198 SPECIAL HISTOLOGY. 



nowise proved that the hairs are pervaded by a peculiar 

 oleaginous fluid (Laer), which might proceed from the medullary 

 substance (Reichert), and which keeps it greasy, for such a fluid 

 has not beeu demonstrated, and the greasiness of the hairs may 

 be more simply explained by the externally adherent sebaceous 

 matter, which is readily visible. The existence of air in the 

 medullary axis and in the cortex can only arise from a dis- 

 proportion between the supply of fluid from the hair-sac and 

 the amount evaporated ; it is owing, as it were, to a drying- 

 up of the hair, which, however, must not be supposed to go so 

 far that the hair contains no fluid in its aeriferous portion. 

 In any case, however, these portions are the most inactive, or 

 relatively dead parts of the hair; the cortex, on the other hand, 

 which is also most readily altered by alkalies and acids, 

 notwithstanding the apparent hardness and density of its 

 elements, is the most rich in juices, and is that in which the 

 nutritive process is most actively going on. Hence it follows, 

 that the hair lives, and is to a certain extent dependent upon 

 the collective organism, particularly on the skin, from whose 

 vessels (i. e. those of the hair-sac) it derives the materials 

 necessary for its maintenance. Therefore, as Henle well says, 

 the condition of the hair is a sort of index of that of the 

 activity of the skin ; if they are soft and shining, the skin is 

 turgescent aud transpires ; if they are dry, brittle, and rough, 

 then it may be concluded that the surface of the body is 

 in a collapsed condition. 



The falling out of the hairs certainly depends, in many cases, 

 — when, for example, it takes place in the course of normal 

 development, — on nothing else than a want of the necessary 

 nutritive material, which in the instance already explained, in 

 speaking of the shedding of the hairs, depends on the detach- 

 ment of the hair from its matrix by the abundant production of 

 cells at the bottom of the hair-sac. In age, perhaps, it arises 

 simply from the obliteration of the vessels of the hair-sacs. 



The whitening of the hairs, which chiefly depends upon a 

 decoloration of the cortex, and less upon that of the almost 

 colourless medulla, should probably be here considered, for its 

 normal occurrence in old age gives it the significance of a retro- 

 gressive development. 



The frequent occurrence of cases, in which the hair grows 



