THE SUPERIOR CAVAL SYSTEM. 



859 



Variations. — As pointed out in the account of the development of the great veins (page 926) , 

 there is at one stage a symmetrical arrangement of the vessels which open into the right auricle 

 from above ; in other words, the left internal jugular is continued directly downward from the 

 point where the left subclavian vein opens into it to the auricle, this downward continuation 

 being usually termed the left superior vena cava. Later a cross-connection, the left innominate 

 vein, forms between the right and left jugulars at the root of the neck, and the left superior vena 

 cava then normally undergoes degeneration, traces of it only persisting as the oblique vein of 

 the left auricle and the coronary sinus. 



Occasionally this normal progress of events fails to occur, the result being the complete 

 absence or imperfect development of the left innominate vein together with a persistence of the 

 left superior vena cava ; or else, even with the perfect development of the left innominate, there 

 may be a failure of the left 



superior vena cava to degen- pj^ _ 



erate. Various gradations 

 between the embryonic and 

 adult conditions may occur, 

 and the annexed diagram 

 (Fig. 752) shows the nature 

 of the anomaly. It may be 

 noted that with the persist- 

 ence of the left superior vena 

 cava there is frequently a 

 retention of the communica- 

 tion with it of the left cardinal 

 vein, which normally be- 

 comes the V. hemi-azygos, — a 

 condition which will be more 

 especially considered in con- 

 nection with the anomalies of 

 the azygos veins ( page 893 ) . 



Left internal jugular 



L. subclavian 

 L. innominate 



L. sup. vena cava 



L. azygos 



L. pulmonary 



arterv 



Coronary sinus 



Right int. jugular 



R. subclavian 



R. innominate 



R. sup. vena cava 

 R. azygos 



R. pulmonary artery 

 R. pulmonary veins 



Practical Consid- 

 erations. —The left in- 

 nominate vein, running 

 horizontally just below the 

 upper border of -the ma- 

 nubrium, lies immediately- 

 above the aortic arch. 



When the latter is unusually high, and occasionally in children, the vein — especially 

 if engorged —may project above the level of the suprasternal notch and may be 

 endangered during a thyroidectomy, the removal of a tumor, or a low tracheotomy. 



Inf. vena cava 



Posterior aspect of heart and great vessels, showing persistence 

 of left superior vena cava ; (semidiagrammatic). 



Tributaries. — In addition to the subclavian and internal jugular veins, by 

 whose union they are formed, each innominate vein receives (i) the deep cervical, 

 (2) the vertebral, (3) the i?iterjial mammary, and (4) the inferior thyroid veins of 

 its side. The left innominate vein receives in addition (5) the superior phrenic, 

 (6) the thymic, (7) the pericardial, (8) the anterior mediastinal, and (9) the 

 left superior intercostal vein. Of these the left superior intercostal vein will be 

 described with the other intercostals. 



I. The Deep Cervical Vein. — The deep cervical vein {\. cervicalis profunda) 

 takes. its origin in a plexus situated in the occipital triangle and having also con- 

 nected with it the vertebral and occipital veins. It passes down the neck, lying be- 

 tween the semispinalis cervicis and the splenius cervicis, and in the upper part of its 

 course accompanies the deep branch of the art. princeps cervicis. Lower down it 

 accompanies the deep cervical branch of the superior intercostal artery and bends 

 slightly outward and forward, passes between the transverse process of the seventh 

 cervical vertebra and the first rib, and opens into the innominate vein either behind 

 the vertebral vein or by a common trunk with that \^essel. 



Tributaries. — In its course down the neck it receives numerous tributaries from the deeper 

 cervical muscles, and opposite each intervertebral foramen which it passes it makes connections 

 with the vertebral vein and the veins of the spinal canal. 



The most iinportant of its tributaries is, however, the occipital vein, which arises in a 

 plexus covering the occipital portion of the skull and communicating with branches of the pos- 

 terior auricular and temporal veins. It passes downward with the occipital artery, pierces tht 



