OF THE HAIRS. 



L95 



Fig. 77. 



<f?" 



at last it passes completely out, and makes its appearance at the 



same opening with the old one, which is more and more pushed 



up. "When the development of the hair has 



gone thus far, the last stage may be readily 



understood. The old hair, which has for a long 



time ceased to grow, and to be connected with 



the bottom of the sac, being thus extruded, 



falls out, while the young hair becomes larger 



and stronger, and fills the gap left by the old 



one. The primary cause of the dying away and 



casting off of the old hair, I consider to be the 



development of the processes of the hair bulb 



and outer sheath from the bottom of the sac, 



which has been described. As the sacs do not 



elongate to a corresponding extent, they push 



upwards all those parts which lie above them, 



and cause a continually increasing space to 



exist between the papilla and the proper hair, 



or the point at which the round cells of the 



bulb begin to elongate and undergo conversion 



into hornv matter. 



The hair thus becomes in a manner detached 

 from the source of its nourishment ; it receives 

 less and less blastema, at last ceasing to grow, and becoming 

 converted into horn in its lowest part. The cells of the pro- 

 cesses, on the other hand, which are connected with the papilla, 

 are incessantly supplied from it with new formative material, 

 which for the time they apply not to the formation of horny 

 matter, but to their own growth. In this manner the pro- 

 cesses continue to grow, and mechanically elevate the cor- 

 nified root of the old hair, with its sheaths, to the aperture of 

 the sebaceous glands, where to all appearance a partial solution 

 of the old sheaths takes place : this may be observed with 

 certainty in the inner sheath, and must be assumed to occur in 

 the outer. 



All that has been said, holds good only with respect to the 



Fig. 77. An eyelash with the root-sheaths from a child one year old, with an old 

 and a growing young hair, x 20 ; the young hair is wholly extruded, and now two 

 hairs appear at one aperture. A sudoriparous canal opens into the hair-sac. The 

 letters have the same signification as in fig. 7G. 



