The combined area of all the capillaries of the body is many times greater than the combined area of the trunk vessels. If this were not so, the high pressure on the arterial system would break the thin capillary walls and also the greater area allows the blood to circulate more slowly which gives time for the liberation of oxygen to the tissues and for the absorption of carbon dioxide. =The Venous System.=—The veins, like the arteries, are tubular vessels, their function being to receive the blood from the capillaries and convey it to the auricles of the heart. There are two classes of veins, =systemic= and =pulmonic=. The systemic veins receive the impure or carbonized blood from the capillaries and convey it to the right auricle of the heart. The pulmonic veins receive the pure oxygenized blood from the lungs and convey it to the left auricle of the heart. The pulmonic veins will be taken up and discussed later under the pulmonary circulation. _Systemic veins_ are divided into superficial and deep veins and sinuses. _The superficial veins_ are found between the layers of the superficial fascia, just beneath the skin, and communicate with the deep veins by branches which pierce the fascia. _The deep veins_ are found deeper down, between the muscles, and are surrounded by the deep fascia. The smaller arteries, such as the radial, brachial, posterior and anterior tibial, and the peroneal arteries, are each accompanied by two veins, one on each side of the artery, which are called venae comites (accompanying veins). The larger arteries, such as the common carotid, the femoral and the iliac, are accompanied by only one vein. [Illustration: FIG. 36—Superficial veins of the head and neck. (Gray)] Veins arise from the capillaries, or, rather, from the minute capillary plexus, formed by a massing or blending of the tiny venules. These small vessels unite to form larger trunks, and as they continue toward the heart increase in size until they finally unite to form the ascending and descending venae cavae. _The Sinuses._—The cerebral veins are small vessels that arise from the capillaries of the brain, and terminate in the sinuses of the dura mater. There are many sinuses in the cranial cavity, and differ from the vein, in that the walls are thinner, having only two walls while the veins have three, and they do not have valves. The outer walls of the sinuses of the brain are formed by a division of the dura mater, while the inner wall is the continuation of the inner wall of the vein. [Illustration: FIG. 37—The internal long saphenous vein. (Gray)] They are of little interest to embalmers, except for the fact that when the brain is injected by any of the so-called needle processes, the fluid is quickly conveyed through these vessels to the tissues of the brain, and that organ is thoroughly preserved.