226 THE HITMAN BODY. 



ries are highly contractile, in the physiological sense of the 

 word. 



Structure of the Capillaries. In the smaller arteries the 

 outer and middle coats gradually disappear, and the elastic 

 layers of the inner coat also go. Finally, in the capillaries 

 the lining epithelium alone is left, with a more or less de- 

 veloped layer of connective-tissue corpuscles around it, repre- 

 senting the remnant of the tunica adventitia. These vessels 

 are thus extremely well adapted to allow of nitration or dif- 

 fusion taking place through their thin walls. 



Structure of the Veins. In these the same three primary 

 coats as in the arteries are found: the inner and middle coats 

 are less developed, while the outer one remains thick, and is 

 made up almost entirely of white fibrous tissue. Hence the 

 venous walls are much thinner than those of the correspond- 

 ing arteries, and the veins collapse when empty while the 

 stouter arteries remain open. But the toughness of their 

 outer coats gives the veins great strength. 



Except the pulmonary artery and the aorta, which possess 

 the semilunar valves at their cardiac orifices, the arteries pos- 

 sess no valves. Many veins on the contrary have such, formed 

 by semilunar pouches of the inner coat, attached by one 

 margin and having the edge turned towards the heart free. 

 These valves, sometimes single, oftener in pairs, and rarely 

 three at one level, permit blood to flow only towards the 

 heart, for a current in that direction (as in the upper dia- 

 gram, Fig. 96) presses the valve close against the side of the 



vessel and meets with no obstruction 



A from it. Should any back-flow be at- 



c j~ T^ H tempted, however, the current closes 



up the valve and bars its own passage 

 _ B_ _ as indicated in the lower figure. These 



valves are most numerous in super- 



ficial veins and those of muscular parts. 



the P OI>tal and W\J Veins. Ustl- 



end of the vessel.' ally the vein is a little dilated opposite 



a valve, and hence in parts where the 



valves are numerous gets a knotted look. On compressing 

 the forearm so as to stop the flow in its subcutaneous veins 

 and cause their dilatation, the points at which valves are 

 placed can be recognized by their swollen appearance. They 

 are most frequently situated where two veins communicate. 



