192 SPECIAL HISTOLOGY. 



described as occurring in those processes of the stratum Malpighii, in 

 which the hairs of the lanugo are developed. The outer cells, in fact, 

 remaining round and colorless, as they were before, the inner ones begin 

 to develop pigment in their interior and to elongate, becoming distin- 

 guished at the same time from the former, as a conical substance with 

 its point directed upwards. At first (Fig. 76 A\ this central substance 

 is quite soft, and like the layers of cells which surround it externally, 

 dissolves readily in solution of caustic soda ; subsequently, however, 

 when, together with the process which incloses it, it has elongated, its 

 elements harden, and separate into two portions, an enternal dark pig- 

 mented, and an external clear part, which are nothing else than a young 

 hair, together with its inner sheath (Fig. 76 B). The young hair, 

 whose point at first does not project beyond its inner root-sheath, now 

 grows gradually, forcing its point through the aperture of the old sac, 

 while at the same time its root-sheath elongates, and thrusts upwards 

 the bulb of the old hair, until 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 

 horny 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 

 processes, 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 processes continue to grow, and mechani- 

 cally elevate the cornified root of the old hair with its sheaths, to the 

 aperture of the sebaceous glands, where to all appearance a partial so- 

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

 tainty 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 eyelashes. 

 The hairs of the head, and the other hairs of the body of the child (al- 

 most a year old) in question, never contained more than one hair, though 



