CIRCULATION OF THE NUTRIENTS 



113 



distinct throbbing, called the pulse. This is due to the 

 enlargement of the artery at each heart beat followed by 

 subsequent contraction. When an artery is cut, therefore, 

 the blood is forced out in spurts at each contraction of the 

 ventricle. 



159. Structure of arteries. If a piece of the aorta of 

 any animal is examined, it will be found that the blood 

 vessel retains its tubular form, and this is due to the presence 



cells of lining 



membrane 



A = artery. B = vein. 



FIG. 35. Cross section of blood vessels. 



of thick layers of muscular and elastic tissue (Fig. 35). 

 It is the elastic tissue that allows the arteries to expand 

 when more blood is forced into them by the contraction of 

 the ventricles. After each pulse these elastic walls squeeze 

 the blood forward into the capillaries; arteries, therefore, 

 are specially adapted to keep the capillaries full of blood. 



The muscular tissue in the walls of the arteries aids in 

 regulating the size of the arteries, and so determines the rel- 

 ative amount of blood supplied to any given organ. For 

 example, when the face is flushed, the muscles in the arteries 

 have relaxed ; pallor, on the other hand, is due to the con- 

 traction of the muscular walls. N 



