HAIRS. 



:42l 



wanting in veiy dark hairs. 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 hair quickly 

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



The medulla or pith, as already remarked, does not exist in all hairs. It is want- 

 ing 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 



Fig. 481. HAIR-FOLLICLE, IN LONGITUDINAL SECTION. MODE- 

 RATELY MAGNIFIED. (Biesiadecki.) 



a, mouth of the hair-follicle ; b, its neck ; c, lower bulbous 

 enlargement ; d, e, dermic coat (outer and inner layers, the 

 innermost or hyaline layer is not shown) ; /, /, epidermic 

 coat (outer and inner root-sheath) ; h, fibrous substance of the 

 hair ; Jk, medulla ; I, hair-knob : m, fat in the subcutaneoiis 

 tissue ; n, arrector pili ; o, papilla of the cutis ; p, papilla of 

 the hair-knob ; s, Malpighian layer of the epidermis ; ep, 

 horny layer, incorrectly represented in the figure as continuous 

 with the inner root-sheath ; t, sebaceous gland. 



it occupies the centre of the shaft and ceases towards 

 the point. It is composed of rows of cells, differing 

 in shape, but generally angular, and in many animals 

 exhibiting regular patterns. When viewed by trans- 

 mitted light, it is black ; by reflected light, on the 

 other hand, it is white, its colour being chiefly due to 

 the contained air-particles which lie in spaces between 

 the cells, but in the hairs of a few animals are 

 within the cells. They are produced by the drying 

 of the originally soft cells of the medulla, on the 

 exposure of the growing hair to the atmosphere. 

 The medulla may be interrupted at parts for a 

 greater or less extent. In the latter case, the axis 

 of the stem at the interruptions is fibrous like the 

 surrounding parts. 



The root of the hair is lighter in colour and 

 softer than the stem ; in young and growing hairs 

 it swells out at its lower end into a bulbous enlarge- 

 ment or knob (figs. 481, 483), but in older hairs 

 which have ceased to grow and are in process of being 

 shed, the termination of the root is not bulbous 

 (fig. 488). The root of the hair is received into a 

 recess of the skin named the hair-follicle, which, 

 when the hair is of considerable size, reaches down 

 into the subcutaneous fat. 



The substance of the hair, of epidermic nature, is. 

 like the epidermis itself, quite extra-vascular, but like 

 that structure also, it is organised and subject to internal 

 organic changes. Thus, in the progress of its growth, 

 the cells change their figure, and acquire greater con- 

 sistency. In consequence of their elongation, the hair, bulbous at the commencement, becomes 

 reduced in diameter, and cylindrical above. But it cannot be said to what precise distance 

 from the root organic changes may extend. Some have imagined that the hairs are slowly 

 permeated by a fluid from the root to the point, but this has not been proved. The sudden 

 change of the colour of the hair from dark to grey, which sometimes happens, has never been 



