THE VASCULAR SYSTEM. H3 



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 

 received by the inferior vena cava are SHOWS THE Di- 

 thirteen in number. The phrenic veins N 



collect the blood from the diaphragm and v, Semilunar val- 

 empty into the vena cava immediately of S the valve. g6 

 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 



