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 famil ies. The plexus consists of a network of spinal veins coursing within and a round the vertebral column and was previously documented only in mammals. T he 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 al ong the length of the vertebral column. We also reveal a posture-induced di fferential flow between the plexus and the jugular veins in two snake speci es with arboreal proclivities. When these snakes are horizontal, the jugula rs are observed fluoroscopically to be the primary route for cephalic drain age and the plexus is inactive. However, head-up tilting induces partial ju gular collapse and shunting of cephalic efflux into the plexus. This postur al discrepancy is caused by structural differences in the two venous system s. The compliant jugular veins are incapable of sustaining the negative int raluminal pressures induced by upright posture. The plexus, however, with t he 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 vertebra l venous plexus is thus an important and overlooked element in the maintena nce of cerebral blood supply in climbing snakes and other upright animals. (C) 2001 Wiley-Liss, Inc.