156 



THE VOYAGE OF H.M.S. CHALLENGER. 



as the coronary sinus of man, and, like it, opens into the posterior part of the right 

 auricle close to the orifice of the posterior vena cava. Towards its termination it is 

 covered by a thin stratum of muscular fibres derived from the muscular wall of the heart. 



The posterior vena cava has the usual mode of termination. 



The pulmonary veins are two in number on each side. 



Veins of the thoracic parietes. 



In the ThyJacine the vena azygos runs forward upon the right side of the 

 vertebral column, and drains the blood from all the intercostal spaces, with the 

 exception of the anterior three upon both sides. It ends by hooking downwards 

 above the root of the right lung, and joining the right anterior vena cava. The three 



U. 



y. ) r\ 



THYLAGINE 



Fig. 5.— Diagram of the veins opening into the right auricle of the heart, 



Vena azygos major. G* C. Great cardiac vein 



Superior intercostal (right). 



AZ. 



S.I. 



E.A.C. Right anterior vena cava. 



L.A.C. Left anterior vena cava. 



S.I. Superior intercostal vein (left). 



P. Posterior cardiac vein. 



P.O. Posterior vena cava. 



A. Aorta. 



P. A. Pulmonary artery. 



anterior intercostal veins of the right side join to form one trunk, which pours its 

 blood into the right anterior vena cava whilst the corresponding veins of the opposite side 

 likewise unite to open into the left anterior vena cava. 



In the Cuscus, Dasyure, Vulpine phalanger, and Phascogale there is also a single 

 azygos vein ; this, however, runs forward upon the left side of the spine. It receives all 

 the blood from the intercostal spaces of its own side, and all the intercostal veins of the 

 right side with the exception of the anterior three. It hooks over the root of the left 

 lung, and joins the left anterior vena cava. The anterior three intercostal veins of the 

 right side unite to pour their blood into the right anterior vena cava. 



