THE CIRCULATION OF THE BLOOD. 



299 



T-he capillaries serve as a medium for the exchange of material - 

 between the blood and tissues.. It is in this region of the vascular 

 apparatus that the blood 

 fulfils its more impor- 

 tant functions. It is 

 here that the nutritive 

 material passes through 

 the walls of the capilla- 

 ries into the tissue spaces 

 and so into direct contact 

 with the tissue-cells; that 

 the waste products, the 



FIG. 136. CAPILLARIES. THE OUTLINES OF THE 

 NUCLEATED ENDOTHELIAL CELLS WITH THE 

 CEMENT BLACKENED BY THE ACTION OF SIL- 

 VER NITRATE. (Landois and Stirling.} 



bolism, partly pass 



through the walls of .the 



capillaries into the 



blo6d. The capillaries 



fulfil their functions by 



reason of the fact that 



they have not only small 



diameters and thin walls, 



but because they pervade 



and penetrate the tissues 



into their inmost recesses. 



Structure of the Veins. The veins serve to collect the blood 



from the capillary areas and return it to the right side of the heart. 



As they emerge from the capillary areas the veins, which in these 



regions are termed vemiles, are quite small. By their convergence 

 and union the veins gradually increase in size in 

 passing from the periphery toward the heart. 

 Their walls at the same time correspondingly in- 

 crease in thickness. The veins from the lower ex- 

 tremities, the trunk, and abdominal organs finally 

 terminate in the inferior vena cava. The veins 

 from the head and upper extremities terminate in 

 the superior vena cava. Both venae cavae empty 

 into the right auricle. 



A typical vein consists of the same three coats as 

 the artery: viz., the tunica intima, the tunica media, 

 and the tunica adventitia. The media, however, 

 edge of the does not possess as much of either the elastic or 

 vidson} ^ Da ~ rnuscle tissues as tne artery, but a larger amount of 

 ^ ie n b ro us tissue^ Hence they readily collapse 



when empty. In virtue of theirstr'ucture the veins also possess both 



elasticity and contractility, though in a far less degree than the arteries. 



FIG. 137. VALVES 

 OF A VEIN. 

 v. Semilunar 

 value, i. Free 



