Inferior thyroid generally described as consisting of two principal trunks, one for each side, these veins very often con sist of as many as four or more branches, which, arising from a plexus of veins in the interior of the thyroid gland, emerge from its lower border and descend on the front of the trachea, covered by the deep layer of fascia and by the infra-hyoid muscles, to open into the left vena innominata. These veins lie in a well-defined triangular space of much sur gical importance, which is bounded on the one side by the innoininata, and on the other by the left carotid artery, whilst its third side, which represents the base of the triangle, is defined by the lower margin of the thyroid body : the apex is constituted by the con vergence of the arteries which form its sides, to the arch of the aorta ; and in this latter direction, the space above described is some, what encroached on by the left vena inner. minata. Sometimes the thyroid veins of op posite sides unite to form an arch across the trachea, and in some few instances they open into the transverse branch of communication which sometimes unites the anterior jugular veins of opposite sides.
Internal mammary veins.—These, which constitute " venm comites" to the trunk of each internal mammary artery, are formed by branches which correspond with those which emanate from that vessel. Very frequently the two vessels unite in a single trunk, which ends, as has been stated, in the vena inno minata on the left side, and in the vena cava on the right.
Vena cava superior.—This great vein re ceives all the blood from the head and neck, and from the upper extremities; it is formed by the union of the two brachio-cephalic veins, and extends from the lower margin of the cartilage of the first rib on the right of the sternum, to the upper and posterior part of the right auricle of the heart where it terminates. Shortly after its formation, the superior vena cava enters the pericardium. That portion of the vein which is without the pericardium (and which is of very small extent) is invested by a sheath of the fibrous layer of that membrane, whilst its intra-pericardial stage is completely invested by the serous membrane, which it serves to convey to the surface of the heart. In its entire course this vein describes a curve, the convexity of which is related to the right pleura with the inter vention of the pericardium, whilst the con cavity touches the aorta ; the pericardium is anterior to it, and the pulmonary artery of the right side passes behind it. The principal collateral branch of the thoracic vena cava is the great azygos vein, which joins that trunk at the moment of its entrance into the cavity of the pericardium.
Azygos veins:*— The azygos veins and their tributaries (which are described by Cruveil bier amongst the superficial rachidian veins) constitute a remarkable system which is of great importance, as it assists in maintaining the circulation through the spinal system of veins, and at the same time connects the branches of the superior with those of the inferior vena cava.
The veins of the azygos system usually consist of three principal trunks, the greater and lesser azygos veins, and the left superior intere stal vein.
The vena azygos major commences in the lumbar region, by the union of several branches from the upper lumbar veins, through which it communicates with the inferior vena cava. Sometimes the azygos vein receives a branch directly from that vessel, and frequently another from the renal vein.
After a very short stage in the abdomen, the great azygos vein enters the thorax through the aortic canal in the diaphragm, and ascends on the vertebral column to the right of the aorta, from which it is separated by the thoracic duct. Having arrived at the third dorsal vertebra, the vein now changes its di rection, and passing forwards mid to the right side, leaves the posterior mediastinum, arches over the right bronchus, and descends to ter minate in the superior vena cava, where that vessel is about to enter the pericardium.
The vena azygos major is joined by all the intercostal veins of the right side, and by the lesser azygos vein, through the medium of which it communicates with the lower inter costal veins of the left side also. Two or three of the middle intercostal veins of the left side open directly into the great azygos N ein, it likewise communicates very freely with the left superior intercostal vein.
The lesser azygos vein. (or left or inferior azygos vein), likewise has its origin in the lumbar region, from some of the upper lnmbar veins on the left side of the spine ; it anas tomoses with the neighbouring veins which ramify on the surface of the vertebra;, and is connected, more frequently even than the greater azygos vein, with the corresponding renal vein (Bresehet). The lesser azygos vein enters the thorax, either through the aortic canal in the diaphragm, or by per forating its left erns along with the left splanchnic nerve, ascends on the left side of the spine, as high as the fifth or sixth dorsal vertebra, where, crossing the front of the column, behind the aorta, to terminate the great azygos vein. The veins from the five or six lower inter costal spaces terminate in this vessel. In some instances there is no venous trunk cor responding to that above described, in which case the left intercostal veins cross the spinal column behind the thoracic aorta, to join the great vein of the right side, which, under these circumstances only, is appropriately termed " azygos." 'The kft superior intercostal vein (left supe rior azygos vein) is formed by the union of the three or four superior intercostal veins of the left side. Its course is subject to some variety, but it always serves to establish a connexion between the azygos system and the deep veins of the neck. In many instances this vein passes upwards, and opens into the left vena innominata, but more frequently it descends, increasing in size as it approaches its termi nation in the great azygos vein. Sometimes the left superior intercostal vein terminates in the azygos minor.