=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 ventricle. 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 (_m_). Branches of the superior vena cava: 1. =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 vertebræ, 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 separately, but the intercostals of the cranial intercostal spaces usually unite two or three together and enter the azygos by one or more common trunks. The azygos also receives =bronchial= and =œsophageal= veins, corresponding to the similarly named arteries. [Illustration: FIG. 129.--THORACIC BLOOD-VESSELS, FROM THE RIGHT SIDE. _a_, aorta; _b_, vena cava superior; _c_, vena cava inferior; _d_, V. azygos; _e_, A. subclavia dextra; _f_, A. mammaria interna; _g_, V. mammaria interna; _h_, costocervical axis; _i_, united trunk of vertebral and costocervical veins; _j_, A. and V. axillaris; _k_, V. jugularis externa; _l_, V. jugularis interna; _m_, the two innominate veins; _n_, intercostal arteries and veins; _o_, thyrocervical axis. 1, heart; 2, trachea; 3, œsophagus; _I_-_XI_, cut ends of the first eleven ribs.]