208 The Alligator and Its Allies 
on account of the large size of that organ, of con- 
siderable caliber. 
The Anterior Vene Cave and their Branches. 
The entrance of the precaval veins into the 
heart was mentioned above; their branches, in 
order from the heart cephalad, will now be de- 
scribed. Since the two precave are alike, it 
will be necessary to describe the branches of only 
one side of the body. After leaving the heart, the 
precava may be traced forward, for a short dis- 
tance, at the side of the trachea and cesophagus, as 
a wide, thin-walled trunk, Fig. 60, vca. The first 
tributaries that it receives are the internal mam- 
mary and vertebral veins, which join it at the base 
of the neck at almost the same place. 
The internal mammary, Fig. 60, im, is a rather 
small vein, bringing blood from the ventral wall of 
the thorax. It may be followed along the inner 
surface of the ribs, near the sternum, in company 
with its corresponding artery. 
The vertebral vein, Fig. 60, v, is also of small dia- 
meter and extends to the dorsal body wall near the 
spinal column, from which region it returns blood 
to the anterior vena cava; it is drawn too large in 
the figure. 
Just cephalad to the vertebral and internal 
mammary, the internal jugular, j, enters the pre- 
cava. The internal jugular may be followed 
directly forward, close to the side of the trachea 
and oesophagus, from which it receives numerous 
