622. The sensitive layer of the skin is thin, soft, uneven, pinkish in hue, and composed of blood-vessels, which confer its various tints of red; and of nerves, which give it the faculty of sensation. The unevenness of this layer is produced by small, elongated, conical prominences, called _Pa-pil´læ_. 623. Each PAPILLA is composed of a minute artery, vein, and nerve. Some of the prominences are arranged in concentric ovals, as may be seen on the ends of the fingers; others are more or less parallel, and pursue a serpentine course; some suddenly diverge, and again reunite, as may be seen in the palm of the hand. Papillæ are found in every part of the skin. Consequently, their number is very great. -=-=-=-=-=-=-=-=-=-=-=-= How is leather formed? 622. What is the appearance of the sensitive layer? What causes the unevenness of this layer? Explain fig. 111. 623. Describe the papillæ. -=-=-=-=-=-=-=-=-=-=-=-= 624. The cutis vera contains not only _Arteries_, _Veins_, and _Nerves_ but _Lymphatics_, _Oil-Glands_ and _Tubes_, and _Perspiratory Glands_ and _Tubes_. [Illustration: Fig. 112. The arteries and veins of a section of the skin. A, A, Arterial branches. B, B, Capillary, or hair-like vessels, in which the large branches terminate. C, The venous trunk, collecting the blood from the capillaries.] 625. The ARTERIES AND VEINS of the skin are very numerous. The larger branches of the arteries pass through the open meshes of the true skin, and are subdivided into a myriad of minute capillary vessels, which form a beautiful net-work on the upper surface of the true skin. This vascular net sends a branch to each of the papillæ, which opens into and terminates in a minute vein. The capillary veins are as numerous as the arteries which they accompany. They unite and form larger trunks, as small springs from the hill side coalesce to form rivulets. -=-=-=-=-=-=-=-=-=-=-=-= 624. What vessels are found in the cutis vera? Explain fig. 112. 625. What is said of the cutaneous arteries? Of the cutaneous veins? -=-=-=-=-=-=-=-=-=-=-=-= 626. The NERVES that are spread over every part of the sensitive layer of the true skin, proceed from the spinal cord. As a proof of the great number of nervous filaments in the skin, no part of this tissue can be punctured with a fine needle without transfixing a nerve, and inducing pain. In some parts of the system, however, the nerves are more abundant than in others; where the sense of feeling is most acute, we find the greatest number of nerves, and those of the largest size. Those parts that are most exposed to injury are most sensitive. _Examples._ 1st. The conjunctiva, or skin of the eye, is pained by the presence of a particle of dust, because it would render vision imperfect.