154 ELEMENTS OF MAMMALIAN ANATOMY. 



and are usually full of blood, while the arteries are 

 empty (Fig. 74). This is due to the fact that the thick 

 muscular coat of the arteries, by its contraction tends 

 to drive the blood into the veins, whose muscular coat 

 is very thin. The three coats composing the walls of 

 the veins are the epithelial, or tunica intima; the middle, 

 or muscular; and the tunica adventitia, or outer elastic 

 coat of fibro-areolar tissue. The veins of the central ner- 

 vous 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 head possess numerous 

 valves. These valves are formed by 

 semilunar folds of the epithelial coat, 

 strengthened by fibrous tissue (Fig. 



75). 



The main deep veins of the limbs ac- 

 company the arteries and take the same 

 F IC 75 --VEIN CUT names as the arteries. A superficial set 

 OPEN. ARROW of veins is present also in the limbs. The 



SHOWS THE Dl- 1 ~ . - ,11 i 



RECTION OF THE l ar ge superficial vein on the lateral aspect 

 BLOOD. O f th e forelimb is the cephalic. The 



v, Semilunar valves; <- . , . ... 



i, free edge of the superficial vein extending along the 

 valve - mesal 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 ventricle. The 

 veins received by the postcava are thirteen in number. 

 The phrenic veins collect the blood from the diaphragm 

 and empty into the vena cava immediately caudad of 

 the diaphragm. The several hepatic veins collect the 

 blood from the liver, which must be partly dissected 



