BLOOD SUPPLY OF THE SPINAL CORD The vessels supplying the cord are the anterior spinal artery and the two posterior spinal arteries which rise at the foramen magnum from the vertebral arteries and are reinforced by cervical, intercostal and lumbar arteries. The anterior spinal artery (a. spinalis anterior) descends along the entrance to the anterior median fissure (Fig. 137); it is formed by the union of two vessels, one from each vetebral. The posterior spinal artery (a. spinalis posterior) of either side, is in reality a pair of vessels which freely communicate, and are so placed as to embrace the posterior nerve roots. The larger vessel of the pair is anterior to the nerve roots, while the smaller is between them and the posterior median fissure (Fig. 137). The spinal arteries give origin to two sets of branches, namely, the fissural or centrifugal and the centripetal arteries. Both sets are end arteries and form rich longitudinal plexuses which overlap each other but do not anastomose.
The fissural or centrifugal arteries rise, first and chiefly, from the anterior spinal artery (Fig. 137). These enter the anterior median fissure and, running lateralward, supply the greater part of the gray matter. Second, a few centrifugal arteries rise from the posterior spinal arteries. Running into the posterior fissure, they are distributed to the posterior white columns, the posterior commissure and to the nucleus dorsalis (Clarki).
The centripetal arteries rise from both the anterior and posterior spinal arteries (Fig. 137). They enter the cord at right angles to the surface and supply the white matter and the peripheral parts of the gray substance, including the tips of the columnw. Those branches to the columnw accompany
Veins.—The veins that carry the blood from the interior of the cord, the venom spinales intern, are the fissural veins, which issue from the fissures, the root-veins, which accompany the ante rior and posterior root-fibers to the surface of the cord, and a small number of veins that issue from other parts of the surface of the spinal cord. All unite in forming the external spinal plexus (venm spinales externx) spread over the entire surface of the cord beneath the arachnoid membrane. According to Cunningham, the plexus includes six longitudinal veins— anterior and posterior median and, on either side, an antero lateral and a postero-lateral vein placed just behind the re spective nerve roots. In the upper cervical region, the plexus forms two or three small veins which empty into the vertebral or inferior cerebellar veins; elsewhere, by a branch along each spinal nerve, the plexus communicates with the internal vertebral plexus (plexus venosi vertebrales intern) outside the dura mater, and is drained into the vertebral, intercostal, lumbar and sacral veins. No valves are found in the spinal veins.
Lymphatics.—Perivascular spaces carry the lymph from the spinal cord. The perineural spaces carry a centripetal stream which empties into the lymph spaces of the cord and its mem branes (Orr and Rows: Brain, Vol. 36). There are no lymphatic . vessels in the cord.