960 THE VASCULAK SYSTEM. 



(3) The oblique vein of the left atrium (Marshall) (Fig. 750) is a small venous 

 channel which descends obliquely, on the posterior wall of the left atrium, and terminates 

 in the coronary sinus. Its orifice is not provided with a valve. It is of special interest, 

 inasmuch as it represents the left superior vena cava of some other mammals, and is 

 developed from the left duct of Cuvier. 



(4) The inferior cardiac vein of the left ventricle runs along the inferior surface of 

 the left ventricle and ends in the coronary sinus. 



(5) The middle cardiac vein commences at the apex of the heart, and, passing 

 posteriorly, in the inferior interventricular sulcus, terminates in the coronary sinus near 

 its right extremity. It receives tributaries from the inferior parts of the walls of both 

 ventricles. 



Veins of the Heart which do not end in the Coronary Sinus. (a) The 

 anterior cardiac veins are two or three small vessels which ascend on the anterior wall 

 of the right ventricle to the coronary sulcus, where they either end in the right 

 atrium or terminate in the small cardiac vein, (b) The venae minimae cordis. A number 

 of small veins, which commence in the substance of the walls of the heart and terminate 

 directly in its cavities, principally in the atria ; some few, however, open into the 

 ventricles. 



VENA CAVA SUPERIOR AND ITS TRIBUTARIES. 



The superior vena cava (Figs. 756 and 757) returns the blood from the head 

 and neck, the upper extremities, the thoracic wall, and a portion of the upper part 

 of the wall of the abdomen. It is formed, at the lower border of the first right 

 costal cartilage, by the union of the two innominate veins, and it descends, with 

 a slight convexity to the right, to the level of the third right costal cartilage, 

 where it opens into the upper and posterior part of the right atrium. It is about 

 75 mm. (three inches) long ; in the lower half of its extent it is enclosed within 

 the fibrous layer of the pericardium, and it is covered in front and on each side by 

 the serous layer. 



Relations. It is overlapped anteriorly by the margins of the right lung and pleural 

 sac and by the ascending aorta. The lung and pleura intervene between it and the 

 second and third costal cartilages, the internal intercostal muscles in the first and second 

 intercostal spaces, and the internal mammary vessels. It is in relation posteriorly with 

 the right margin of the trachea, the right vagus nerve, the vena azygos, which opens into 

 it at right angles, the right bronchus, the right pulmonary artery, and the upper right 

 pulmonary vein. On its left side are the ascending portion of the aorta, and the commence- 

 ment of the innominate artery, whilst on the right side it is in close relation with the 

 right pleura, the phrenic nerve and the pericardiaco-phrenic (O.T. comes nervi phrenici) 

 vessels intervening. 



Tributaries. In addition to the two innominate veins, by the union of which it is 

 formed, the superior vena cava receives only one large tributary, viz., the vena azygos ; 

 but several small pericardial and mediastinal veins open into it 



VENA AZYGOS AND ITS TRIBUTARIES. 



The vena azygos (O.T. vena azygos major) (Fig. 798) commences either from 

 the posterior aspect of the inferior vena cava, at the level of the right renal vein, 

 or as the direct upward continuation of an anastomosing channel which connects 

 together the lumbar veins of the right side, and which is known as the right 

 ascending lumbar vein. It ascends through the aortic orifice of the diaphragm, 

 and is continued upwards through the posterior mediastinum. In the upper part 

 of its course, it first passes posterior to the root of the right lung, and then arches 

 anteriorly, above the root, to its termination in the posterior part of the superior 

 vena cava, immediately before the latter vessel pierces the pericardium and at the 

 level of the second costal cartilage. It frequently possesses imperfect valves. 



Relations. In the abdomen it lies on the anterior surfaces of the bodies of the 

 upper lumbar vertebrae, posterior to the right crus of the diaphragm and the inferior 

 vena cava, and to the right side of the cisterna chyli. 



In the thorax it lies on the anterior surfaces of the bodies of the lower eight thoracic verte- 

 brae, the intervening fibro-cartilages, and the anterior longitudinal ligament; and anterior to 



