140 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



[Fig. 34. 



mon people, almost always present a lighter hue ; and in some of the islands 

 of the Polynesian Archipelago, bordering on the Equator, they are not darker 

 than the inhabitants of Southern Europe. An occasional development of dark 

 pigment-cells takes place during pregnancy, in some females of the fair races; 

 thus it is very common to meet with an extremely dark and broad areola 

 round the nipple of pregnant women ; and sometimes large patches of the 

 cutaneous surface, on the lower part of the body especially, become almost 

 as dark as the skin of a Negro. On the other hand, individuals are occasion- 

 ally seen with an entire deficiency of pigment-cells, or at least of their proper 

 secretion ; and this not merely in the skin, but in the eye ; such are termed 

 Albinoes ; and they are met with alike among the fair, and among the dark 

 races. The absence of colour usually shows itself also in the hair ; which 

 is almost white. 



165. The Nails, like Hoof, Horn, &c., may be regarded as nothing more 

 than an altered form of Epidermis. When their newest and softest portions 

 are examined, they are found to consist of nucleated particles, resembling 

 those of the newer layers of Epidermis ; in the more superficial lamina?, 

 however, no distinct structure can be made out; but, when treated with acetic 

 acid, some traces of nuclei may be detected in them. The Nail is produced 

 from the surface of the true skin that lies beneath it, which is folded into a 



groove at its root; this surface is highly vascular. 

 The increase in length is effected by successive 

 additions at the root, causing the whole nail to 

 shift onwards ; but as it moves, it receives ad- 

 ditional layers from the subjacent skin, which 

 increases its thickness. The nail is continuous 

 with the true Epidermis at every part, except 

 its free projecting edge ; and in the fetus, the 

 continuity is maintained there also. 



166. The Hair, as originally consisting of 

 Epidermic cells, may be properly described 

 here ; although, when fully formed, it departs 

 widely (in Man at least) from the cellular type. 

 It has been imagined until recently, that the 

 Hair, in common with the other Epidermic tis- 

 sues, is a mere product of secretion ; its mate- 

 rial, which is chiefly horny matter of the same 



composition with that of the Epidermis and its appendages, being elaborated 

 from the surface of the pulp at its base. It is not known, however, to con- 

 tain a distinctly organized structure ; and to be formed by the conversion of 

 a cellular mass at its root. The Hair originates within a follicle, which is 

 formed by a little depression of the Skin, and which is lined by a continua- 

 tion of the Epidermis. From the bottom of this follicle, there rises up a 

 cluster of cells, which may be regarded as an increased development of Epi- 

 dermic cells ; the exterior of this cluster, which is the densest part, is known 

 as the butt) ; whilst the softer interior is termed the pnlp. The follicle itself 

 is extremely vascular ; and even the bulb is reddened by minute injection, 

 though no distinct vessels can be traced into it. Although the Hairs of differ- 

 ent animals vary considerably in the appearances they present, we may gene- 

 rally distinguish in them two elementary parts ; a cortical or investing sub- 

 stance, of a fibrous horny texture ; and a medullary or pith-like substance, 

 occupying the interior. The fullest development of both substances is to be 

 found in the spiny Hairs of the Hedgehog, and in the quills of the Porcu- 

 pine ; which are but hairs on a magnified scale. The cortical substance 

 forms a dense horny tube, to which the firmness of the structure seems chiefly 



Section of ihe skin on the end of 

 the finger: The cuticle, and nail, n, 

 detached fromlhe cutis and matrix, m.} 



