New vascular system in reptiles: anatomy and postural hemodynamics of the vertebral venous plexus in snakes.

Using corrosion casting, we demonstrate and describe a new vascular system--the vertebral venous plexus--in eight snake species representing three families. The plexus consists of a network of spinal veins coursing within and around the vertebral column and was previously documented only in mammals. The spinal veins of snakes originate anteriorly from the posterior cerebral veins and form a lozenge-shaped plexus that extends to the tip of the tail. Numerous anastomoses connect the plexus with the caval and portal veins along the length of the vertebral column. We also reveal a posture-induced differential flow between the plexus and the jugular veins in two snake species with arboreal proclivities. When these snakes are horizontal, the jugulars are observed fluoroscopically to be the primary route for cephalic drainage and the plexus is inactive. However, head-up tilting induces partial jugular collapse and shunting of cephalic efflux into the plexus. This postural discrepancy is caused by structural differences in the two venous systems. The compliant jugular veins are incapable of sustaining the negative intraluminal pressures induced by upright posture. The plexus, however, with the structural support of the surrounding bone, remains patent and provides a low-pressure route for venous return. Interactions with the cerebrospinal fluid both allow and enhance the role of the plexus, driving perfusion and compensating for a posture-induced drop in arterial pressure. The vertebral venous plexus is thus an important and overlooked element in the maintenance of cerebral blood supply in climbing snakes and other upright animals.