THE FLOW OF BLOOD IN THE BODY 183 



DEMONSTRATIONS 



73. The flow of blood in the veins and the action of the valves of 

 the vein can be shown by placing a finger upon a vein in the skin upon 

 the back of the hand. Then press out the blood by running another 

 finger a few inches up the vein. When the second finger is removed, 

 notice that the blood does not return in the vein, for the valves stop 

 the backward flow ; but if the first finger is removed, the vein at once 

 fills up. This is one of the proofs which Harvey used to prove the 

 circulation of the blood. 



74. The position of the main arteries upon the limbs should be 

 shown upon the body. Remember that they are usually over the middle 

 of a joint upon the side toward which it can be bent. Explain that 

 wherever a beating can be felt there is a pulse and an artery. 



75 . Examine an artery and vein prepared for the microscope. Notice 

 its smooth and thin inner layer puckered because of the contraction of 

 its outer coats. Next is the muscular layer, each cell wrapped around 

 the tube. The next and outermost layer is composed of connective 

 tissue. Notice that the main difference between the artery and the 

 vein is that the artery is thicker. 



76. Tie a string or a rubber band rather tightly around the finger. 

 Notice that in a few minutes the finger becomes purple, cold, swollen, 

 and painful. Explain that the string does not exert enough pressure 

 to close the thick arteries which are under high pressure, but that it 

 readily closes the veins. 



77. Show the capillary circulation in a frog's foot. Place the frog 

 in a covered glass of water to which a teaspoonful of ether has been 

 added. When it ceases to move, spread its web over a hole cut in card- 

 board. A ring of dried mucilage will hold it in place. Examine it 

 under a microscope with a magnifying power of about 200 diameters. 

 Oval cells will be seen shooting through a network of capillaries. The 

 tail of a small fish also will show the circulation. 



REVIEW TOPICS 



i. Describe the tubes which conduct blood to the cells 

 of the body, their structure, situation, arrangement, 

 action, and pulse. 



