Circulation of the Blood. 63 



caval from the upper and post-caval from the lower part of 

 the body, return the blood to the heart. The veins, like 



Walls of Capillaries 



Fig. 39. Part of Frog's Web (highly magnified). 



the arteries, are smooth inside and elastic (though less 

 elastic than the arteries). They are thinner-walled than 

 the arteries (see Fig. 40) and collapse when empty, 



