CAPILLARY VESSELS. 95 



made between the heart and the wound ; if a vein, the 

 wound must be between the heart and the pressure. 

 But I confess, I know not how I should ascertain wheth- 

 er it was a vein or an artery that was wounded. 



j) rf B. When blood flows from an artery, it is by 

 jets, like air from the nose of the bellows ; but from a 

 vein, it is in a steady stream. The superficial veins 

 however, which are the ones most frequently injured, 

 are so small that they seldom give much trouble, or 

 when cut oft, they contract and thus prevent the escape 

 of blood. 



Emily. I do not yet understand enough of the sub- 

 ject to see why a physician, when he bleeds a person, 

 always puts a cord around the arms just above where 

 the opening is to be made. 



Dr. B. By so doing, the blood is prevented from 

 passing along to the heart, and accumulates below the 

 ligature, so that when an opening is made, it gushes out 

 very freely. If no ligature were put round the limb, a 

 little blood it is true, would escape from the opening, 

 but the greater portion would pass by towards the heart. 

 Hence, if after the physician is gone, the bandage should 

 get loose, and the blood start, afresh, you would stop it 

 by putting the cord around the limb on the other side of 

 the opening. But it is time for us to see how the biood 

 is affected in its passage through the lungs. 



Emily. But first let me ask what purpose is an- 

 swered by the blood's going to every part of the body, 

 if it passes from the arteries directly into the veins with 

 all possible speed. 



Dr. B. The minute arteries do not terminate direct- 

 ly in the veins at least, all of them do not but be- 

 tween these two sets of vessels, we find a third set form- 

 ing a most delicate and intricate net work, and from 

 their extreme minuteness, are called capillary vessels. 

 Though they communicate with both arteries and veins, 

 yet from their infinite division and ramification, and their 

 peculiar powers, they require us to consider them as an 



