250 



OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



degree with the common 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 occasionally 

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

 tion ; and this not merely in the skin, but in the eye : such persons, termed 

 Albinoes, are met with alike among the fair, and among the dark races. The 

 absence of color usually shows itself also in their hair, which is almost white. 



244. 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 ex- 

 amined, they are found to consist of nucleated cells (Fig. 40, B), resembling 



Fig. 40. 



Fig. 41. 



Oblique section through the Matrix of the Nail: A, Section of the skin on the end of the 



Cutis of the bed of the nail ; B, mucous layer of the finger : The cuticle, and nail, n, detached 



nail;-c, horny layer of the same, or true nail-substance; from the cutis and matrix, m. 



a, papillae of the nail-matrix; 6, cells of the Malpighian 

 gtratum of the nail ; c, ridges of the true nail-substance ; 

 d, deepest layer of perpendicular cells of the mucous por- 

 tion of the nail ; e, upper layer of flattened cells of the 

 same ; /, nuclei of the true nail-substance. 



those of the newer layers of Epidermis ; but in the more superficial laminae (c) 

 no distinct structure can be distinguished without the assistance of reagents. 

 When, however, a thin slice of the nail is immersed for some little time in a 

 dilute solution of caustic potash or soda, its tissue swells up, and its component 

 cells, though previously flattened and compacted together, reassume their sphe- 

 roidal form, and display themselves in the most beautiful manner (as was first 

 pointed out by Donders); their nuclei, however, are no longer distinguishable 

 in the most superficial layers. The Nail is produced from the surface of the 

 Corium that lies beneath it, which is folded into a groove at its root (Fig. 41); 

 this surface is highly vascular, and is furnished with longitudinal elevated ridges 

 (Fig. 40, a, A), to which bloodvessels are copiously distributed, and between 

 which the soft inner layer of the nail dips down (6), like the Malpighian layer 

 of the cuticle between the sensory papillae. The increase of the Nail in length 

 is effected by successive additions to its root, causing the whole nail to shift 

 onwards; but as it moves, it receives additional layers from the subjacent skin, 

 which increase its thickness. According to the observations of M. Beau, the 



