CIRCULATION OF THE BLOOD. 



31 



Capillary Plexus in a Portion of the Web of a Frog's Foot (magnified 110 diameters) : 1, 

 trunk of vein ; 2, 2, 2, its branches; 3, 3, pigment-cells. 



firm and elastic ; veins collapse, while arteries remain open when 

 not distended by blood. Valves occur in most of the veins ; these 

 are so placed as to prevent the blood from tending to flow back- 

 ward. The valves are so placed as to aid the onward progress of 

 the blood in the veins, the pressure of neighboring muscles forcing 

 forward the blood, which cannot regurgitate past the valves. 



What is the relative area of arteries, capillaries, and veins ? 



With the division of the arteries into branches the sectional 

 area of the branches is greater than of the stem. Of the veins 

 the same is true, while the total sectional area of the capillary 

 system is much greater than of either. For purposes of simile the 

 comparison may be made of two funnels placed base to base. In 

 numbers one may consider the sectional area of aorta v as 1 ; of 

 vense cavae, '2 or 3 ; of capillaries, (about) 800. 



