The ridges and furrows serve, like the papillæ in other parts of the skin, to increase the surface of the cutis; and, by affording more space for the distribution of the vessels and nerves, they contribute to the sensitiveness of the part, and account for the severe pain which is caused when any foreign body is thrust under the nail. The pulp in the interior of a tooth, and the frog of a horse’s foot, are also instances in which an exquisitely sensitive structure is placed beneath a hard or horny substance. The object, in each case, is the same, viz. to give the power of taking cognizance of impressions which are made upon the surface.
Hairs.
Hairs may also be regarded as modifications of the cuticle, because, like the nails, they are continuous with the cuticle, and are formed from the rete mucosum. Each hair (figs. 79 and [83]) is received into a depression of the cutis, which is called a “follicle,” and which is lined, as far as the bottom, by cuticle (a), and rete mucosum (b). At the bottom of the follicle (d) the cuticle is absent, and the hair rests, directly, upon the rete; and, at this part, the rete, instead of being converted into cuticle, as it is at the sides of the follicle, becomes transformed into hair, in the following way.
The cells of which the rete is composed swell out as they ascend, and so form the soft “bulb” of the hair. The outermost cells are gradually flattened, and assume an imbricated arrangement, overlapping one another like the tiles upon a roof (fig. 79, e, and fig. 80); and those in the interior are elongated, so as to be converted into more or less distinct fibres. The cells nearest the middle, or axis, of the hair remain moister and softer than those nearer the exterior, and form what is sometimes called the “marrow” of the hair (figs. 81 and 82).
The colour of the hair is given by the presence of minute grains of colouring matter, like those in the cuticle of the Negro. They are formed in the cells at the root of the hair, and pass up with them into its structure. The quantity of colouring matter is usually slight in infancy and childhood, and increases during adolescence. Hence the hair becomes darker as we grow up. It is more or less deficient in the grey hair of old age; and in the instance of Marie Antoinette, and others whose hair is said to have turned grey in a few hours, the colouring matter is supposed to have been destroyed by some fluid, formed from the blood, and passing, through the pulp, into the hair.
The hairs serve to protect the skin; and, as a general rule, they are most abundant upon the parts which are most exposed, and which, therefore, stand most in need of such protection. They are scattered over the back of the hand. On the palmar surface they are not required, and they would have interfered with the sense of touch; and we do not, accordingly, find them there, nor upon the sole of the foot, nor upon the edges of the lips. In certain parts of some animals, however, they serve as valuable adjuncts to the tactile organs by extending the range within which the contact of surrounding substances is felt. Thus the whiskers of the cat are set upon papillæ so sensitive that the slightest touch upon any part of the hair is felt; and the animal is thereby assisted in threading its way in the dark. This provision, added to the mode in which their feet are muffled with soft hair and their claws are retracted, enables the members of the feline tribe to steal with almost absolute stillness upon their prey.
Oil-glands.
Fig. [83].
Hair, and Oil-glands.