The dark layer, marked 2, 7, 8, is that portion of the true skin which gives the external color marking diverse races. In the portion of the dark layer marked 3, 4, is seen a net-work of nerves which run from two branches of the nervous trunks coming from the spinal marrow. These are nerves of sensation, by which the sense of touch or feeling is performed. Fig. 56 represents the blood-vessels, (intermingled with the nerves of the skin,) which divide into minute capillaries, that act like the capillaries of the lungs, taking oxygen from the air, and giving out carbonic acid. At _a_ and _b_ are seen the roots of two hairs, which abound in certain parts of the skin, and are nourished by the blood of the capillaries. [Illustration: Fig. 56.] [Illustration: Fig. 57.] At Fig. 57 is a magnified view of another set of vessels, called the _lymphatics_ or _absorbents_. These are extremely minute vessels that interlace with the nerves and blood-vessels of the skin. Their office is to aid in collecting the useless, injurious, or decayed matter, and carry it to certain reservoirs, from which it passes into some of the large veins, to be thrown out through the lungs, bowels, kidneys, or skin. These _absorbent_ or _lymphatic vessels_ have mouths opening on the surface of the true skin, and, though covered by the cuticle, they can absorb both liquids and solids that are placed in close contact with the skin. In proof of this, one of the main trunks of the lymphatics in the hand can be cut off from all communication with other portions, and tied up; and if the hand is immersed in milk a given time, it will be found that the milk has been absorbed through the cuticle and fills the lymphatics. In this way long-continued blisters on the skin will introduce the blistering matter into the blood through the absorbents, and then the kidneys will take it up from the blood passing through them to carry it out of the body, and thus become irritated and inflamed by it. There are also oil-tubes, imbedded in the skin, that draw off oil from the blood. This issues on the surface, and spreads over the cuticle to keep it soft and moist. But the most curious part of the skin is the system of innumerable minute perspiration-tubes. Fig. 58 is a drawing of one very greatly magnified. These tubes open on the cuticle, and the openings are called pores of the skin. They descend into the true skin, and there form a coil, as is seen in the drawing. These tubes are hollow, like a pipe-stem, and their inner surface consists of wonderfully minute capillaries filled with the impure venous blood. And in these small tubes the same process is going on as takes place when the carbonic acid and water of the blood are exhaled from the lungs. The capillaries of these tubes through the whole skin of the body are thus constantly exhaling the noxious and decayed particles of the body, just as the lungs pour them out through the mouth and nose. [Illustration: Fig. 58.]