178 THE CIRCULATION. 



tractile power apart from elasticity, can of themselves 

 exercise no direct influence on the movement of their 

 contents : yet that the constant interchange of relations 

 between the blood and the tissues outside the vessels does 

 in some measure facilitate the movement of blood through 

 the capillary system, and thus constitute one of the assist- 

 ant forces of the circulation. 



THE VEEN'S. 



In structure the coats of veins bear a general resem- 

 blance to those of arteries. Thus, they possess an outer, 

 middle, and internal coat. The outer coat is constructed of 

 areolar tissue like that of the arteries, but is thinner. In 

 some veins it contains muscular fibre cells. 



The middle coat is considerably thinner than that of the 

 arteries ; and, although it contains circular unstriped 

 muscular fibres or fibre-cells, these are mingled with a 

 larger proportion of yellow elastic and white fibrous 

 tissue. In the large veins entering the heart, namely, the 

 venae cavce and pulmonary veins, the middle coat is replaced 

 by circularly arranged striped muscular fibres, continuous 

 with those of the auricles. 



The internal coat of veins is less brittle than the cor- 

 responding coat of an artery, but in other respects, 

 resembles it closely. 



The chief influence which the veins have in the circula- 

 tion, is effected with the help of the valves, which are 

 placed in all veins subject to local pressure from the 

 muscles between or near which they run. The general 

 construction of these valves is similar to that of the semi- 

 lunar valves of the aorta and pulmonary artery, already 

 described (p. 118); but their free margins are turned in 

 the opposite direction, i.e., towards the heart, so as to stop 

 any movement of blood backward in the veins. They are 

 commonly placed in pairs, at various distances in different 

 veins, but almost uniformly in each (fig. 53). In the smaller 



