ABSENCE OF THE VENA CAVA INFERIOR 397 
ferior?” Dwight believed that the vein which extends through 
the diaphragm to empty into the lower part of the atrium is the 
inferior cava, regardless of the arrangement of its abdominal 
tributaries. If the vein, as in certain abnormal cases, receives 
only the branches from the liver, and if this vein is called the 
common hepatic vein (following Kaestner, ’00), then consist- 
ency requires that in normal cases the cava be described as emp- 
tying into the common hepatic vein and not into the heart. 
The work of Lewis, however, on rabbits has fully established the 
importance of the subcardinal veins and the essential nature of 
their anastomosis with the sinusoids of the liver in the formation 
of the inferior vena cava. In fact, this seems to be the crux of 
the whole matter. If this anastomosis takes place, a cava is 
present; if it does not, then a cava is wanting, even though the 
other parts be present in proper sequence from heart to pelvis. 
In this sense only, the embryo about to be described is consid- 
ered a case of absence of the vena cava. 
The subeardinal veins may play a very important part in the 
production of anomalies, as shown by Johnson (10), while con- 
siderable importance is attached by Huntington and McClure to 
the supracardinal veins in the production of various anomalies 
in the cat. There seem to be, therefore, a number of channels 
through which blood may drain from the pelvic limbs and ab- 
dominal parietes in order to reach the heart, namely, the cardinal 
veins, the subcardinal anastomosis with hepatic sinusoids, and 
the supracardinal veins, to which must be added their derivatives, 
the azygos veins (Parker and Tozier, Sabin). 
To facilitate the interpretation of conditions in the embryo 
the azygos system has been studied in a series of young pigs after 
birth and in older embryos by means of injections and dis- 
sections. Figure 1 shows one such preparation drawn as seen 
from the left side. The vena hemiazygos is seen to lie somewhat 
to the left and behind the aorta throughout the greater part of 
its course, receiving the lateral wall veins from both sides. The 
connection with the vena cava at the lower end of the hemiazygos 
(X in figure) is not usual, but does occur. At the upper end the 
vein turns sharply forward, passing to the left cf the aorta and 
over the root of the left lung where it turns downward in close 
