THE VASCULAR SYSTEM 1 65 



metatarsals to the deep plantar region, where it receives 

 an anastomosing branch from the saphenous forming the 

 plantar arch, and sends off branches to the digits. {Aa digitales 

 plantares.) 



THE VENOUS SYSTEM 



The veins are the vessels returning the blood to the heart. 

 As a rule, veins carry only impure blood, but the pulmonary 

 veins returning blood from the lungs to the left atrium carry 

 pure blood. After death the veins can readily be distinguished 

 from the arteries by the fact that they have much thinner walls 

 than the arteries and are usually full of blood, while the arteries 

 are empty (Fig. 82). 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 epi- 

 thelial, or tunica intima; the middle or muscular; and the tunica 

 adventitia, or outer elastic coat of fibro-areolar 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. 83). 



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 or atrium. The veins received 

 by the inferior vena cava are thirteen in number. The phrenic 



