188 SPECIAL HISTOLOGY. 



generally be separated only in shreds with it, from the cutis, rarely al- 

 together: this I regard as the first indication of the fibrous layers of 

 the hair-sacs. In the sixteenth and seventeenth weeks, the processes of 

 the mucous layer, which I will henceforward simply call " hair-rudi- 

 ments," increase in size up to -004-0-06 of a line in length, and 0-03- 

 0-04 of a line in breadth, and acquire thicker coverings, but as yet ex- 

 hibit no trace of a hair. In the eighteenth week these first appear in 

 the eyebrows, as hair-rudiments of 0-1-0-2 of a line, their central cells 

 becoming somewhat elongated, and arranging themselves with their lon- 

 gitudinal axes parallel to that of the rudiment, whilst the peripheral 

 cells are disposed with their now longer diameter transversely. A variety 

 of shade in the hitherto homogeneous hair-rudiment arises in this 

 manner, and a central substance, broad below, running above into a 

 sharp point, becomes marked off from an outer portion, which is narrow 

 below and thick above. When the rudiment has attained a length of 

 0-22 of a line, this marking off is still more distinct, the rather longer 

 and especially broader, inner cone having a somewhat clearer appear- 

 ance (Fig. 74). Finally, in rudiments of hair of 0'28 of a line, the 

 inner cone is divided into two structures, a central portion somewhat 

 darker, and an external, perfectly transparent and glassy, the hair 

 and the inner root-sheath, whilst the peripheral cells which have re- 

 mained opaque, constitute unmistakably the outer root-sheath (Fig. 

 75 A). At the same time the papilla, which was even before (Fig. 74) 

 just traceable, becomes more distinct, and the proper hair-sac also more 

 recognizable, as the cells which lie external to its structureless membrane 

 begin to pass into fibres which may, even at this time, be known by their 

 decussation. The hair-sacs and hairs arise, in other places, exactly in 

 the same manner as in the eyebrows, except that their development takes 

 place somewhat later. In the fifteenth week, no rudiments of hairs are 

 visible, except on the forehead and eyebrows ; in the sixteenth and 

 seventeenth week they appear all over the head, back, chest, and ab- 

 domen ; and not till the twentieth week on the extremities. The hairs 

 themselves never make their appearance earlier than 3-5 weeks after 

 that of the rudiments ; in the nineteenth week, for example, the com- 

 mencement of hairs is nowhere to be seen, except on the forehead and 

 eyebrows ; and in the twenty-fourth week they are still absent upon the 

 hand and foot, and partly on the forearm and leg. 



Once formed, the hairs and hair-sacs continue to grow. The former 

 sometimes penetrate the epidermis immediately (eyebrows, eyelashes, 

 Fig. 75), sometimes their points are insinuated between the horny layer 

 and the stratum Malpiglm, or among the elements of the horny layer 

 itself, and grow for a time covered by the epidermis (chest, abdomen, 

 back, extremities [?]), through which they eventually make their pas- 

 sage. Involutions of the skin growing towards the hairs as they pass 



