Fig. 430.—Vertical section of skin and subcutaneous tissues, showing the sweat-glands and fat-globules, ducts passing upwards to the epidermis or external cuticle. Magnified 250 diameters.
The Dermis, or true skin, consists of an interlacing network of connective tissue, yellow elastic tissue corpuscles, vessels, and nerves. There are also small muscular fibres in connection with the hair follicles, and beneath the subcutaneous tissues contain an abundant supply of fat adipose tissue. Numerous ridges are seen on the surface, especially on the palm of the hand and sole of the foot, caused by rows of little elevations of the cutis vera, termed papillæ. These are more or less conical, and contain a capillary loop, nerve, and touch corpuscle, which serve to increase the sensitiveness of the part, lodging a touch corpuscle in a favourable position for receiving sensations of touch, [Fig. 430].
Sweat glands are situated in the subcutaneous tissue, and consist of fine tubes, which form the duct (seen in the section, [Fig. 430]); these are continuous with a blind extremity, coiled up into a ball one-sixtieth of an inch in diameter, and surrounded by a plexus of capillaries to form the gland ([Fig. 431], No. 2). Between the layer of columnar cells and the limiting membrane is a layer of non-striated muscle, and beneath the rite mucosum there are several layers of polyhedral cells, and an external and internal limiting membrane; the epithelium of the duct is at its mouth continuous with the epithelium of the epidermis.
Fig. 431.
1. Blood vessels of papillæ supplied to cutis; 2. Perpendicular section through the scalp, with two hair-sacs; a. epidermis; b. cutis; c. muscles of the hair follicles.
Nails consist of a root and body, the lunular of which is the whitish portion of the body near the root, where the skin beneath is less vascular than any other portion of the finger. The nail closely resembles the epidermis, and consists of hard and thin layers of cells on the surface, and round, moist cells beneath. Posteriorly the nail fits into a groove which lodges its root. The part to which the nail is attached is known as the nail-bed. The stratified appearance produced by the coalescence of the cells, and their lying over each other, is shown in [Plate VII]., No. 149, the toe of the mouse; while the special arrangement of tissue is better seen under polarised light ([Plate VIII]., No. 174).
Hairs consist of a shaft and root. The shaft is cylindrical, and covered with a layer of imbricated scales, arranged with their edges upwards. The substance of the hair consists of fibres, or elongated fusiform cells, in which nuclei are seen. There are present in some hairs ([Fig. 432]) small air spaces or lacunæ. In the coarser hair of the body there is a pith (medulla), occupied by small angular cells and fat granules.
Fig. 432.