THE CIRCULATION. 11 



which is nearly perpendicular coincides with the contrac- 

 tion of the ventricles ; while the wavy portion marks their 

 dilatation. 



36. The Veins. The vessels which convey the blood 

 on its return to the heart are the veins. They begin in 

 the several organs of the body, and at first are extremely 

 small; but uniting together as they advance, they con- 

 stantly increase in size, reminding us of the way in which 

 the fine rootlets of the plant join together to form the 

 large roots, or of the rills and rivulets that flow together 

 to form the large streams and rivers. In structure, the 

 veins resemble the arteries, but their walls are compara- 

 tively inelastic. They are more numerous, and commu- 

 nicate with each other freely in their course, by means of 

 interlacing branches. 



37. But the chief point of distinction is in the presence 

 of the valves in the veins. These are little folds of mem- 

 brane, disposed in such a way, that 



they only open to receive blood 

 flowing toward the heart, and close 

 against a current in the opposite 

 direction. Their position in the 

 veins on the back of the hand may 



be readily observed, if we first ob- pio 3( _ THF VALVBR op THE 

 struct the return of blood by a cord VEINS. 



tied around the forearm or wrist. In a few minutes the 

 veins will appear swollen, and upon them will be seen 

 certain prominences, about an inch apart. These latter 

 indicate the location of the valves, or, rather, they show 

 .that the vessels in front of the valves are distended by the 

 blood, which cannot force a passage back through them. 



38. This simple experiment proves that the true direction 

 of the venous blood is toward the heart. That the color 



36. What are the vein* ? How do they form ? What do they resemble ? 



37. Valves in the veins ? What are they ? Their position ? Experiment with 

 the cord ? 



38. What will be proved by the experiment ? What inference is drawn ? 



