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.
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.