THE AZYGOS SYSTEM. 893 



neous nerve (lying in front of the vein) may give rise to chronic traumatic neuritis 

 (Tillaux), while injury of the cutaneous branches of the musculo-cutaneous nerve 

 (in closer relation to the median cephalic vein), or entanglement of those branches 

 in the cicatrix, may, by reflex irritation acting through the motor fibres, cause tonic 

 spasm of the biceps and brachialis anticus, "bent arm" (Hilton). 



The Arm. — The cephalic vein and its anomalies should be studied in relation 

 to ligation of the axillary artery {q.v.^, the first portion of which it crosses (sepa- 

 rated from it by the clavi-pectoral fascia), on its way to reach the axillary vein. 



It may be remembered that the basilic vein pierces the brachial aponeurosis a 

 little below the middle of the arm and ceases to be superficial. 



THE AZYGOS SYSTEM. 



The principal trunks of the azygos system of veins are persistent portions of 

 the embryonic cardinal veins which drained the thoracic and abdominal walls, as well 

 as the paired viscera of the abdomen, and united above with the jugular trunks to 

 form the Cuvierian ducts (page 926). On the development of the inferior vena 

 cava their importance diminished greatly, and in the adult they serve principally to 

 collect the blood from the intercostal spaces. The reduction of the lower part of the 

 left jugular vein (page 927) brought about further modifications of the left cardinal, 

 its original connection with the left jugular being dissolved and a new one formed 

 with the right cardinal. This latter vein forms what is termed the azygos vein of the 

 adult, while what persists of the left one is known as the hemiazygos and accessory 

 hemiazygos. 



The Azygos Vein. 



The azygos vein (v. azygos) (Fig. 765), sometimes called the azygos major, 

 begins immediately below the diaphragm, where it is directly continuous with the right 

 ascending lumbar vein, formed by the anastomosis of branches of the lumbar veins and 

 connecting below with the ilio-lumbar or common iliac. The azygos vein passes up- 

 ward into the thoracic cavity, traversing the diaphragm either by the cleft between the 

 medial and intermediate portions of the right crus or else by the aortic opening. It 

 then continues its way upward in the posterior mediastinum, resting upon the ante- 

 rior surfaces of the bodies of the thoracic vertebrae a little to the right of the middle 

 line, passing over the right intercostal arteries and having the thoracic aorta and the 

 thoracic duct immediately to the left of it. When it reaches the level of the fourth 

 vertebra it bends forward and somewhat to the right, and, curving over the right 

 bronchus and the right pulmonary artery, it descends slightly to open into the pos- 

 terior surface of the superior vena cava, just above the level at which that vessel 

 becomes invested by the pericardium, The terminal portion of the vein from the 

 fourth vertebra onward is sometimes termed the azygos arch. 



The azygos vein in a considerable proportion (22 per cent., Gruber) of cases is 

 entirely destitute of valves, and when present they rarely exceed four in number, are, 

 apparently, never exactly paired, and are usually insufficient. They occur more fre- 

 quently in the arch than in the vertical portion of the vein. 



Tributaries. — The azygos vein at its origin has usually some small connections 

 with the vena cava inferior, but its principal tributaries are the right intercostal veins. 

 In addition it receives branches from the oesophagus (vv. oesophageae), from the are- 

 olar tissue and lymph-nodes of the posterior mediastinum and from a plexus which 

 surrounds the thoracic aorta, from the posterior surface of the pericardium, and from 

 the substance of the right lung, these last bronchial veins (vv. bronchiales posteri- 

 ores) issuing from the hilum of the lung and opening into the azygos at the beginning 

 of its arch. They anastomose with the pulmonary veins both along the course of the 

 smaller bronchi and also outside the lung, and they receive some smaller bronchial 

 veins (vv. bronchiales anteriores) situated upon the anterior surface of the bronchi. 

 The azygos vein furthermore receives the hemiazygos vein ; this and the intercostal 

 veins will be described below. 



Variations. — Since the cardinal veins, from which the azygos and hemiazygos are formed, 

 are primarily symmetrical, it may happen, just as was the case with the aortic arches, that it is 

 the left one that is more fully retained and therefore becomes the azygos vein, the right becoming 



