ANATOMY OF THE HEART AND BLOOD-VESSELS 303 



while the smaller arteries are highly contractile, in the physio- 

 logical 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 developed layer of connective- 

 tissue corpuscles around it, representing the remnant of the tunica 

 adventitia. These vessels are thus extremely well adapted to 

 allow of filtration or diffusion 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 de- 

 veloped, 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 corresponding 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 possess no 

 valves. Many veins, on the contrary, have such, formed by semi- 

 lunar pouches of the inner coat, attached by one margin and hav- 

 ing 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 c 

 diagram, Fig. 109) presses the valve close 



,,.,,.,, , , FIG. 109. Diagram to 



against the side of the vessel and meets illustrate the mode of ac- 

 with no obstruction from it. Should any 



back-flow be attempted, however, the CUr- and H, the heart end of the 



rent closes up the valve and bars its own 



passage as indicated in the lower figure. These valves are most 

 numerous in superficial veins and those of muscular parts. They 

 are absent in the venae cavae and the portal and pulmonary veins. 

 Usually 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. 



