THE LYMPH VESSELS lU) 



coats. The structure seems to be influenced by gravitational diffi- 

 culties, so there appears to be more muscle in veins of the extremities 

 than in those nearer the heart. 



Veins differ from arteries in that all except very small veins 

 possess valves which are pocket-like extensions of the intima with 

 a core of subendothelial connective tissue. (Fig. 70.) Such valves 



r^-iSr^;/ Adventitia 

 Fig. 70. — Diagram of a vein showing a valve. 



open toward the heart so that a backward flow of blood is pre\'ented. 

 In the connective tissue that supports an artery there is usually a 

 companion vein which has a greater volume and usually appears 

 collapsed in microscopic preparations. In the adventitia of arteries 

 and \-eins, but not in arterioles and ^■enules, there occur blood- 

 vessels called vasa vasorum, W'hich provide a capillary network for 

 the wall tissue of the vessel. Similarly a nerve supply, known as 

 the nervi vasorum, is present. The outer coat of the larger arteries 

 and \'eins supports a system of small lymph vessels also. 



THE LYMPH VESSELS. 



In general, these vessels resemble veins in structure but have 

 thinner walls and more valves. Lymph capillaries are larger than 

 blood capillaries, are thin walled, and not commonly seen in ordinary 

 preparations. In the largest lymphatics, three coats may be differ- 

 entiated. When preserved uncollapsed, the smaller ones appear 

 as endothelial-lined spaces in loose fibroelastic connective tissues. 

 These vessels are larger than the veins whose distribution the\' 



