150 INTRODUCTION. 



pith : it would seem, besides, that the hair is not extra-vascular, as 

 has been supposed by many, although, undoubtedly, its growth is the 

 result of additions to its root, by the secreting power of the bulb. 

 The principle of vitality in the hair, may, indeed, be at a low ratio ; 

 nevertheless, in that singular disease, the plica polonica, the hair not 

 only enlarges in bulk, becoming swollen, and morbidly sensitive, but 

 it often admits the passage of the red arterial blood, and will bleed 

 when divided by the scissors ; a fact, in itself, sufficient to prove its 

 vascularity.* With regard to the difference between hair and wool, it 

 may be observed, that, in chemical composition, mode of growth, and 

 organic texture, they are the same ; but that they differ from each other 

 in certain points, till lately but little understood. 



Hair is of two kinds, as respects its growth : one sort grows continually, 

 or, having attained its full extent, is not periodically shed ; if cut, it continues 

 to grow, like the incisor teeth of rodents, thereby demonstrating a persistent 

 activity of pulp the mane of the Horse is an example. The second 

 sort, of which the fur of animals in general consists, grows to its full 

 extent, and, whether cut or not, is shed and renewed periodically. 



The following illustration (fig. 149) will serve to shew the mode in 

 149 which hairs and bristles are developed 



and pass through a membrane-line canal, 

 which traverses the corium. 



The bulb, or root, consists of an in- 

 ner vascular pulp, , invested with a 

 capsule, b. From the latter of these is 

 produced the outer part, or rind, of the 

 shaft of the hair, c ; from the former, the 

 inner, or central part, d. The whole bulb 

 is enclosed in a membranous cell, e, 

 to which it is not attached, but through 

 Growth of hairs. which the vessels and nerves, f, pass 



to its pulp, a. This membranous cell, which encloses the bulb, is pro- 

 longed, as the shaft of the hair grows, to the surface of the skin. That a 

 tubular structure of hair results from its mode of growth (the base of the 

 tube in the bulb being filled with pulp), seems to be very apparent ; but 

 the tube in the shaft of the hair is, perhaps, often obliterated, or filled with 

 a dry, spongy pith. When hair falls away, it is in consequence of the 

 cessation of vital energy in the bulb, its blood-vessels becoming obliterated, 

 and, consequently, the supply of nourishment being cut off. Hair, how- 

 ever, is renewed, as are feathers after the moulting of birds ; a new bulb 



* Grief, sudden terror, or long illness, turn the hair white ; and, as will be noticed more par- 

 ticularly in a future part of this work, its colour is affected by temperature. 



