24 COMPARATIVE ANATOMY. 



Each hair arises first as a proliferation of the epidermic cells in 

 the region of the Malpighian layer, which projects inwards towards 

 the derma (Fig. 13, A and B, Sc, SM, 0). In this manner the 

 hair-germ is formed. The thickening of the epidermis then 

 grows out into the form of a papilla, and becomes surrounded 

 by the cells of the derma, so that, as in the case of the feather, 

 it comes to lie within a kind of pocket, the hair- follicle (Fig. 13, 

 C, D, F). The originally uniform mass of cells of the hair-gerrn 

 later becomes differentiated into a peripheral and a central 

 portion (Fig. 13, E, F, PZ, GZ}. The latter consists of more elong- 

 ated cells, and gives rise later to the hair-s-haft with its medulla 

 or pith, and to the cortex, as well as to the cuticle of the shaft, 

 and to the so-called inner root-sheath ; the former gives rise to 

 the outer .root-sheath (comp. Fig. 14, which represents the 

 fully-formed hair). The base of the hair-shaft which fills up the 

 bottom of the follicle is broadened out to form the hair- knob 

 (Fig. 13,E,F,^AT), and the richly vascular hair-papilla (Fig. 13, E, 

 F, P, P 1 ), which arises comparatively late, extends into it from 

 below. At Dr, in Fig. 13, the sebaceous glands are seen 

 arising by a proliferation of the Malpighian cells. The hair usually 

 breaks through the skin in an oblique direction. The character of 

 the medulla varies so much that upon it principally depend the 

 differences observable in the hair of Mammals. 1 The colour of the 

 hair is due to three causes : Firstly, to the greater or less accumula- 

 tion of pigment in the cells of the cortical layer ; secondly, to the 

 air contained in the intercellular spaces of the medulla; and lastly, 

 to the nature of the surface of the hair, i.e. whether it is rough 

 or smooth. 



The hair-sheaths consist of an outer, or dermic coat (Fig. 

 14, F, F 1 ), and of an inner or epidermic coat, the so-called 

 root- sheath (WS, WS 1 ). The first of. these is formed entirely 

 from the connective-tissue of the derma that is, from the above- 

 mentioned hair-sac or follicle, while the latter is a product of 

 the Malpighian layer, with which it remains in connection. 

 Between the two hair-sheaths lies a homogeneous and thin 

 hyaline-layer (GIT). 



The follicular tissue, which is richly provided with blood- 

 vessels, extends into the knob -like base or root of the hair-shaft 

 (bulb us), and gives rise to the hair -papilla (Fig. 14, HP). The 

 latter is the real matrix of the hair, and from this region a new 

 hair-shaft may develop when the hair is shed, periodically or 

 non-periodically as the case may be, by the formation of a new 

 papilla. Whether new hairs arise in the same manner as in 

 the embryo ("primary hair-formation") cannot be stated with 

 certainty. 



Smooth muscles (arrectores pili) and nerves, as well as seba- 



1 In Bats, tho hairs arc usually distinguished by scale-like projections of their 

 surface. 



