610 



THE BLOOD-VASCULAR SYSTEM. 



The Left Innominate Vein, about two and a half inches in length, and larger 

 than the right, passes from left to right across the upper and front part of the chest, 



at the same time inclining 

 downward, to unite with its 

 fellow of the opposite side. 



Anterior jugular. 



Superior thyroid. 



External jugular. 



forming the superior vena 

 cava. It is in relation, in 



of the sternum, from which 

 it is separated by the Stern o- 

 hyoid and Sterno-thyroid 

 muscles, the thymus gland 

 or its remains, and some 

 loose areolar tissue. Be- 

 hind, it lies across the roots 

 of the three large arteries 

 arising from the arch of the 

 aorta. This vessel is joined 

 by the left vertebral, left 

 internal mammary, left in- 

 ferior thyroid, and the left 

 superior intercostal veins, 

 and occasionally some thy- 

 mic and pericardiac veins. 

 There are no valves in the 

 innominate veins. 



Peculiarities. Sometimes 

 the innominate veins open sep- 

 irately into the right auricle ; in 

 such cases the right vein takes 

 the ordinary course of the supe- 

 rior vena cava ; but the left vein 

 left superior veiia cava, as it 

 is termed after communicating 

 by a small branch with the right 

 one, passes in front of the root 

 of the left lung, and, turning to 

 the back of the heart, receives 

 the cardiac veins, and termin- 

 ates in the back of the right 

 auricle. This occasional con- 

 dition in the adult is due to the 

 persistence of the early foetal 

 condition, and is the normal 

 state of things in birds and 

 some mammalia. 



The internal mammary 

 veins, two in number to 

 each artery, follow the 

 course of that vessel, and 

 receive branches corre- 

 sponding with those derived 

 from it. The two veins of 

 each side unite into a single 

 trunk, which terminates in 

 the innominate vein. 



The inferior thyroid 

 veins, two, frequently three 

 or four, in number, arise in the venous plexus on the thyroid body, communicating 

 with the middle and superior thyroid veins. They form a plexus in front of the 



Lnttrrna 



FIG. 331. The venae cavce and azygos veins, with their formative 

 branches. 



