AZYGOS. 



365 



process and muscle quoted, each is composed 

 of parts that were originally double or sym- 

 metrical., which have coalesced in the middle 

 line so completely as to appear single ; as to 

 the vessel, the description of which will form 

 the subject of the present article, there is very 

 frequently an analogous trunk, only somewhat 

 smaller, on the opposite side of the spine. 



AZYGOS VEIN, Posterior thoracic, Prelum- 

 bo-thoracique, Vena sine part, Azygos major. 

 This vein exists in the posterior part of the 

 cavity of the thorax, on the right side of the 

 bodies of the dorsal vertebrae; it serves to 

 receive the blood from most of the intercostal 

 spaces, from the phrenic, bronchial, and medi- 

 astinal veins, as also from the vertebne and 

 vertebral sinuses, and to convey it into the 

 superior vena cava ; it also establishes a com- 

 munication between this last-named vessel and 

 the inferior cava through some of its lumbar 

 branches, and thus connects the veins of the 

 upper and lower segments of the body, in the 

 same manner as the internal mammary and 

 epigastric, and several others of the thoracic 

 and abdominal arteries inosculate. 



In the present article we shall consider not 

 only the greater and lesser vena azygos, but 

 also the principal branches which each receives 

 namely, the intercostal and bronchial veins. 

 The right or great vena azygos presents many 

 varieties as to the size and number of its 

 branches, as well as in its exact origin; it 

 usually commences very small opposite the 

 first or second lumbar vertebra, on the upper 

 extremity of the right psoas muscle from the 

 confluence of several minute veins, which com- 

 municate with branches from the superior 

 lumbar, capsular, renal, and spermatic veins, 

 and thus indirectly with the abdominal cava ; 

 it not unfrequently, however, arises by a branch 

 from the cava itself, in which case it appears 

 even in this region as a vessel of considerable 

 size. The abdominal portion of the vena azygos 

 is but short, ascends obliquely inwards, crosses 

 the right cms of the diaphragm, and enters the 

 posterior mediastinum between the crura of 

 this muscle in company with, and to the right 

 side of the thoracic duct and aorta ; it is here 

 surrounded by so much cellular and adipose 

 tissue as to be frequently very indistinct; it 

 sometimes enters the chest along with the right 

 splanchnic nerve through an opening in the 

 right crus itself, or external to the latter, between 

 the attachments of the diaphragm to the body 

 and transverse process of the first lumbar 

 vertebra. The thoracic portion of the vena 

 azygos ascends along the right side of the 

 vertebral column in front of the right inter- 

 costal arteries, and covered by the right pleura, 

 to which it is closely connected, being, in 

 fact, contained in the subserous cellular 

 tissue ; the aorta is to its left, and in the in- 

 tervening adipose matter the thoracic duct is 

 placed ; the right splanchnic nerve is external 

 to it or on its right side. Opposite to about 

 the fourth dorsal vertebra the vein leaves the 

 spine, increases very much in size, arches 

 forwards and to the right, around and above 



the right pulmonary artery and bronchial tube, 

 and opens into the back part of the superior 

 vena cava, immediately above the reflection of 

 the serous layer of the pericardium on that 

 vessel. A small fold of the lining membrane 

 of the azygos vein, a mere rudiment of a valve, 

 exists at its junction with the cava ; sometimes, 

 however, this fold is well developed, it is even 

 observed to be double. Similar folds or valves 

 are occasionally found lower down in the vena 

 azygos, but generally it is destitute of valves. 

 The vena azygos has been seen by Cheselden 

 to open into the vena cava within the pericar- 

 dium close to the right auricle ; it also occa- 

 sionally opens into the cava at a point higher 

 than that which has been stated as its regular 

 termination, and it now and then joins the right 

 or even the left vena innominata. 



The vena azygos receives several veins; in 

 the abdomen and in its passage through the 

 diaphragm it is joined by one or two of the 

 superior Inmbars, and by small branches from 

 the diaphragm; in the thorax it receives the 

 intercostals; the seven or eight inferior inter- 

 costals of the right side enter it distinctly ; the 

 corresponding number of the left side some- 

 times join it in a similar manner, but most com- 

 monly they first unite into a trunk, called the 

 left or minor azygos, of which we shall speak 

 presently. The three or four superior inter- 

 costal veins of the right side unite into one or 

 two branches which end in the convexity at the 

 upper extremity of the azygos major, which 

 also receives the right bronchial veins in the 

 same situation, and at a lower point the ceso- 

 phageal ; the latter, like the arteries of the 

 same name, are irregular in number and in 

 situation. 



The left vena azygos, azygos minor, semi- 

 azygos, is smaller, but in other respects similar 

 to the right ; it commences by small branches 

 from the superior left lumbar, capsular, and 

 renal veins, which unite into a delicate vessel 

 that sometimes communicates with the right 

 azygos, and sometimes with the inferior cava ; 

 it then passes through the aortic opening in 

 the diaphragm, or through or external to its 

 left crus in company with the left splanchnic 

 nerve, and ascends along the anterior and left 

 side of the dorsal vertebrae as high as the 

 seventh or eighth ; it then crosses the spine 

 behind the aorta, oesophagus, and thoracic 

 duct, to join the right or great vena azygos. 

 The azygos minor receives the six or seven 

 inferior left intercostal veins, and as it is 

 passing across the spine it is generally joined 

 by a large descending branch which is formed 

 by the confluence of some of the superior of 

 these vessels; the azygos minor also receives 

 the left bronchial veins as well as some branches 

 from the diaphragm, oesophagus, and medias- 

 tinum. In some subjects this vein is wanting; 

 in such cases the left intercostals join the proper 

 azygos either individually, or by two or three 

 uniting into a large branch. 



The intercostal veins are eleven or twelve in 

 number on each side ; in their course and dis- 

 tribution they correspond to the intercostal 



