THE FLOW OF BLOOD IN THE BODY 



173 



artery causes it to contract, forcing the blood onward in 

 a steady stream. But the artery can exert no more power 

 in contracting upon its blood than the heart exerted in 

 distending the artery, and so it is really the heart's force 

 which propels the blood. 



300. The pulse. The extra distention of the aorta by 

 each systole of the heart produces a wave in the blood 

 which runs along the arterial tubes. Wherever an artery 

 runs near the surface, as in the wrist, the wave may be 

 felt, and is called the pulse. The pulse is not a sudden 

 current of blood shot through the artery, but is a wave 

 in the steady stre'am. By means of the pulse the fre- 

 quency and regularity of the heart beats may 



be determined. When an artery is cut, a 

 continuous jet of blood spurts out to a con- 

 siderable distance, which momentarily in- 

 creases in size with each wave beat. 



301. Capillaries. The smallest arteries 

 suddenly divide into an extremely fine net- 

 work of tubes, called capillaries. Each capil- 

 lary tube is from ^Ao" to soW of an inch in 

 diameter, and from y0W to -^j- of an inch in 

 length. It is composed of the same kind of 

 smooth and flat cells as those which line the 

 arteries ; in fact, the capillaries are the pro- 

 longation of the linings of the arteries. They 



* ing the platelike 

 penetrate the spaces between the cells of the ceils of which it 



body in such a close network that several is com P sed ( x 



500). 

 capillaries may be in contact with each cell, 



and the point of a fine needle cannot be thrust into the 

 body without wounding some. The blood in the capillaries 

 gives the pink tinge to the skin, which disappears when 

 the blood is pressed out. The total capacity of the capil- 



Diagram of a 



