328 



NORMAL HISTOLOGY. 



sac in the vicinity of the neck. The glands usually lie on the side towards 

 which the hair inclines, but sometimes, especially in the case of the smaller 

 hairs, they may completely surround the follicle. Since these glands are 

 outgrowths from the same tissue that lines the follicles, their ducts pierce the 

 outer root-sheath, bringing their oily secretion into direct relation with 

 the hairs. 



Most of the larger hair-follicles, particularly those of the scalp, are 

 provided with ribbon-like bundles of involuntary muscle, called the arrec- 

 tores pilorum in recognition of their effect on the hairs. They arise from 

 the superficial part of the corium, pass obliquely downwards to be inserted 

 into the sheath of the hair-follicle near the junction of corium and subdermal 

 tissue, and on the side corresponding with the inclination of the hair and the 

 situation of the sebaceous glands. Since the latter are closely embraced by 

 the muscular bands, contraction of the muscles exerts pressure upon the 



glands and facilitates the dis- 

 charge of their secretion, the 

 sebum. 



The blood-vessels sup- 

 plying the hair-follicle, which 

 in a sense constitute a special 

 system for each sac, include 

 the capillary loops ascending 

 within the hair-papilla and 

 the network of capillaries sur- 

 rounding the follicle immedi- 

 ately outside the glassy mem- 

 brane. The first are derived 

 from a small special twig that 

 ascends to the follicle, and the 

 second from the subpapillary 

 network of the corium. With 

 the exception of those draining 

 the papilla, which are tributary 

 to the deeper stems, the veins 

 join the subpapillary plexus. 



The nerves distributed 

 to the follicles follow a fairly 

 definite arrangement. Usually each hair-sac is supplied by a single fibre, 

 sometimes by two or more, which approaches the follicle immediately below 

 the level of the mouth of the sebaceous glands. After penetrating the fibrous 

 sheath as far as the glassy membrane, the nerve-fibre separates into two 

 divisions that encircle more or less completely the follicle and on the opposite 

 side break up into terminal arborizations. The nerve-endings usually lie 

 on the outer surface of the glassy membrane within the middle third of the 

 follicle and only exceptionally are found within the outer root-sheath or the 

 hair-papilla. 



Development. The primary development of the hair begins, about 

 the end of the third month of foetal life, as localized proliferations of the 

 epidermis. In section these appear as lenticular thickenings and on the 

 surface as slight projections. Very soon solid epithelial cylinders sprout from 

 the deeper surface of these areas and invade the subjacent corium to form the 

 rudiments of the hair-follicles. The original uniform outline of these proc- 

 esses is early replaced by a flask-shaped contour in consequence of the 



Papillary 

 twig 



FIG. 372. Section of injected scalp, showing capillary net- 

 works surrounding hair- follicles and twigs entering papillae. 



