The vertebral and azygos venous systems, and some variations in systemic venous return.

Abstract

The venous system of man, in accord with conventional anatomic description, is divided into two main groups: the pulmonary veins and the systemic veins. With the exception of the cardiac veins, the systemic venous drainage returns to the heart through two great vessels, the superior and the inferior vena cava. From the cranial cavity and the upper extremities the blood converges on the superior vena cava, and from the abdominal viscera and lower extremities on the inferior vena cava. Obstruction of either of these trunks is not incompatible with life in man (5, 36, 39). The development of a large by-pass mechanism, effectively establishing communication between the two vast caval drainage areas, is accomplished by collateral vessels, of which the vertebral veins and the azygos system form an important component (34, 37, 39, 40). In the dog, both venae cavae may be occluded for thirty minutes, with survival, if the azygos vein is patent (3). This is accompanied by a fivefold increase in azygos blood flow. How this is possible becomes apparent after consideration of the intercommunications of the vertebral veins (4, 37). At the base of the brain, they anastomose extensively with the venous trunks of the cranium; in the neck, with the deep cervical veins; in the thorax and abdomen, with the intercostal and lumbar veins; in the pelvis, with the large venous plexuses anterior to the sacrum. In turn, the sacral and lumbar veins communicate directly with the inferior vena caval system, the lumbar and intercostal veins with the azygos system, and the azygos system with the superior vena cava and its branches (26). In the early 1940's, Batson revived interest in the vertebral system of veins by demonstrating that a thin opaque medium injected into the dorsal vein of the penis of a cadaver would spread into the sacral canal, fill the veins in the wings of the bony pelvis, and finally move up the vertebral system as far as the cranial cavity. The mode of spread was similar to that of carcinoma of the prostate, and Batson suggested that in the great venous lakes formed by these plexuses, tumor emboli might well spread from origin to final site of deposition. He also showed that the vertebral veins filled, following injection in a live monkey, if the inferior vena cava were compressed (6–8). Shortly after Batson described the anatomy of the vertebral veins and expounded his theory of their role in metastases, Harris went to great lengths to prove that Batson's ideas were neither new nor original (24). As a matter of fact, the vertebral venous system was by no means unknown to anatomists prior to Batson's time, although Franklin in his Monograph on Veins, written in 1937, did not even mention the vertebral plexuses (20). Willis in 1664 (48) and Winslow in 1732 (49) characterized the structure of the spinal veins. Bock in 1823 described the rich plexuses within the bony canal, the posterior venous plexus, and the azygos system (9).