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STUDIES IN ADVANCED PHYSIOLOGY. 



coats give to the arteries much strength and elasticity, 

 while the development of the muscular coat makes contrac- 

 tion and dilatation possible. 



In veins the inner and middle coat are much reduced, 

 but the outer coat of strong, white fibrous tissue is present. 

 For this reason veins have much thinner walls and easily col- 

 lapse when empty, and, of course, have practically no 

 power of muscular contraction and dilatation. However, 

 owing to the strength of the outer coat, they are, neverthe- 

 less, not easily torn. With the exception of the larger veins 

 near the heart, veins have valves in their course. These 

 valves are semilunar folds fastened in such a way as to allow 

 the flow of blood towards the heart, but preventing it in the 



Fig. 79. VALVES IN VEINS. 

 a, laid open to show pockets; b, inner view with valves closed; c, as seen from outside. 



opposite direction. These valves may occur singly, but are 

 more often in pairs, while in some instances three occur at 

 the same point. By compressing the veins near the wrist 

 the positions of the valves in the congested portions may 

 be recognized by the swollen points along the veins. 



Capillaries have all these coats absent, except only the 

 innermost membrane of flattened epithelial cells. This ex- 

 treme thinness of wall, it being only one cell thick, permits 

 readily the phenomena of osmosis and the migration of the 

 white blood corpuscles. 



The boundary lines between arteries and capillaries on 

 the one hand, and capillaries and veins on the other, are 



