186 SPECIAL HISTOLOGY. 



broader than the cells which have already been described (0-014 to 

 0-018 of a line long, 0-006 to 0-009 of a line broad), but are also poly- 

 gonal, and always possess, at least in the lower half of the root-sheath, 

 distinct elongated nuclei, often prolonged into points of 0-004-0-006 of 

 line. The diameter of the whole inner root-sheath is, upon the average, 

 0-006-0-015 of a line, whence it follows that its cells, of which there are 

 never more than three layers, are at least 0-002-0-005 of a line thick. 

 They are recognizable at once in their natural position, and by the teasing 

 out of the root-sheath, and are readily isolated by the use of soda and 

 potassa (Fig. 72), but without swelling up, a character which no less than 

 their great resistance to alkalies altogether, distinguishes their cells, in 

 common with the epidermic scales of the hairs, from all others. 



At the bottom of the hair-sac, the inner root-sheath consists only of 

 a single layer of beautiful, large, polygonal, nucleated cells, without any 

 intermediate openings (Fig. 72, (7), which becoming at last soft, delicate, 

 and rounded, pass without defined limits into the outer layers of the 

 round cells of the hair-bulb. Superiorly, this membrane not unfrequently 

 becomes somewhat separated from the hair, and ends, not far from the 

 apertures of the sebaceous glands in a sharp, notched edge, formed by 

 its separate more or less projecting cells. Thence upwards, it is re- 

 placed by a layer of cells, in some cases at first nucleated, but at other 

 times not, which gradually approximates more and more, as it is traced 

 higher up, to the horny layer of the epidermis, into which it passes con- 

 tinuously ; it is not, however, any direct continuation of the inner root- 

 sheath. 



62. Development of the Hairs. The first rudiments of the hairs are 

 flask-shaped, solid processes of the mucous layer of the epidermis formed 

 by its growth inwards, in which the internal and external cells subse- 

 quently become differentiated in such a manner, that the former, a 

 gradual conversion into horn going on, are, in the axis of the rudiment, 

 metamorphosed, in the first place into a small delicate hair, and secondly, 

 around this into its internal sheath ; while the latter, undergoing less 

 alteration and remaining soft, constitute the outer root-sheath and the 

 soft cells of the hair-bulb. Hence the hairs and their sheaths arise at 

 once in their totality. The former, as minute hairs with root, shaft, 

 and point, and are therefore not developed point first, as the teeth are, 

 with their crown first, and still less as Simon has supposed, from their 

 root first. The elements of the youngest hairs are nothing but elongated 

 cells similar to those of the cortex of the later hairs, which are deve- 

 loped by the lengthening and chemical alteration of the innermost cells 

 of the rudiments of the hairs. Medullary cells are entirely wanting, 

 but the cuticle is clearly visible. The inner sheath is striated, presents 

 no openings, and consists of elongated cells, which have been developed 



