SYSTEM.] ANATOMY 90; tar artery to tlio sides of the toes, and constitute the digital arteries. From the large arterial trunks in the leg many branches proceed, to carry blood to the different structure in the limb. Structure The wall of an artery consists of several coats. The of arteries, outermost is the tunica adventitia, composed of connective tissue ; immediately internal to this is the yellow elastic coat ; within this again the muscular coat, formed of invol untary muscular tissue, the contractile fibre-cells of which are for the most part arranged transversely to the long axis of the artery ; in the larger arteries the elastic coat is much thicker than the muscular, but in the smaller arteries the muscular coat is relatively strong ; the vaso-motor nerves terminate in the muscular coat. Internal to the muscular coat is the elastic fenestrated coat, formed of a smooth elastic membrane perforated by small apertures. Most internal of Fio. 91. Dingram of the structure of an artery. A, tunica adventitia; E, elastic co.it ; M, muscular coat; F, fenestrated coat; En, endothtiium continuous with the endothelial wall of C, the capillaries. all is a layer of endothelial cells, which form the free surface over which the blood flows. The arteries are not nourished by the blood which flows through them, but by minute vessels, vasa vasorum, distributed in their external, elastic, and muscular coats. The Capillaries. These are the minute tubes which con nect together the terminal branches of the arteries and the rootlets of the veins. They vary in diameter in different localities from y-gVo-^ to 3*W tn mc ^- They are arranged in more or less compact networks, which lie in the interstices between the tissues of the part or organ. The vascularity F;o. 92. Capillary Network in the Web of the Foot of the Frog (A. Thomson). of a tissue depends upon the relative proportion of the capillaries that it contains. Some tissues, as adult cartilage, the cornea, epithelium, and endothelium, are destitute of capillaries, i.e., are non-vascular. The capillary wall is very simple in structure : in the smallest capillaries it consists merely of a layer ui eadothelial cells, continuous with the endothelial lining of the arteries and veins ; in the larger capillaries a delicate tunica adventitia is superadded. The transition from a capillary to a small artery or a small vein is marked by the development of a muscular and an elastic coat in the wall of the blood-conveying tube. The Veins. The veins convey the blood from the peri- Vei phery back to the heart, and in their course incieasein size, by junction or anastomosis with each other. In most of the veins delicate valves are found, each of which consists of two semicircular segments, and a pouch-like dilatation of the wall of the vein is opposite each segment. When the blood flows along the veins, the valves lie against the wall of the vessel, but if pressure be applied to a vein so as to obstruct the onward flow of the circulation, then the blood passes into the pouch between the wall of the vein and the valve adjacent to the seat of pressure, when the valve closes so as to stop regurgitation. The valves are found especially in those veins where the circulation is likely to be interfered with either by the pressure of the muscles on the veins during their action, or by the pres sure of blood caused by gravity, and are usually seated at the points of confluence of veins. They are absent in the veins of the lungs, of the brain, and of several of the abdominal viscera. Some of the veins lie in the sub cutaneous fat, and are called superficial veins, others lie amidst the muscles, and form the deep veins. The deep veins accompany the arteries and are named after them ; the superficial veins do not accompany arteries ; frequent anastomoses take place between the superficial and deep veins. The veins are arranged primarily into two groups the Pulmonary veins and the Systemic veins. The distribution of the pulmonary veins will be given in the anatomy of the lungs. The Systemic veins consist of the coronary venous system ; of the system of the superior vena cava ; of the system of the inferior vena cava ; and associated with the inferior vena cava is the portal venous system. The arrangement of the coronary vein has been described in the anatomy of the heart. The system of the Superior Vena Cava consists of both superficial and deep veins, and is arranged as follows : The superficial veins of the hand commence at the tips and sides of the fingers, from which they proceed along tho back of the hand, beneath the skin of which they may be distinctly seen. They then ascend along the fore-arm, form ing three large veins : the radial, on the outer side ; tho ulnar, on the inner; and the median, in the middle of the front of the fore-arm. At the bend of the elbow the median divides into two branches, of which one joins the radial to form the cephalic, the other joins the ulnar to form the basilic. Into one or other of the two branches of the median the surgeon generally makes an opening when he is desirous of drawing blood from the patient. The cephalic and basilic veins terminate by joining the deep or axillary vein. The communications between the superficial and deep veins are not, however, confined to the point of ter mination of the former, but occur at various parts of their course. The deep veins of the hand commence at the tips of the fingers, and pass as digital veins up the sides of the fingers to the palm of the hand, where they form an arch cor responding to the arterial arch of the palm ; from this they extend upwards along the front of the fore -arm, as far as the bend of the elbow, closely accompanying the arteries of the fore-arm, and receiving from the muscles numerous small branches corresponding to the small arteries sent to those muscles. At the bend of the elbow two brackial veins result from the junction of these different veins of the fore-arm, which pass up the inner side of the upper-arm, closely accompanying the brachial artery as far as the armpit, where they join to form a single large vein, the
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