SUBCLAVIAN INNOMINATE. 547 



chial; and near its termination it receives the cephalic vein. This vein is pro- 

 vided with a pair of valves, opposite the lower border of the Subscapularis 

 muscle; valves are also found at the termination of the cephalic and sub- 

 scapular veins. 



The Subclavian Vein, the continuation of the axillary, extends from the outer 

 margin of the first rib to the inner end of the sterno-clavicular articulation, 

 where it unites with the internal jugular, to form the vena innominate. It is 

 in relation, in front, with the clavicle and Subclavius muscle; behind, with the 

 subclavian artery, from which it is separated internally by the Scalenus Anticus 

 and phrenic nerve. Below, it rests in a depression on the first rib and upon 

 the pleura. Above, it is covered by the cervical fascia and integument. 



The subclavian vein occasionally rises in the neck to a level with the third 

 part of the subclavian artery, and in two instances has been seen passing with 

 this vessel behind the Scalenus Anticus. This vessel is provided with valves 

 about an inch from its termination in the innominate, just external to the 

 entrance of the external jugular vein. 



Branches. It receives the external and anterior jugular veins and a small 

 branch from the cephalic, outside the Scalenus ; and on the inner side of that 

 muscle, the internal jugular vein. 



The Venae Innominatse (Fig. 808) are two large trunks, placed one on each 

 side of the root of the neck, and formed by the union of the- internal jugular 

 and subclavian veins of the corresponding side. 



The right vena innominata is a short vessel, about an inch and a half in length, 

 which commences at the inner end of the clavicle, and, passing almost vertically 

 downwards, joins with the left vena innominata just below the cartilage of the 

 first rib, to form the superior vena cava. It lies superficial and external to the 

 arteria innominata; on its right side the pleura is interposed between it and the 

 apex of the lung. This vein, at its angle of junction with the subclavian, 

 receives the right vertebral vein, and right lymphatic duct; and, lower down, 

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

 costal veins. 



The left vena innominata, about three inches in length, and larger than the 

 right, passes obliquely from left to right across the upper and fore part of the 

 chest, to unite with its fellow of the opposite side, forming the superior vena 

 cava. It is in relation, in front, with the sternal end of the clavicle, the sterno- 

 clavicular articulation, and the first piece of the sternum, from which it is sepa- 

 rated by the Sterno-hyoid and Sterno-thyroid muscles, the thymus gland or its 

 remains, and some loose areolar tissue. Behind, 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 inferior thyroid, left internal mammary, and the left 

 superior intercostal veins, and occasionally some thymic and pericardiac veins. 

 There are no valves in the venae innorninatae. 



Peculiarities. Sometimes the innominate veins open separately into the right auricle ; in 

 such cases the right vein takes the ordinary course of the superior vena cava, but the left vein, after 

 communicating by a small branch with the right one, passes in front of the root of the left lung, 

 and turning to the hack of the heart, receives the cardiac veins, and terminates in the back of 

 the right auricle. This occasional condition of the veins in the adult, is a regular one in the 

 foetus at an early period, and the two vessels are persistent in birds and some mammalia. The 

 subsequent changes which take place in these vessels are the following : 'The communicating 

 branch between the two trunks enlarges and forms the future left innominate vein; the remain- 

 ing part of the left trunk is obliterated as far as the heart, where it remains pervious, and forms 

 the coronary sinus ; a remnant of the obliterated vessel is seen in adult life as a fibrous band 

 passing along the back of the left auricle and in front of the root of the left lung, called by Mr. 

 Marshall the vestigial fold of the pericardium. 



The internal mammary veins, two in number to each artery, follow the course 

 of that vessel, and receive branches corresponding with those derived from 

 it. The two veins unite into a single trunk, which terminates in the innomi- 

 nate vein. 



