AZYGOS, 'tryoc, jugum.) The term azygos is applied by anatomical writers to cer tain parts of the human body, which, being situated in or near the mesial line, appear singly, and not symmetrically or in pairs : thus we read of the azygos process of the sphenoid bone, of the azygos uvula muscle, of the azygos artery, vein, &c. This term, however, (strictly speaking,) is seldom very correctly applied, for in the cases of the bony 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.
Az YG OS VEIN, Posterior thoracic, Prelim bo-thoracique, Vena sine pari, 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 vertebra ; it serves to receive the blood from most of the intercostal spaces, from the phrenic, bronchial, and medi astinal veins, as also from the vertebra 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 crus 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.