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THE POPULAR SCIENCE MONTHLY. 



sectional area. The capillaries have a sectional area several times 

 that of the aorta, the purpose of this being to delay the blood at the 

 time it is broucjht into most intimate contact with the tissues. 



The walls of the capillaries are of extreme tenuity, and easily per- 

 meable under the physical action called osmosis. Even the corpuscles 

 can pass outward through the walls. 



Fig. 7. Diagram of the Four Cavities of the Human Heart, o rf, right auricle ; n d, right 

 ventricle ; o g, left auricle ; v g, left ventricle. The arrows indicate the course of the blood. 



To what degree the heart is aided by other forces is yet a matter of 

 investigation. Probably there are several forces assisting. The elas- 

 ticity of the arteries increases their carrying capacity. They are firm, 

 elastic tubes, which expand under the pressure from each heart-con- 

 traction, and then by their own elasticity contract and help the on- 

 ward flow of the blood. In the smaller arteries the flow loses the in- 

 termittent character it possesses in the larger arteries, and becomes a 

 steady stream. The elasticity of the arteries serves precisely the same 

 purpose as the air-chamber of any force-pump, that of equalizing the 

 flow, and so increasing the amount delivered. The whole force is de- 

 rived from the heart ; the arteries cause the force to act continuously. 



The veins are lax tubes, somewhat larger than the arteries, and 

 capable of holding all the blood of the body. They convey the same 

 amount of blood as the latter, but more slowly. In the larger veins, 

 however, near the auricles, the velocity may be two hundred millimetres 

 per second. They are provided with valves which effectually prevent 

 the blood from flowing l^ackward toward the heart. Any compres- 

 sion, produced by muscular contraction, or otherwise, will therefore 

 assist the forward flow of venous blood. This is one explanation why 



