CAVA AND INNOMINATE VEINS. 329 



The left vein (g} is twice as long as the right, and is directed obliquely 

 downwards above the level 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 several nerves de?ciMd- 

 ing over the arch. 



The branches of the veins are nearly alike on the two sides. Each re- 

 ceives the internal mammary, the inferior thyroid, and the superior inter- 

 costal of its own side ; and the left vein is joined in addition by some 

 small thymic and pericardiac veins. 



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

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

 right auricle. When such a condition exists, the right vein takes the 

 course of the upper cava in front of the root of the right lung ; but the left 

 vein 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 connects the two above the heart. 1 



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

 period of the growth of the fo3tus ; and two vessels are also persistent in 

 some mammalia. 



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

 the growth of the foetus, to produce the arrangement common in the adult, con- 

 cern the trunk on the left side. The following is an outline of them. First the 

 cross branch between the two trunks enlarges, and forms the future left innomi- 

 nate vein. Next the left trunk below the cross branch disappears at its middle, 

 and undergoes transformations at each end : At the upper end it becomes con- 

 verted into the superior intercostal vein. At the lower part it remains pervious 

 for a short distance as the coronary sinus ; and even the small oblique vein open- 

 ing into the end of that sinus in the adult (p. 314), is a remnant of the trunk of 

 the vein that lay beneath the heart. 



In the adult there is a vestige of the occluded vessel in the form of a fold of the 

 serous membrane of the pericardium in front of the root of the left lung ; this Mr. 

 Marshall names the vestigial fold of the pericardium (p. 311 ). 2 



The INFERIOR or ASCENDING CAVA enters the right auricle as soon as 

 it lias pierced the diaphragm. No branches join the vein in the thorax. 

 The anatomy of this vein will be given with the vessels of the abdomen. 



The PULMONARY VEINS are two on each side. They issue from the 

 fissure of the root of the lung, and end in the left auricle : their position 

 to the other vessels of the root has been noticed at p. 310. 



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

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

 upper and middle pulmonic lobes, and the inferior vein is formed by 

 branches of the lower lobe. 



The left veins cross in front of the descending aorta; and one springs 

 from each lobe of the lung. 



NERVES OF THE THORAX. 



The pneumogastric and the sympathetic nerves supply the viscera of the 

 thorax. Through the cavity courses the phrenic nerve to the diaphragm. 



1 An example of two large vessels, " double vena cava," opening into the right 

 auricle in the adult, is contained in the Museum of University College. 



2 See a paper by Mr. Marshall on the development of the veins of the neck 

 (Philosoph. Transac., 1850). 



