THE V E I X S. 



141 



THE VEINS. 



In structure the coats of veins bear a general resemblance to 

 those of arteries. Thus, they possess an outer, middle, and in- 

 ternal coat. The outer coat is constructed of areolar tissue 

 like that of the arteries, but is thicker. In some veins it con- 

 tains muscular fibre-cells. 



The middle coat is considerably thinuer 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 

 near the heart, namely, the vence eavce and pulmonary veins, 

 the middle coat is replaced, for some distance from the heart, 

 by circularly arranged striped muscular fibres, continuous with 

 those of the auricles. 



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

 ing coat of an artery, but in other respects resembles it closely. 



The chief influence which the veins have in the circulation, 

 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 semiluuar valves of the aorta and pul- 

 monary artery, already described (p. 96) ; but their free mar- 

 gins 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 veins, single valves are often met with ; and three or 



Diagrams showing valves of veins. A. Part of a vein laid open and spread out, 

 with two pairs of valves. B. Longitudinal section of a vein, showing the apposition 

 of the edges of the valves in their closed state. C. Portion of a distended vein, ex- 

 hibiting a swelling in the situation of a pair of valves. 



