The venous system of seals, with new ideas on the significance of the extradural intravertebral vein.

The structural features of the venous system of seals, including such specialties as a caval sphincter, a huge posterior caval vein and hepatic sinuses, venous plexuses and a huge extradural intravertebral vein, are described and functional aspects of these features in relation to diving habits are discussed. A number of old concepts are discarded and new ones are proposed, particularly with regard to the significance of the extradural intravertebral vein, which, contrary to common current belief, probably is of little importance during diving.