THE VASCULAR SYSTEM. 143 
and the tunica adventitia, or outer elastic coat of fibro-areo- 
lar tissue. The veins of the central nervous system and its 
membranes have no muscular coat. While the only valves 
in the arteries are found at their origin from the heart, the 
veins of the limbs, neck, and the head possess numerous 
valves. These valves are formed by semilunar folds of the 
epithelial coat, strengthened by fibrous tissue (Fig. 77). 
The main deep veins of the limbs accompany the arteries 
and take the same names as the arteries. A superficial set 
of veins is present also in the limbs. The large superficial 
vein on the lateral aspect of the forelimb is the cephalic. 
The superficial vein extending along the 
medial aspect of the hind-limb is the 
saphenous. 
The Veins of the Trunk, Head, and 
Neck.—There are two chief venous 
trunks: the precava, or superior vena 
cava, and the postcava, or inferior vena 
cava. Both vessels open into the dorsal 
aspect of the right auricle. The veins gee ee 
received by the inferior vena cava are sHows THE D1- 
thirteen in number. The phrenic veins Bion 
collect the blood from the diaphragm and v, Semilunar_val- 
empty into the vena cava immediately Fine ea a 
caudad of the diaphragm. The several 
hepatic veins collect the blood from the liver, which must be 
partly dissected away to see their entrance into the vena 
cava. The two suprarenal veins return the blood from the 
suprarenal bodies and two renal veins carry the blood from 
the kidneys. The left ovarian or spermatic vein is a tribu- 
tary to the left renal, but the right ovarian empties directly 
into the vena cava. <A pair of small iliolumbar veins col- 
lecting blood from the lumbar muscles empty into the vena 
cava a centimeter or more craniad to the large common 
