172 



ANATOMY FOR NURSES 



[Chap. X 



the side of the vein, and the free edge points toward the heart. 

 Their function is to keep the blood flowing in the right direction, 

 i.e. toward the heart, and prevent regurgitation. 



Should the blood on its onward course toward the heart be for 

 any reason driven backward, the refluent blood, getting behind 

 the wall of the vein and the flaps of the valve, will press them 

 inward until their edges meet in the middle 

 of the channel and close it up. 



The valves are most numerous in the 

 veins where regurgitation is most likely to 

 occur, i.e. the veins of the extremities. For 

 the same reason a greater number are found 

 in the lower than in the upper limbs. They 

 are absent in many of the small veins, in 

 the large veins of the trunk, and in veins not 

 subjected to muscular pressure. The veins, 

 like the arteries, are supplied with both 

 blood-vessels and nerves ; the supply, how- 

 ever, is far less abundant. 



It must be remembered that although the 

 arteries, capillaries, and veins have each the 

 distinctive structure above described, it is at the same time diffi- 

 cult to draw the line between the smaller artery and larger capil- 

 lary; and between the larger capillary and smaller vein. The 

 veins on leaving the capillary networks only gradually assume 

 their several coats, while the arteries dispense with their coats in 

 the same imperceptible way as they approach the capillaries. 



Fiu. 108. — Diagram 

 SHOWING Valves of 

 Veins. A, part of a vein, 

 laid open, with two pairs 

 of valves ; B, longitudinal 

 section of vein, showing 

 valves closed. (Sharpey.) 



LYMPH VASCULAR SYSTEM 



As the process of transudation described on page 153 is con- 

 tinual, it follows that oedema would result from the accumulation 

 of lymph if some system of drainage were not provided. This 

 drainage system is provided by the lymph vascular system. 



Lymph spaces 



Lymph Vascular 

 System 



Lymph vessels < 



Lymph nodes 



LjTiiph capillaries. 



L\Tiiphatics. 



Thoracic duct. 



Right hanphatic duct. 



Lacteals. 



Serous sacs. 



