1408 



VENOUS SYSTEM. 





rests upon the first rib, the scaleni muscles, 

 and the phrenic nerve, which descends be- 

 tween the vein and the scalenus anticus mus- 

 cle. Contrary to what is observed in the 

 case of the axillary and of most other veins 

 of large size, the branches which join the 

 subclavian vein do not correspond to those 

 which are given off by the subclavian artery. 

 The collateral branches of the subclavian vein, 

 are the external and internal jugular veins ; 

 the former joins it on the outer, and the latter 

 on the inner side of the sterno-mastoid mus- 

 cle. The subclavian vein not uncommonly 

 receives the vertebral also, although, as has 

 been stated, this vein is most frequently a 

 tributary of the brachio-cephalic trunk. 



Brachio-cephalic veins. {Ven<s mnoinmatcB t 

 Meckel.) These veins, one on each side, 

 are formed by the union of the internal jugu- 

 lar with the subclavian vein ; they are appa- 

 rently of the same size, but are contrasted in 

 other essential particulars, as they are ex- 

 amined on the right and left side : thus they 

 will be found to differ from each other in 

 length and in direction, as well as in their 

 connexions with surrounding parts, as also in 

 the number of collateral branches which they 

 respectively receive. 



The right vena innominata is comparatively 

 short, and nearly vertical in direction ; it is 

 placed behind the cartilage of the first rib, 

 and is related externally to the right pleura 

 and internally to its corresponding artery, the 

 arteria innominata ; the vein and artery how- 

 ever are not in close apposition, and the right 

 pneumogastric nerve is found in the interval 

 between them, but deeper than both ; fila- 

 ments of the cardiac nerves also are inter- 

 posed between them.* 



The left vena innominata, more than twice 

 as long as the preceding, traverses the upper 

 part of the anterior mediastinum almost hori- 

 zontally from left to right, but at the same 

 time with an obliquity downwards and slightly 

 forwards. It is placed behind the first bone 

 of the sternum, from which it is separated by 

 the sterno-hyoid and sterno-thyroid muscles, 

 and by the thymus gland ; and bending across 

 the three primary branches of the aorta, 

 slightly overlaps the anterior surface of the 

 arch itself towards the right side. Imme- 

 diately below the cartilage of the first rib and 

 on the right side of the mesian line, the 

 innominate veins unite to form the superior 

 vena cava. 



Collateral branches. The vertebral is in 

 general the only vein which terminates in the 

 right vena innominata, for although it does 

 occasionally occur that the inferior thyroid 

 and internal mammary veins join the vena in- 

 nominata on their respective sides, yet in the 

 great majority of instances these veins ter- 

 minate differently on the right and left side : 

 on the left they almost invariably open into 



* At the moment of its formation the right vena 

 innominata is placed anterior, and inferior, to the 

 first stage of the subclavian artery ; between them, 

 however, we find the mammary artery, and the 

 phrenic and pneumogastric nerves. 



the corresponding vena innominata, whilst on 

 the right they as constantly open into the 

 vena cava superior. It is not very uncom- 

 mon for the left vena innominata to receive 

 all the inferior thyroid veins ; and the left 

 superior intercostal vein, which is usually 

 connected by a large collateral branch with 

 this vein, sometimes, but more rarely, termi- 

 nates directly in that vessel. 



Inferior thyroid veins. Although generally 

 described as consisting of two principal trunks, 

 one for each side, these veins very often con- 

 sist of as many as four or more branches, 

 which, arising from a plexus of veins in the 

 interior of the thyroid gland, emerge from its 

 lower border and descend on the front of the 

 trachea, covered by the deep layer of fascia 

 and by the infra-hyoid muscles, to open into 

 the left vena innominata. These veins lie in 

 a well-defined triangular space of much sur- 

 gical importance, which is bounded on the 

 one side by the innominata, and on the other 

 by 4 the left carotid artery, whilst its third 

 side, which represents the base of the triangle, 

 is defined by the lower margin of the thyroid 

 body : the apex is constituted by the con- 

 vergence of the arteries which form its sides, 

 to the arch of the aorta ; and in this latter 

 direction, the space above described is some- 

 what encroached on by the left vena inno- 

 minata. Sometimes the thyroid veins of op- 

 posite sides unite to form an arch across the 

 trachea, and in some few instances they open 

 into the transverse branch of communication 

 which sometimes unites the anterior jugular 

 veins of opposite sides. 



Internal mammary veins. These, which 

 constitute " venae comites " to the trunk of 

 each internal mammary artery, are formed by 

 branches which correspond with those which 

 emanate from that vessel. Very frequently 

 the two vessels unite in a single trunk, which 

 ends, as has been stated, in the vena inno- 

 minata on the left side, and in the vena cava 

 on the right. 



Vena cava superior. This great vein re- 

 ceives all the blood from the head and neck, 

 and from the upper extremities ; it is formed 

 by the union of the two brachio-cephalic 

 veins, and extends from the lower margin of 

 the cartilage of the first rib on the right of 

 the sternum, to the upper and posterior part 

 of the right auricle of the heart where it 

 terminates. Shortly after its formation, the 

 superior vena cava enters the pericardium. 

 That portion of the vein which is without the 

 pericardium (and which is of very small extent) 

 is invested by a sheath of the fibrous layer 

 of that membrane, whilst its intra-pericardial 

 stage is completely invested by the serous 

 membrane, which it serves to convey to the 

 surface of the heart. In its entire course this 

 vein describes a curve, the convexity of which 

 is related to the right pleura with the inter- 

 vention of the pericardium, whilst the con- 

 cavity touches the aorta ; the pericardium is 

 anterior to it, and the pulmonary artery of 

 the right side passes behind it. The principal 

 collateral branch of the thoracic vena cava is 



