1066 ORGANS OF THE SENSES AND THE COMMON INTEGUMENT 





seen on the free surface of the epidermis. Each ridge contains two rows of papillae, 

 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 papillae, 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 subcuta- 

 neous fat appears about the fourth month, and the papilla? 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 papillae, 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 sub- 

 cutaneous 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 SKIN. 



The 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 papillae a*re 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 



