THE INNOMINATE VEINS. 469 



The INNOMINATE VEINS are two in number, right and left ; and innominate 

 each is formed near the inner end of the clavicle by the junction of ^ e 

 the subclavian and internal jugular veins. Below, they are united 

 in the superior cava. The trunks differ in length and direction, 

 and in their relations tc surrounding parts (fig. 171). 



The right vein is about one inch long, and descends almost right, 

 vertically, on the right side of the innominate artery, to its junction 

 with the opposite vein. On the outer surface the pleura covers it, 

 and along it the phrenic nerve is placed. 



The left vein is nearly three inches in length, and is directed and left ; 

 obliquely to the right, along the upper border of the arch of the 

 aorta. It crosses behind the sternum, and the remains of the 

 thymus gland ; and it lies on the three large branches of the aortic 

 arch, as well" as on the nerves descending over the arch. 



The tributaries of the veins are nearly alike on the two sides, their 

 Each receives the vertebral and the internal mammary of its own 

 side, and occasionally the inferior thyroid, though these veins more 

 often blend into one trunk below, which opens into the junction 

 of the two innominate veins or into the left. The left vein also 

 is joined in addition by the superior intercostal, and some small 

 thymic and pericardia! veins. 



Occasionally the innominate veins are not united in the vena cava, but Sometimes 

 descend separately to the heart, where each has a distinct opening in the they open 

 right auricle. When such a condition exists, the right vein takes the course fntothe 3 ^ 

 of the upper cava in front of the root of the right lung ; but the left vein heart, 

 descends in front of the root of the left lung, and turning to the back of the 

 heart, receives the cardiac veins, before it opens into the right auricle. A 

 cross branch generally connects the two above the arch of the aorta. 



This occasional condition in the adult is a regular one at a very early period 

 of the growth of the foetus ; and the two vessels are also persistent in some 

 mammalia. 



Change of the two veins into one. The changes taking place in the veins How two 

 during foetal growth, to produce the usual arrangement in the adult, concern are changed 

 the trunk on the left side. The following is an outline of them. First a into one> 

 cross branch is formed between the two trunks, and this enlarging gives rise 

 to the left innominate vein. Then the left trunk below the cross branch dis- 

 appears at its middle, and undergoes transformation at each end : At the 

 upper end it becomes converted into a part of the superior intercostal vein, and coro- 

 At the lower end it remains pervious for a short distance as the coronary sinus ; narv ^| nus 

 and the small oblique vein opening into the end of that sinus in the adult is a * ' 

 remnant of the trunk as it lay beneath the heart. 



In the adult there is a trace of the occluded vessel in the form of a small 

 fibrous band in the vestigial fold of the pericardium (p. 451). 



The INFERIOR OR ASCENDING VENA CAVA enters the right Vena cava 



auricle as soon as it has pierced the diaphragm. No branches join 

 the vein in the thorax. 



The PULMONARY VEINS are two on each side, upper and lower. Fourpui- 

 They issue from the hilum of the lung, and end in the left auricle : ? e ?ns. ry 

 their position in regard to the other vessels of the root has been 

 noticed at p. 449. 



The right veins are longer than the left, and lie beneath the Right veins 

 right auricle of the heart. The superior receives its roots from 



