1142 GENERAL ANATOMY OR HISTOLOGY. 



opaque and deeper colored when viewed by transmitted light than the fibrous part ; 

 but when viewed by reflected light it is white. It is composed of rows of poly- 

 hedral cells, which contain granules of eleidin and frequently air-bubbles. The 

 fibrous portion of the hair constitutes the chief part of the shaft ; its cells are 

 elongated and unite to form flattened fusiform fibres. Between the fibres are found 

 minute 'spaces which contain either pigment-granules in dark hair or minute air- 

 bubbles in white hair. In addition to this there is also a diffused pigment con- 

 tained in the fibres. The cells which form the cortex of the hair consist of a 

 single layer which surrounds those of the fibrous part ; they are converted into 

 thin, flat scales, having an imbricated arrangement. 



Connected with the hair-follicles are minute bundles of involuntary muscular 

 fibres, termed arrectores pili. They arise from the superficial layer of the corium, 

 and are inserted into the outer surface of the hair-follicle, below the entrance of 

 the duct of the sebaceous gland. They are placed on the side toward which the 

 hair slopes, and by their action elevate the hair (Fig. 679). 1 



The sebaceous glands are small, sacculated, glandular organs, lodged in the sub- 

 stance 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 terminates in a cluster of small secreting pouches or saccules. 

 The sacculi connected with each duct vary, as a rule, in number from two to five, 

 but in some instances may be as many as twenty. They are composed of a 

 transparent, colorless membrane, enclosing a number of epithelial cells. Those 

 of the outer or marginal layer are small and polyhedral, and are continuous with 

 the lining cells of the duct. The remainder of the sac is filled Avith larger cells, 

 containing fat, except in the centre, where the cells have become broken up, leav- 

 ing a cavity containing their debris and a mass of fatty matter, which constitutes 

 the sebaceous secretion. The orifices of 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 the glands are of large size, 

 distinctly lobulated, and often become much enlarged from the accumulation of 

 pent-up secretion. The largest sebaceous glands are those found in the eyelids 

 the Meibomian glands. 



The sudoriferous or sweat-glands are the organs by which a large portion of 

 the aqueous and gaseous materials is excreted by the skin. They are found in 

 almost every part of this structure, and are situated in small pits on the under sur- 

 face of the corium, or, more frequently, in the subcutaneous areolar tissue, sur- 

 rounded by a quantity of adipose tissue. They are small, lobular, reddish bodies, 

 consisting of a single convoluted tube, from which the efferent duct proceeds upward 

 through the corium and cuticle, becomes somewhat dilated at its extremity, and 

 opens on the surface of the cuticle by an oblique valve-like aperture. The efferent 

 duct, as it passes through the epidermis, presents a spiral arrangement, being 

 twisted like a corkscrew, in those parts where the epidermis is thick ; where, how- 

 ever, it is thin, the spiral arrangement does not exist. In the superficial layers 

 of the corium the duct is straight, but in the deeper layers it is convoluted or even 

 twisted. The spiral course of these ducts is especially distinct in the thick cuticle 

 of the palm of the hand and sole of the foot. The size of the glands varies. 

 They are especially large in those regions where the amount of perspiration is 

 great, as in the axillae, where they form a thin, mammillated layer of a reddish 

 color, which corresponds exactly to the situation of the hair in this region ; they 

 are large also in the groin. Their number varies. They are most numerous on 

 the palm of the hand, presenting, according to Krause, 2800 orifices on a square 



1 Arthur Thomson 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 im- 

 pressed on the hair which is moulded within it, so that the hair, on emerging through the skin, will 

 be curled. Curved hair-follicles are characteristic of the scalp of the Bushman. 



