Metazoa Ran a. 



28; 



in the presence (in most veins) of valves or pocket-like flaps 

 formed by the tunica intima and tunica media. These 

 valves are so arranged that a current of blood flowing in the 

 normal direction into the heart encounters no opposition, 



FIG. 150. TRANSVERSE SECTION OF WALL OF A TYPICAL VEIN. (Quain.) 



a, epithelial layer ; l>, elastic membrane ; c, tunica media ; d, tunica adventitia. 



whilst a regurgitation from the heart causes the pockets to 

 open and press against each other, and so prevent the back- 

 ward passage of impure blood to the tissues (fig. 151). 



FIG. 151. -VALVES IN A VEIN. (Quain.) 



A, vein cut open ; n, longitudinal section of vein showing the valve in 

 action ; c, the same seen from outside. 



Capillaries are exceedingly delicate vessels, which form 

 the connection between the artery and its corresponding 

 vein, and whose walls have been reduced to a tunica intima of 



