The principal vein, which receives the refluent blood, and conveys it to the heart, is thus named, from its size.

The vena cava is generally described as being two; viz. the ascending and the descending; the right auricle receives them both, one at its upper, the other at its lower part.

The superior vena cava receives the blood principally from the thorax, head, and upper extremities, with a very small proportion from the parts below the diaphragm.

The inferior vena cava receives the blood principally from the abdomen and lower extremities, and very little from the parts above the diaphragm.

The ancients called the vena cava superior, the vena cava ascendent; and the vena cava inferior, vena cava descendens.

According to Winslow, who is extremely accurate in his description of the blood vessels, the superior vena cava runs down to the right auricle of the heart, almost in a direct course, for about two fingers' breadth within the pericardium, on the right side of the aorta, but a little more anteriorly. Above the pericardium, it runs down from the cartilage of the first true rib, and a little higher than the curvature of the aorta; here it receives two branches, viz. the right and left subclavian veins. The trunk of this upper vena cava, above the pericardium, to the just named bifurcation, receives anteriorly the vena mediastina, pericardia, diaphrag-matica superior, thymica, mammaria interna, and trachealis. All these are called dextrae. Their fellows on the other side are called sinistrae; they do not fall into the trunk of the vena cava, but into the left subclavian vein. Posteriorly, a little above the pericardium, the trunk of the vena cava receives a capital branch, called vena azygos. It runs down by the vertebrae dorsi, almost to the diaphragm, composed of the greatest part of the venae intercostales and lumbares su-periores.

Hardly a quarter of an inch of one side of the vena cava inferior is contained in the pericardium; from thence it immediately perforates the diaphragm, receiving the venae diaphragmaticae inferiores, orphrenicae: it passes behind the liver, through the great sinus of that viscus, from which it receives venae hepaticae. In this course it inclines towards the spina dorsi and aorta inferior, the trunk and ramifications of which it accompanies all the way to the os sacrum, the arteria coeliaca and the two mesentericae excepted. Arrived at the os sacrum, the two iliacae unite to form its trunk, joined by the hy-pogastricae, and some other branches distributed into the pelvis. Under the ligamentum Fallopii they take the name of crurales, each of which receives the blood from the lower extremities.