734 MR. R. H. BURNE ON THE [Juiie 15, 



to the voluntary organs might without much disadvantage be 

 held more or less in abeyance, and their circulation be stopped 

 or i-etarded for the time. As a matter of fact the visceral 

 circulation, even if the venous sphincter were completely closed, 

 would probably not become quite stagnant, at least for some 

 considerable time, for the returning venous current would be 

 5 accommodated in the sub-diaphragmatic sinus. When the animal 

 returns to the surface and breathes again, the temporary block 

 on the vena cava inferior would be removed and the normal 

 circulation restored. 



In this connection it should be noticed that in the Walrus 

 (Murie, I. c. p. 431) the artei-ies for the head and fore part of 

 the body and for the alimentary canal are remarkably large in 

 compai-ison with those for the hind limbs, so that evidently the 

 two most important venous return currents are from the viscera 

 by the inferior cava and from the fore part of the animal by the 

 superior cava, that from the hind end of the body being insigni- 

 ficant. The removal of one of these chief sources of impurity 

 from the circulation would necessarily greatly enhance the 

 effective purification of the other. 



Although this may be, and I think is, the primary function of 

 this muscle, there is also the probability, as pointed out to me by 

 my cousin (Mr. T. W. Burne), that the temporary closure of the 

 vena cava inferior would help to relieve the pressure in the right 

 heart consequent on the suspension of respiratory movements. 



This pi-essure must be very great in diving creatures, and the 

 paramount importance of its reduction is shown by the presence 

 even in animals not aquatic, including man, of a well-known 

 mechanism in connection with the tricuspid valve designed to 

 act as a safety-valve and allow of the backflow of blood into the 

 veins when the right ventricle becomes unduly distended thi-ough 

 cessation of active respiration or by an increased flow of venous 

 blood to the heart. This mechanism was exhaustively described 

 many years ago by Mr. T. W. King * (for the knowledge of whose 

 paper I am much indebted to Dr. James Mackenzie), and his argu- 

 ments, as he himself emphasizes, apply with added force to diving 

 air-breathing creatures, for not only is the pulmonary circulation 

 checked by the cessation of the respiratory movements while the 

 animal is still otherwise in full activity, but as has been shown 

 by Mr. Houston t the great pressure of the surrounding water 

 tends to drive the blood from the surface of the body and con- 

 centrate it upon the heart. To obviate this and to retard the 

 return flow of the blood, there are in most if not all diving 

 air-breathing animals (both birds and mammals) great sinuses 

 in connection with the chief veins in which the blood may collect 

 on its way to the heart and into which it may regurgitate through 

 the imperfectly closed tricuspid when the over-distended right 

 ventricle contracts. But except in the case of- the inferior vena 



* King, Guy's Hospital Keports, vol. ii. 1837, pp. 104-178. 

 t Houston, Brit. Assoc. Reports, 1836, p. 81. 



