196 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



crushing the hair, after it has been macerated for some time in dilute 

 acid. In dark hairs, the pigmentary granules are frequently scattered 

 between th fibres : but they are frequently found in greater abundance 

 in the central cells, where they form a dark spot in the middle of the 

 transverse section (c). Sometimes, however, no such collection is seen ; 

 and whatever pigmentary matter exists in the hair is equally diffused 

 through the whole of it, or, is even accumulated rather towards its exte- 

 rior (D). This colouring matter seems related to Hsematine ; it is 

 bleached by chlorine ; and when it gives a dark hue to the hair, it usually 

 contains a good deal of iron. 



330. The fibres of which the chief part of the shaft is made up are 

 probably cells, which have become elongated by the process already 

 noticed ( 193), and which have at the same time secreted horny matter 

 into their interior. This change is continually going on in the bulb of 

 the hair, at the base of the part previously completed ; and by the pro- 

 gressive formation of new cells in the bulb, a constant growth of the 

 shaft is provided for. The central medullary substance is rather 

 derived from the cells of the pulp, in which a continuous growth goes 

 on, at the same rate with that of the bulb. The imbricated layer of 

 cells which forms the true cortical substance may be said to be a pro- 

 longation of the ordinary Epidermis over the surface of the hair, being 

 developed from the external layer of the bulb, where it is continuous 

 with the epidermic lining of the follicle. Thus the Hair is constantly 

 undergoing elongation by the addition of new substance at its base ; 

 precisely in the same manner as the teeth of certain Mammals grow 

 from persistent pulps. The part once formed usually undergoes no 

 subsequent alteration ; but there is evidence that it may be affected by 

 changes at its base, the effect of which is propagated along its whole 

 extent. Thus, it is well known that cases are not unfrequent, in which, 

 under the influence of strong mental emotion, the whole of the hair has 

 been turned to gray, or even to a silvery white, in the course of a single 

 night ; a change which can scarcely be accounted for in any other way 

 than by supposing that a fluid, capable of chemically affecting the 

 colour, is secreted at the base of the hair, and transmitted by imbibition 

 through the medullary substance to the opposite extremity. The know- 

 ledge of the organized structure of hair enables us better to understand 

 some of the effects of disease, and especially of that peculiar affection 

 termed Plica Polonica. The hair of individuals suffering from it is dis- 

 posed to split into fibres, often at a considerable distance from the 

 roots, and to exude a glutinous substance ; and these two causes unite 

 in occasioning that peculiar matting of the hair, which has given origin 

 to the name of the disease. In the hair thus affected, there is evidently 

 a power of transmitting fluid absorbed at the roots : and it is said that 

 even blood exudes from the stumps, when the hairs are cut off close to 

 the skin. 



6. Of Cells coalesced into Tubes with Secondary Deposit. 



331. Most of the tissues which have been hitherto described, differ in 

 no essential particulars from those of Plants ; the chief departure from 



