3i6 THE CIRCULATORY SYSTEM. 



Vena cordis magna (Fig. 1 18, 3). — This arises as a number 

 of scattered branches on the surface of both ventricles. These 

 unite to form two main trunks. One of these, on the ventral 

 side of the heart, marks externally the septum between the 

 two ventricles. It passes beneath the left auricular appendage, 

 then turns to the left and passes along the sulcus coronarius 

 to the dorsal side of the heart. Here it unites with the second 

 main trunk which ramifies over the surface of the left ven- 

 tricle. Arriving at the base of the right auricle it enters 

 this by the coronary sinus, which lies just beneath the opening 

 of the inferior vena cava. 



In addition to the vena cordis magna and its branches 

 there are a number of small veins, especially in the wall of the 

 right ventricle. These enter the right auricle by a number of 

 small separate openings along the sulcus coronarius. 



2. Vena cava superior (Fig. 129, b). 



The superior vena cava is the great vein returning the 

 blood from the head, fore-limb, and cranial part of the trunk. 

 It extends from the level of the first rib on the right side of the 

 vertebral column to the right auricle. Its caudal end lies 

 dorsad of the aortic arch. Opposite the right rib it is formed 

 by the junction of the two innominate veins {in). 



Branches of the superior vena cava: 



I. V. azygos {d). — The first branch of the superior vena 

 cava is the azygos vein, which enters the vena cava on the 

 right side a centimeter or less craniad of the root of the right 

 lung. It is formed in the abdominal cavity by the confluence 

 of two or three small veins, which collect the blood from the 

 muscles of the dorsal wall of the abdomen. The small median 

 trunk thus formed enters the thoracic cavity between the crura 

 of the diaphragm and lies on the ventral surface of the centra 

 of the thoracic vertebra?, slightly to the right of the middle 

 line. The azygos receives the intercostal veins (n), which 

 correspond to the intercostal arteries and have the same course 

 and distribution. In the caudal part of the thoracic cavity the 

 intercostals enter the azygos separatel)', but the intercostals of 



