. A practical treatise on diseases of the skin, for the use of students and practitioners. f the skin to its extremity; thelatter, when uncut, always tapers to a perfectly acuminate point, asillustrated by the uncut hairs of the eyelids, and those of the lower ANATOMY AND PHYSIOLOGY OF THE SKIN 39 Fig. 14. animals. The hair-shaft is either straight, curled, wavy, or alternatelyvaried in diameter, producing the peculiar character of the growthseen upon the scalp of the negro, these va-riations being due to the different degreesof flattening of the hair-shaft in a trans-verse direction. The colo
. A practical treatise on diseases of the skin, for the use of students and practitioners. f the skin to its extremity; thelatter, when uncut, always tapers to a perfectly acuminate point, asillustrated by the uncut hairs of the eyelids, and those of the lower ANATOMY AND PHYSIOLOGY OF THE SKIN 39 Fig. 14. animals. The hair-shaft is either straight, curled, wavy, or alternatelyvaried in diameter, producing the peculiar character of the growthseen upon the scalp of the negro, these va-riations being due to the different degreesof flattening of the hair-shaft in a trans-verse direction. The color of the hair is dependent uponthe pigment it contains, the proper color ofthe hair-cells, and the quantity of air con-tained in the medulla. Variation in thesethree factors produces the wide range be-tween a snowy whiteness and an ebony-black. The membrane which invests the shaftof the hair is the cuticle composed of nu-merous flattened plates, regularly overlaidso as to resemble fish-scales when viewedunder the microscope on the flat side, andthe overlapping shingles of the roof of a. house when seen on the ecW. Transverse section 01 hair andfollicle. The Cortical, or main, Substance of the Shaft of the hairis composed of flat, nucleated, fusiform, epidermal cells. The strength,elasticity, and extensibility of the hair are chiefly due to the corticalsubstance, and in particular to the firmness with which these epidermalcells are attached to one another. The Medullary Substance of the Shaft of the hair is foundbest developed in the short, strong hairs of the beard and eyelashes,being wanting in the lanugo hairs. It consists of a loosely packedmass of epidermal elements, differing in shape, developed in the centreof the axis of the shaft. This part of the hair contains also the pig-ment and fatty matters, which are here arranged as in the rete of theepidermis. Seen under the microscope, the medulla appears as a con-tinuous or interrupted longitudinal band, extending from the bu
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