254 



OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



Fig. 44. 



thus, it is well known that cases are not unfrequent, in which, under the influ- 

 ence 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 % supposing that a fluid, capa- 

 ble of chemically affecting the color, is secreted at the base of the hair, and 

 transmitted by imbibition through the medullary substance, to the opposite ex- 

 tremity. Another evidence of their retention of a degree of vitality, is found in 

 the fact of Hairs having a tendency to become pointed, after having been cut 

 short off. In the hairs of some animals (particularly the whiskers of the Seal 

 and other Carnivora) the base is hollow, and contains a large papilla, or eleva- 

 tion of the cutis, furnished with nerves and bloodvessels : this is separated, by a 

 layer of basement membrane, from the proper tissue of the Hair. In such cases 

 there is bleeding from the stumps of the hairs, when they are shaved off close 

 to the skin. We have seen that there is an approach to this papillary structure 

 in man j and it may perhaps be an abnormal development of it, which occasions 

 the hair to bleed in the disease termed Plica Polonica. The hair of individuals 

 affected with this, is further disposed to split into fibres, often at a considerable 

 distance from the roots, and to exude a glutinous substance ; these two causes 

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

 the name of the disease. 



247. The history of the embryonic development of the Hair has recently 



been made the subject of careful study 

 by Prof. Kblliker j and the following is 

 the substance of his account of it. The 

 hair-rudiments may be said to be com- 

 posed of little processes of the Malpig- 

 hian layer of the epidermis, which are 

 received into corresponding depressions 

 in the corium (Fig. 44, A, I, I); these 

 are soon perceived to be inclosed in a 

 limiting membrane (B, t), which sepa- 

 rates the contained cells (m, m) from 

 the interior of the follicle, just as the 

 basement membrane of the Skin with 

 which it is continuous, separates the 

 Malpighian layer of the Epidermis from 

 the corium. The hair-matrix now 

 lengthens and swells out at the bottom, 

 so as to assume a flask shape. Cells 

 are deposited outside the limitary mem- 

 brane, which are eventually converted 

 into, or give place to, fibres ; and thus 

 the dermic coats of the follicle are pro- 

 duced. But whilst this is going on out- 

 side, the cells within the follicle under- 

 go changes. Those in the middle 

 lengthen out conformably with the axis 

 of the follicle, and constitute a short 

 conical miniature hair, faintly distin- 

 guishable by difference of shade from 

 the surrounding mass of cells, which 

 are also slightly elongated, but trans- 

 versely with regard to the follicle (Fig. 

 45, A.). The papilla (B, h) makes its 

 appearance at the swollen root of the 



A. Development of the Hair-bulbs in the Epider- 

 mis of the forehead, in a human foetus of sixteen 

 weeks, as seen from the under side : B, a single 

 hair-matrix more enlarged, as seen laterally: a, 

 horny layer of the epidermis ; b, mucous layer of 

 the same ; i, structureless membrane surrounding 

 the hair-matrix, prolonging itself from betwixt the 

 mucous layer and the corium; m, rounded with 

 some elongated cells, forming the matrix of the 

 hair. 



