1898.1 SYSTEM OE THE C'HIROPTBRA. 73 



is continued onward as the precaval. Immediately dorsal to the 

 vessel there is the right subclavian artery. 



The precaval vein then runs inward and backward to join the 

 anterior part of the right auricle. On its right side are placed the 

 corresponding internal mammary artery, the upper lobe of the 

 right lung, and the right phrenic nerve. To the left lie the right 

 innominate artery, the ascending aorta, and posteriorly the left 

 vagus nerve. On the ventral surface the precaval, below the 

 entrance of the common mammary vein, is covered by the upper 

 lobe of the right lung. Its total length is 10 mm. 



The tributaries of the right precaval are : — 



1. The vena azygos major, joining the dorsal surface of the vessel 

 4 mm. from the surface of the auricle. 



2. The common mammary vein, joining the ventral surface. 



3. Small mediastinal and thymic veins enter at various places. 

 The course of the Vana Azygos Major is forward, lying on the left 



side of the centra of the vertebrae, till it reaches the fourth inter- 

 vertebral disc. Here it turns downward and ends by joining the 

 right precaval, as already described. The intercostal veins of both 

 sides join the vena azygos major (those of the left side passing 

 above the aorta), except in the first intercostal space on the right 

 side, and the first and second on the left, the veins from these 

 spaces passing to the vertebral. 



Common Mammary Vein. — The right and left internal mammary 

 veins arise by tributaries which correspond closely to the arteries, 

 one vena comes accompanying each branch. Both veins run 

 forward, lying immediately behind the junction of the ribs with 

 the mesosternum, immediately internal to the artery (fig. 7). At 

 the middle of the first intercostal space, having been joined by the 

 highest perforating vein, the left internal mammary inclines to the 

 right, crossing behind the presternum, and joins the vein of the 

 right side to form tlie common mammary '. This continues the 

 direction of the left vein, and, receiving a tributary corresponding 

 to the presternal artery, joins the right precaval, 6*5 mm. from the 

 surface of the auricle. 



In fig. 3 (p. 65), which represents the heart and gi-eat vessels of 

 Pteropus edulis, the internal mammary veins are seen to join the 

 corresponding presternal veins in the usual manner. 



Left Precaval Vein. — Formed by the same tributaries and in the 

 same manner as the corresponding trunk on the right side. The 

 vessel lies at first between the subclavian artery above and the 

 upper lobe of the left lung below, pursuing a slightly arched course 

 backward and a litde inward, till it reaches the auriculo-ventricular 

 groove, 19 mm. from its origin. Here it turns to the right, and 

 much increased in calibre ends in the right auricle, at a point 

 corresponding to the opening of the coronary sinus in man. 



The left precaval vein receives small mediastinal and thymic 

 branches, and close to its termination the dorsal cardiac veins. 



Postcaval Vein. — The thoracic portion of this vessel runs directly 



'■ A very similar arrangement is to be met with in the Capybara {Hydroehotrus 

 cupyhxra) an:l alsD in the Common For {Cinis vidpes)- 



