188 SPECIAL HISTOLOGY. 



cells, without any intermediate openings (fig. 72, C), which be- 

 coming 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 replaced 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 continuously; it is not, however, 

 anv 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 epi- 

 dermis formed by its growth inwards, in which, the internal and 

 external cells subsequently 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 developed 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 from those lying between the 

 hair and the outer sheath. The proper hah*-sac is formed, in its 

 fibrous layers, essentially in loco, out of the formative cells 

 which surround the rudiment of the hair ; possibly, however, 

 they may be considered as an involution of the cutis, pro- 

 duced by the ingrowing process of the epidermis. Its 



