THE CIRCULATION OF THE BLOOD. 187 



arteries, but is thicker. In some veins it contains muscular fibre-cells, 

 which are arranged longitudinally. 



The middle coat is considerably thinner than that of the arteries; it 

 contains circular unstriped muscular fibres, mingled with a large pro- 



Fig. 161. Diagram 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, exhibiting a swelling in the situation of a pair 

 of valves. 



portion of yellow elastic and white fibrous tissue. In the large veins, 

 near the heart, namely the vence cavce 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. 



Fig. 162. A, vein with valves open. B, vein with valves closed: stream of blood passing off by 



lateral channel. (Dalton.) 



The internal coat of veins consists of a fenestrated membrane, which 

 may be absent in the smaller ones, lined by endothelium. 



Valves. The chief influence which the veins have in the circula- 

 tion, is effected with the help of the valves, contained in all veins sub- 

 ject to local pressure from the muscles between or near which they run. 



