Now, in vessels larger than capillaries, outside of this layer of cells there are various coats of fibrous, elastic, or muscular tissue, according to the size of the vessel and the strength which it is necessary for it to possess.
The Structure of have as their innermost coat—the coat next to the channel of the vessel—a delicate membrane con sisting of cells precisely similar to the wall of the capillaries. Outside of this is a coat of fibrous tissue with an abundance of elastic fibres in it. Outside of this again is what is called the middle coat, consisting of elastic tissue and fibres of in voluntary muscle disposed circularly round the vessel. These coats are indicated in Fig. 138, which represents an ar tery dividing into two branches. a a point to the inner lining mem brane of cells, b b to the middle muscular coat, which has been acted on by acetic acid to show the nuclei of the cross muscular fibres ; and c c point to the outer coat. Fig. 137 c represents a small artery or vein in which the nuclei of mus cular fibres running both lengthwise and across are shown. The outer coat of an artery consists mainly of fibrous tissue, also with elastic fibres. In the large arteries the elastic tissue pre dominates, while in the small arteries the mus cular coat is more abundant. Thus the feature of the large arteries is their elasticity, while that of the small is their contractility. The importance of this is spoken of on p. 310.
The structure of veins is practically the same as that of arteries. They have the same coats, but they are much thinner and more soft. There is also this other difference, that the veins, with some exceptions, have valves, which are directed towards the heart and per mit the blood to flow in that direction, while preventing its flow in the opposite direction. In a dead animal an artery may be distinguished from a vein by the stoutness and firmness of its walls, while those of the vein are soft and yield ing. Also, on cutting through the artery, owing to the thickness of its walls, the tube will be seen to remain open, while the walls of the vein are collapsed and folded on one another. Arteries are in the dead animal generally found empty, the veins only containing blood. It was this observation that led old anatomists to give the name arteries (arteria, an air - vessel) to the strong open vessels, because, finding them always apparently empty, they thought that in life they contained the animal spirits.
The functions of the various vessels will be noted in discussing the details of the circu lation.
THE DISTRIBUTION OF THE BLOOD-VESSELS.
(Plate XVII.) The Arteries of the Head and Neck are among the earliest that spring from the main trunk, arising from the heart—the aorta. The aorta passes upwards a short way in the chest, as high as the level of the upper border of the second rib, and it then arches backwards to wards the back-bone, which, having reached, it turns downwards and passes down through the chest, lying close against the left side of the back-bone. At the arch three branches come off to supply the neck, head, and upper limbs. They are seen in Fig. 134 (p. 298). The first of the three (a in the figure) is called the inno minate artery (the unnamed artery). It passes towards the right and is very short, splitting into two branches when behind the junction of the collar-bone and breast-bone. One of the branches (t) is the subclavian artery, which arches across the lower part of the neck behind the collar-bone on its way to the right arm, giving off branches to the head, neck, and chest in its course. In Fig. 139 the number 12 points to a part of it that is least covered by muscles. The other branch (11 in Fig. 134) is the common carotid artery of the right side. It passes up the side of the neck, running alongside of the windpipe and larynx, to the level of the angle of the jaw. Here it divides into two. An ex ternal portion—the external carotid—passes up in front of the ear, where it ends in branches to the neck, face, and outer parts of the head. The other branch—internal carotid—passes deeply into the neck, and, through an opening in the skull behind the ear, enters the brain, supplying it and the eye with blood. Fig. 139 shows slightly the ramifications of these vessels over head and neck. It is to be noted that in the neck the carotid artery lies deeply under muscles so as to be well protected from injury, but that the external carotid conies more near the surface about the angle of the jaw. The arteries of the left side are similarly disposed, but their origin is slightly different. The com mon carotid artery and the subclavian artery of the left side arise, each separately and not by a common trunk, from the arch of the aorta, as shown in Fig. 134, p. 298.