THE SUPERIOR CAVAL SYSTEM. 86i 



as the vena thyreoidea ima, corresponding to the artery of the same name, which, 

 however, need not be present with it. It opens usually into the left innominate 

 vein, but occasionally is prolonged inward to terminate in the superior vena cava. 



Tributaries. — The plexus thyreoideus impar receives communications from the superior 

 thyroid veins and also has opening into it the inferior laryngeal veins (vv. laryngeae inferiores) 

 which descend from the larynx. The inferior thyroid veins receive directly branches from the 

 trachea (vv. tracheales) and from the oesophagus (vv. oesophageae). 



Practical Considerations. — An incision across the inferior thyroid vein, 

 whose walls, being imbedded in inflamed tissue, could not collapse, has caused 

 sudden death by the entrance of air. Parise, in attempting to seize the divided 

 inferior thyroid vein during tracheotomy, lifted the superficial wall only, thus per- 

 mitting air to enter the vein with a fatal result (Allen). 



5. The Superior Phrenic Vein. — The superior phrenic vein (v. phrenica 

 superior) has its origin upon the upper surface of the diaphragm and ascends through 

 the thorax, lying between the pericardium and pleura and accompanying the phrenic 

 nerve and the superior phrenic artery, of which it is a companion vein. Usually the 

 veins of both sides are double. They open above into the left innominate vein, fre- 

 quently uniting with the thymic, pericardial, and mediastinal veins before their termi- 

 nation. They are provided with valves both at their orifice and along their course. 



6. The Thymic Veins. — The thymic veins (vv. thymicae) are rather insig- 

 nificant in the adult and are usually two or three in number. They arise in the 

 adipose tissue which replaces the thymus gland and empty above into the left innomi- 

 nate vein, frequently uniting with the superior phrenic veins. In the child they are 

 of considerable size in correlation with the development of the thymus gland. 



7. The Pericardial Veins. — The pericardial veins (vv. pericardiacae) vary 

 considerably in number. They are all small, and empty in part into the left innomi- 

 nate vein and in part into the azygos and internal mammary veins. 



8. The Anterior Mediastinal Veins. — The anterior mediastinal veins (w. 

 mediastinales aoteriores), like the preceding, are variable in number and small. They 

 arise in the anterior mediastinum and open above into the left innominate vein. 



The Internal Jugular Vein. 



The internal jugular vein (v. jugularis interna) (Figs. 753, 760) is the principal 

 venous trunk of the neck. It is. the continuation of the lateral sinus at the jugular 

 foramen, and descends the neck in company with the internal and common carotid 

 arteries to a point a little external to the sterno-clavicular articulation, where it unites 

 with the subclavian to form the innominate vein. At its origin it rests upon the anterior 

 sloping surface of the jugular process of the occipital bone, and usually presents at 

 this point a distinct bulbous enlargement (bulbus venae jugularis superior) measuring 

 about 1.5 cm. in diameter. Below the bulbus superior, at its exit from the jugular 

 foramen, the diameter of the vein averages about 9 mm. , although subject to consider- 

 able variation, and usually differing on the two sides, since the lateral sinuses, of 

 which the veins are the continuations, differ on the two sides, that of the right being 

 in the majority of cases the larger. As it descends the neck the vein gradually 

 increases in size as it receives its various tributaries, and just before its union with the 

 subclavian vein it presents a more or less pronounced spindle-shaped enlargement 

 (bulbus venae jugularis inferior). This dilatation is usually much more distinct in the 

 right vein than in the left, and at its upper end is provided with a pair of valves or else 

 with a single one, the cavities of the valves being directed downward as if to prevent 

 an upward flow of blood. Even when a pair is present they are insufficient, but they 

 may nevertheless play an important part in preventing the blood from flowing into 

 the innominate through the subclavian vein and from producing, during the systole 

 of the auricle, a back pressure in the cerebral veins which are in connection with the 

 internal jugular. Since the right innominate is much more nearly in a line with the 

 vena cava superior than is the left, the greater development of the inferior bulb in 

 the right internal jugular can be readily understood. 



