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316 THE CIRCULATORY SYSTEM. 
Vena cordis magna (Fig. 118, 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, 4). 
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 (zz). 
Branches of the superior vena cava: 
1. V. azygos (@).—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 vertebre, slightly to the right of the middle 
line. The azygos receives the intercostal veins (x), 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 separately, but the intercostals of 
