THE HAIR. 265 



And since it is certain that in full health, the hairs are constantly 

 falling out, doubtless the formation of new ones, as just described, 

 is also simultaneously taking place. Hence not unfrequently, two 

 hairs are seen coming out of the same aperture, as before stated 

 the old and the new one. When a hair is pulled out, however, it 

 breaks off just above the bulb, and another is produced from the 

 latter directly, as at first. Heusinger found that on pulling out the 

 whiskers of dogs, they were reproduced in a few days from the old 

 sacs. And when the hairs fall out after sickness, it is probable that 

 they are reproduced from the old sacs, since the latter remain for 

 a long time. (E. H. Weber) 



Uses and Physiological Relations of the Hair. 

 The uses of the hair are various, depending partly upon its phy- 

 sical and partly on other characteristics. 



1. The hair is for protection, whether against cold, or other 

 agents, as on the head, &c. ; against exposure to light, as in case of 

 the eyebrows. 



2. It is for concealment, as on the pubes, &c. 



3. It prevents the ingress of foreign bodies into the passages 

 opening externally; as the vibrissae of the nostrils, the cilia of the 

 eyelids, and the hairs sometimes existing in the external meatus 

 auditorius. 



4. The hair gives character and expression, as the beard ; which 

 is also protective against changes of temperature in circumstances 

 requiring its agency in this respect. A tendency to affections of 

 the bronchial mucous membrane is, therefore, frequently removed 

 by allowing the beard to grow. 



5. The hair is for ornament, as that of the head. 



The hairs, like the nails, grow again if cut or worn away ; other- 

 wise they remain at their typical length in the various parts of the 

 body. Berthold found the hairs of the heads of females from 16 to 

 24 years old, grew about 7 lines a month. If the beard were shaved 

 every 12 hours, it would grow to from 5 J to 12 inches, and if every 

 24 hours, to from 5 to 7J inches, per annum. Shaving once in 36 

 hours would reduce the annual growth to from 4 to 6J inches. 

 The beard grows I faster by day than during the night; and in 

 18 days, about ^ more in summer than in winter. Kolliker 

 supposes that each sac is supplied by the vessels of the papilla 

 with a sufficiency of nourishment to develop the hair to its typical 



