266 THE TISSUES. 



length, and to keep the whole hair in a state of moisture and vi- 

 tality ; and that if the hair be cut, the then superfluous amount of 

 nourishment develops the hair till it again attains to the previous 

 length. This is equivalent to saying that the hair-sacs have, each, 

 the power to develop a hair of a determinate length, it being also 

 the function of each to maintain this length; so that if the hair is 

 cut, it again attains to it. Kolliker states that cut hairs do not pro- 

 duce new points, while others have asserted the contrary. When- 

 ever they become pointed again after being cut, as is quite certain 

 in many cases, it is doubtless from mere mechanical causes already 

 suggested, and not from developmental agencies. The growth 

 takes place in the hair-sac, and at the root; the shaft being thus 

 constantly protruded, till the hair attains its normal length. 



Though the hairs are not vascular they are not a dead substance. 

 Fluids are, doubtless, effused through them which serve for the 

 maintenance of their vitality, ascending from the bulb through the 

 fibrous portion and the medulla to every part, probably by mere 

 imbibition. After accomplishing their object, they pass off by eva- 

 poration, and another supply is afforded. From without, the hair 

 can absorb fluids only in the form of vapor. The oily matter of 

 the sebaceous follicles is spread upon the cuticle of the hair, but 

 does not, probably, penetrate it at all; nor is there any greasy fluid 

 afforded within by the medullary cells. 



The existence of air- vesicles in the medullary axis, can arise only 

 from a diminished supply of the fluids from the sac, compared with 

 the amount evaporated. It is thus due to a partial drying up of 

 the hair. The fibrous portion appears to be the most actively nou- 

 rished, and is the most rich in fluids, though comparatively so hard. 

 Gray hairs contain more of the air-vesicles, and to them its silvery 

 appearance is due. That their vitality is not, however, essentially 

 diminished, is proved from the fact that they grow rapidly when 

 cut. 



Thus the hairs live, and must, of course, be modified in their de- 

 velopment and growth by the vital conditions of the skin. The 

 condition of the hair is, therefore, an index of that of the skin. If 

 they are soft and shining, it may be inferred that the skin is tur- 

 gescent and active; if dry and harsh, that it is in a collapsed and 

 inactive condition. 



Any essential modification, therefore, in the circulation of the 

 skin, and hence of the blood supplying the hair-sacs, modifies the 



