; they are best seen too in the former, in 

 which there is no risk of deception from pigment-specks. Viewed by 

 transmitted light they are dark, but brilliantly white by reflected light. 

 "When a white hair has been boiled in water, ether, or oil of turpentine, 

 these cavities become filled with fluid, and are then quite pellucid; 

 but when a hair which has been thus treated is dried, the air quickly 

 finds its way again into the lacuna3, and they resume their original aspect. 



The medulla or pith, as already remarked, does not exist in all hairs. 

 It is wanting in the fine hairs over the general surface of the body, and 

 is not commonly met with in those of the head ; nor in the hairs of 

 children under "five years. When present it occupies the centre of the 

 shaft and ceases towards the point. It is more opaque and deep- 

 coloured than the fibrous part ; in the white hairs of quadrupeds it is 

 Avhite, but opaque and dark when seen by transmitted light. It seems 

 to be composed of little clusters of cells, differing in shape, but gene- 

 rally angular, and containing minute particles, some resembling pigment- 

 granules, and others like very fine fat granules, but really for the most 

 part air ]3articlcs, apparently included in some, solidified tenacious sub- 

 stance. The whole forms a continuous dark mass along the middle of 

 the stem, interrupted at parts for a greater or less extent. In the latter 

 case, the axis of the stem at the interruptions may be fibrous like the 

 surrounding parts, or these intervals may be occupied by a clear colour- 

 less matter; and, according to Henle, some bairs present the appear- 

 ance of a sort of canal running along the axis and filled in certain parts 

 with opaque granular matter, and in others with a homogeneous trans- 

 parent substance. 



The root of the hair is lighter in colour and softer than the stem ; 



