|Henry Gray (18251861). Anatomy of the Human Body. 1918.|
|seen on the free surface of the epidermis. Each ridge contains two rows of papillæ, between which the ducts of the sudoriferous glands pass outward to open on the summit of the ridge. Each papilla consists of very small and closely interlacing bundles of finely fibrillated tissue, with a few elastic fibers; within this tissue is a capillary loop, and in some papillæ, especially in the palms of the hands and the fingers, there are tactile corpuscles.|
Development.The epidermis and its appendages, consisting of the hairs, nails, sebaceous and sweat glands, are developed from the ectoderm, while the corium or true skin is of mesodermal origin. About the fifth week the epidermis consists of two layers of cells, the deeper one corresponding to the rete mucosum. The subcutaneous fat appears about the fourth month, and the papillæ of the true skin about the sixth. A considerable desquamation of epidermis takes place during fetal life, and this desquamated epidermis, mixed with sebaceous secretion, constitutes the vernix caseosa, with which the skin is smeared during the last three months of fetal life. The nails are formed at the third month, and begin to project from the epidermis about the sixth. The hairs appear between the third and fourth months in the form of solid downgrowths of the deeper layer of the epidermis, the growing extremities of which become inverted by papillary projections from the corium. The central cells of the solid downgrowths undergo alteration to form the hair, while the peripheral cells are retained to form the lining cells of the hair-follicle. About the fifth month the fetal hairs (lanugo) appear, first on the head and then on the other parts; they drop off after birth, and give place to the permanent hairs. The cellular structures of the sudoriferous and sebaceous glands are formed from the ectoderm, while the connective tissue and bloodvessels are derived from the mesoderm. All the sweat-glands are fully formed at birth; they begin to develop as early as the fourth month.
| The arteries supplying the skin form a net-work in the subcutaneous tissue, and from this net-work branches are given off to supply the sudoriferous glands, the hair follicles, and the fat. Other branches unite in a plexus immediately beneath the corium; from this plexus, fine capillary vessels pass into the papillæ, forming, in the smaller ones, a single capillary loop, but in the larger, a more or less convoluted vessel. The lymphatic vessels of the skin form two net-works, superficial and deep, which communicate with each other and with those of the subcutaneous tissue by oblique branches.|
| The nerves of the skin terminate partly in the epidermis and partly in the corium; their different modes of ending are described on pages 1059 to 1061.|
The Appendages of the SkinThe appendages of the skin are the nails, the hairs, and the sudoriferous and sebaceous glands with their ducts.
| The Nails (ungues) (Fig. 943) are flattened, elastic structures of a horny texture, placed upon the dorsal surfaces of the terminal phalanges of the fingers and toes. Each nail is convex on its outer surface, concave within, and is implanted by a portion, called the root, into a groove in the skin; the exposed portion is called the body, and the distal extremity the free edge. The nail is firmly adherent to the corium, being accurately moulded upon its surface; the part beneath the body and root of the nail is called the nail matrix, because from it the nail is produced. Under the greater part of the body of the nail, the matrix is thick, and raised into a series of longitudinal ridges which are very vascular, and the color is seen through the transparent tissue. Near the root of the nail, the papillæ are smaller, less vascular, and have no regular arrangement, and here the tissue of the nail is not firmly adherent to the connective-tissue stratum but only in contact with it; hence this portion is of a whiter color, and is called the lunula on account of its shape.|
| The cuticle as it passes forward on the dorsal surface of the finger or toe is attached to the surface of the nail a little in advance of its root; at the extremity of