OF THE I! A IKS. 197 



accord, the process does not appear to he exactly the same as 

 in man. 



§64. 



Physiological Observations. — The hairs have a definite length, 

 dependent upon locality and sex, but if they arc cut they grow- 

 again, and consequently exhibit the same conditions as the 

 other horny textures. The place from whence the growth of the 

 hair proceeds is unquestionably the bottom of the hair-sae. 

 Here there arise around the papillae, -with the co-operation of a, 

 blastema formed out of its vessels or those of the hair-sac, new 

 elements, by the continual multiplication of the existing cells, 

 while those which are already present, somewhat higher up pass 

 uniuteruptedly, the middle ones into medullary cells, the next 

 into cortical plates, the outermost into epidermic scales, and 

 thus the hornv part of the hair is continually forced from below 



* L a/ 



upwards, and elongates. In the latter no formation of elemen- 

 tary parts takes place, but at most a certain metamorphosis of 

 those which are already existent, which produces 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-shcaths and the outer layer of 

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



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

 substance. Although the processes which go on in it are not 

 at all understood, we may suppose that fluids are diffused through 

 it which subserve its nutrition 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 vapour, 

 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 



