THE APPENDAGES OF THE SKIN 



1069 



found, 



parts, 

 in the 



Hyaline layer > 



Cortex 

 of hair 



Medulla 

 of fiair 



Huxley's _ _ 

 layer 



Henle's layer - "^ 



the center of the bulb; this gives rise to the dark tract of pigment often 

 of greater or less length, in the axis of the hair. 



The shaft of the hair (scapus pili) consists, from within outward, of three 

 the medulla, the cortex, and the cuticle. The medulla is usually wanting 

 fine hairs covering the surface of 

 the body, and commonly in those 

 of the head. It is more opaque 

 and deeper colored than the cor- 

 tex when viewed by transmitted 

 light; but when viewed by re- 

 flected light it is white. It is 

 composed of rows of polyhedral 

 cells, containing granules of elei- 

 din and frequently air spaces. 

 The cortex constitutes the chief 

 part of the shaft; its cells are 

 elongated and united to form 

 flattened fusiform fibers which 

 contain pigment granules in dark 

 hair, and air in white hair. The 

 cuticle consists of a single layer 

 of flat scales which overlap one 

 another from below upward. 



Connected with the hair fol- 



Outer or 

 dermic coat 



licles are minute bundles of in- 

 voluntary muscular fibers, termed 

 the Arrectores pilorum. They 

 arise from the superficial layer 

 of the corium, and are inserted 

 into the hair follicle, below the 

 entrance of the duct of the seba- 

 ceous gland. They are placed on 

 the side toward which the hair 

 slopes, and by their action diminish the obliquity of the follicle and elevate the 

 hair (Fig. 944). * The sebaceous gland is situated in the angle which the Arrector 

 muscle forms with the superficial portion of the hair follicle, and contraction of the 

 muscle thus tends to squeeze the sebaceous secretion out from the duct of the gland. 

 The Sebaceous Glands (glandules sebaceae) are small, sacculated, glandular 

 organs, lodged in the substance of the corium. They are found in most parts of 

 the skin, but are especially abundant in the scalp and face; they are also very 

 numerous around the apertures of the anus, nose, mouth, and external ear, but are 

 wanting in the palms of the hands and soles of the feet. Each gland consists of a 

 single duct, more or less capacious, which emerges from a cluster of oval or flask- 

 shaped alveoli which vary from two to five in number, but in some instances there 

 may be as many as twenty. Each alveolus is composed of a transparent basement 

 membrane, enclosing a number of epithelial cells. The outer or marginal cells 

 are small and polyhedral, and are continuous with the cells lining the duct. The 

 remainder of the alveolus is filled with larger cells, containing fat, except in the 

 center, where the cells have become broken up, leaving a cavity filled with their 

 debris and a mass of fatty matter, which constitutes the sebum cutaneum. The 

 ducts open most frequently into the hair follicles, but occasionally upon the general 

 surface, as in the labia minora and the free margin of the lips. On the nose and face 



1 Professor Arthur Thomson, of Oxford, suggests that the contraction of these muscles on follicles which contain 

 weak, flat hairs will tend to produce a permanent curve in the follicle, and this curve will be impressed on the hair 

 which is moulded within it, so that the hair, on er 



Fio. 945. Transverse section of hair follicle. 



characteristic of the scalp of the Bushman. 



emerging through the skin, will be curled. Curved hair follicles are 



