420 INNERVATION. [CHAP. XIV 



in the quills of the porcupine. An approach to this papillary pro- 

 jection may be frequently seen in the hairs of man ; but its real size 

 appears to have been much overrated, from the basement membrane 

 having been overlooked. Where a papilla exists, the basement mem- 

 brane is of course continued over it, and separates it from the true 

 hair, which is never penetrated by either vessels or nerves. 



The sebaceous glands of the skin very generally open into the 

 hair follicles at a short distance from the surface. 



The hair follicle is fixed more or less firmly in its place, according 

 to the size and stiffness of the hair, by the dermoid and subdermoid 

 tissues uniting intimately with it on its deep or convex surface, where 

 also are spread out the capillary vessels which furnish the materials 

 of growth. These latter are adapted in number to the dimensions 

 of the follicle. 



Thus the hairs, like the cuticle, are beautifully organized, and 

 maintain a vital, though not a vascular, connexion with the body. 

 Some evidence of their retaining a degree of vitality is found in 

 the fact, first pointed out by Mandl, and verified in some instances 

 by ourselves, that hairs have a tendency to become pointed after 

 having been cut short off. The process is very slow, and seems to 

 consist in a further condensation and elongation of the elementary 

 cells at the new extremity. 



Well-authenticated instances have occurred, in which the hair has 

 grown white in a single night, from the sudden influence of some 

 depressing passion ; and some have held this circumstance a proof 

 that fluids circulate through them. It seems most probable that 

 this phenomenon results from the secretion, at the bulb, of some 

 fluid perhaps an acid, as Vauquelin supposes which percolates 

 the tissue of the hair, and chemically destroys the colouring mat- 

 ter. The ordinary gray hairs of age resemble other hairs in every 

 respect but colour, and the process of change from dark to gray 

 seems to take place rapidly in each individual hair. 



According to Vauquelin, the colour of hair depends on the" pre- 

 sence of a peculiar oil, which is of a sepia tint in dark hair, blood- 

 red in red hair, and yellowish in fair hair. When extracted, as it 

 may be by alcohol or sether, the hair is left of a grayish yellow. 

 The colour is destroyed by chlorine, and probably otherwise resem- 

 bles closely that of the cuticle in the dark races. The substance of 

 hair is similar in chemical composition to that of horn. After being 

 softened by maceration in cold nitric acid, it is soluble in boiling 

 water, and the solution after evaporation becomes a gelatinous 

 mass on cooling. The horny matter is said to be distinguishable 



