• 1The venous system has been examined by dissection and by making Marco resin casts of the vascular system of young and adult specimens of Phoca vitulina, Halichoerus grypus, Hydrurga leptonyx, Mirounga leonina and Phocaena phocaena. Embryos and foetuses of the above species and of Leptonychotes weddelli, Lobodon carcinophagus, Zalophus californianus, Delphinus delphis, Globiocephala melaena, Balaenoptera physalus and Megaptera novaeangliae have been dissected, injected with coloured gelatin or serially sectioned.
  • 2A duplicated abdominal posterior vena cava, thin-walled and of large size, was found in all species examined. A well-defined sphincter supplied by the right phrenic nerve was found surrounding the posterior vena cava anterior to the diaphragm in all species of Pinnipedia; the sphincter generally consisted of circular bundles of striated muscle, separated from the diaphragm by a fibrous ring. In the Zalophus foetus there was a thin caval sphincter composed of two slips of diaphragmatic muscle. A sphincter-like “sling” of diaphragmatic muscle was found in Phocaena.
  • 3A large hepatic sinus was present in all the Pinnipedia examined except Zalophus, but was absent in the Cetacea.
  • 4A marked stellate plexus of veins on the surfaces of the kidney was present in all species of Pinnipedia. No renal vein emerges from the hilum and the stellate plexus drains by several large vessels into the corresponding limb of the posterior vena cava. The stellate plexus has numerous communications, particularly with an extradural intravertebral vein. A superficial plexus is present, though less marked, in Cetacea, and the main drainage is by one or more renal veins emerging from the mesial slit.
  • 5The extradural intravertebral venous system is well-developed in all specimens of Pinnipedia and is present in all adult and foetal Cetacea examined. In Phoca it is in the form of a large vein lying dorsal to the cord, which communicates posteriorly with the stellate renal plexus and the pelvic plexuses, and anteriorly with the intracranial venous sinuses. In Phocaena there is a large vein placed ventral to the cord, but its main communication is with the anterior vena cava. In the Balaenoptera foetus there are two small veins lying ventral to the cord, which communicate with the anterior vena cava via the dorsal thoracic veins. In the latter two species the vertebral canal anterior to the thorax is filled with retial tissue surrounding the cord.
  • 6In all specimens examined there is only a poorly developed jugular venous system. The intracranial venous drainage would appear to be almost entirely via veins, sinuses or retial tissue in the vertebral canal.
  • 7The Pinnipedia have a well-developed azygos system; the Cetacea either a poorly developed one or none at all.
  • 8All specimens of Pinnipedia, except Zalophus, and those Cetacea examined show a venous plexus or a single vein encircling the pericardium and projecting into the pleural cavity and draining into the thoracic posterior vena cava.
  • 9Certain experiments carried out on anaesthetized seals are described. Stimulation of the right phrenic nerve in Phoca caused contraction of the caval sphincter, which was recorded manometrically and also by X-rays after intravenous injection of radio-opaque substances. A fibre count was made of the phrenic nerve and its terminal branches.
  • 10Estimations of blood volume (117 ml. per kg. body weight in a Common Seal pup) and of other blood constants are recorded.
  • 11The possible function of the peculiarities of the venous system in Pinnipedia and Cetacea is discussed.