270. The middle or the fibrous tunic is composed of yellowish flattened fibres which pass in an oblique direction around the calibre of the vessel, forming segments of circles, which, uniting, produce complete rings (fig. CXVIII. 2). This tunic is thick, consisting of several layers of fibres which it is easy to peel off in succession. They form a firm, solid, elastic, but, at the same time, brittle membrane. 271. The inner tunic, thin, colourless, nearly transparent, and perfectly smooth, is moistened by a serous fluid, and is thence called the serous coat (fig. CXVIII. 1). To the naked eye it presents no appearance of fibres, yet notwithstanding its extreme delicacy, it is so strong that, after the other coats of the artery have been entirely removed in a living animal, it is capable of resisting the impetus of the circulation, and of preventing the dilatation of the artery. The arteries themselves are supplied with arteries, vessels that nourish their tissues, and which are sent to them from neighbouring branches, seldom or never from the vessel itself to which they are distributed. Each individual part of an artery is supplied by its own appropriate vessels, which form but few communications above and below, so that if care be not taken in surgical operations to disturb these nutrient arteries very little, the vessel will perish for want of sustenance. 272. The vein, the tube that carries back the blood to the heart, is composed of the same number of tunics as the artery, which, with the exception of the middle, are essentially the same in structure, but they are all much thinner. The external tunic consists of a less dense and strong cellular membrane; the middle tunic, instead of being formed of elastic rings, is composed of soft and yielding fibres, disposed in a longitudinal direction; while the inner coat, which is still more delicate than that of the artery, is arranged in a peculiar manner. The inner coat of most veins, at slight intervals, is formed into folds (fig. CXX. 5), one margin of which is firmly adherent to the circumference of the vessel, while the other margin is free and turned in the direction of the heart. These membranous folds are termed valves. In all veins the diameter of which is less than a line the valves are single; in most veins of greater magnitude they are placed in pairs, while in some of the larger trunks they are triple, and in a few instances quadruple, and even quintuple. The veins, like the arteries, are supplied with nutrient vessels and nerves.