THE CIRCULATION 



163 



36. The Veins. The vessels by which the blood returns to 

 the heart are the veins. At first they are extremely small; 

 but uniting together as they advance, they constantly 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 comparatively inelastic. They are more numerous, 



FIG. 43. VEIN WITH VALVES 

 OPEN 



FIG. 44. VEIN WITH VALVES CLOSED ; 

 THE STREAM OF BLOOD PASSING OFF 

 BY A LATERAL BRANCH 



and communicate 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 membrane, 



ninety-six times ; of a tapir, forty-four times ; of a horse, forty times ; of 

 a wolf, forty -five times ; of a fox, forty-three times ; of a bear, thirty- 

 eight times ; of a monkey, forty -eight times ; of an eagle, one hundred 

 and sixty times. It was impossible to determine the beatings of the 

 elephant's pulse. A butterfly, however, it was discovered, experienced 

 sixty heart pulsations in a minute." 



36. What are the veins ? How do they form ? What do they resemble ? 



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



