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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
the center of the bulb; this gives rise to the dark tract of pigment often found, of greater or less length, in the axis of the hair.
  The shaft of the hair (scapus pili) consists, from within outward, of three parts, the medulla, the cortex, and the cuticle. The medulla is usually wanting in the fine hairs covering the surface of the body, and commonly in those of the head. It is more opaque and deeper colored than the cortex when viewed by transmitted light; but when viewed by reflected light it is white. It is composed of rows of polyhedral cells, containing granules of eleidin 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 follicles are minute bundles of involuntary 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 sebaceous 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). 1 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.


FIG. 945– Transverse section of hair follicle. (See enlarged image)

  The Sebaceous Glands (glandulæ sebaceæ) 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
Note 1.  Clark and Lhamon, Anatomical Record, 1917, xii. [back]

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