194 SPECIAL HISTOLOGY. 



tical plates, the outermost into epidermic scales, and thus the horny part 

 of the hair is continually forced from below upwards, and elongates. 

 In the latter no formation of elementary parts takes place, but at most 

 a certain metamorphosis of those which are already existent, which pro- 

 duces a gradual thinning of the root from the bulb upwards, until it 

 acquires the thickness of the shaft. Higher up still, these changes 

 of the elementary parts cease, whence cut hairs, for example, do not 

 produce new points. The root-sheaths and the outer layer of the epi- 

 dermis take no part in the growth of the cut hairs. 



The complete hair, though non-vascular, is not a dead substance. Al- 

 though the processes which go on in it are not at all understood, we 

 may suppose that fluids are diffused through it which subserve its nu- 

 trition and maintenance. These fluids are furnished from the vessels of 

 the papilla and sac of the hair, in all probability ascend (particularly 

 from the bulb) ; without any special canals through the cortex upwards, 

 and thus reach all parts of the hair. Having served for the nutrition 

 of the hair, they evaporate from its outer surface and are replaced by a 

 fresh supply. Perhaps the hairs also absorb fluids from without, though 

 of course only in the condition of vapor, like a hair used as a hygrometer ; 

 on the other hand I cannot believe that, as many authors would seem to 

 suppose, the secretion of the sebaceous glands passes from without into 

 the hairs, since the perfectly closed cuticle is probably impervious 

 to it. In the same way it seems to be in 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 been 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 hair in the medullary 

 axis and in the cortex can only arise from a disproportion 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 sup- 

 posed 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 and transpires ; if they are dry, brittle, 

 and rough, then it may be concluded that the surface of the body is in 

 a collapsed condition. 



