THE CIRCULATION 



103 



flow through the muscles, and the muscles are so extensive 

 and use so much blood that the digestion may surfer be- 

 cause of the vigorous exercise. 



170. Massage. So effective and necessary is exercise in 

 aiding the circulation, that some people employ others who 

 are skilled in the art called massage, to come regularly and 

 squeeze and knead every muscle like dough. Thus fresh 

 blood is brought and the removal of waste material from 

 the tissues is aided. If a boy has so-called "growing 

 pains " which may come from over-fatigue, or which may 

 be a light form of rheumatism 



from exposure to wet and cold, 

 his pains can be relieved by 

 thoroughly rubbing the aching 

 part. 



171. The arteries lie deep 

 under the muscles near the 

 bones, and are likewise pressed 

 upon by muscles, but their 

 walls are much stiff er than the 

 walls of the veins. In 

 many of them in passing 

 through the muscles have tough, 



fibrous sheaths. It is well that the arteries are not 

 so much affected by exercise, for if they were squeezed 

 by the contracting muscles, the blood would be pressed 

 backward as well as forward since they are destitute 

 of valves, and this would not be favorable to the 

 circulation. 



172. The Lungs aid the Heart to circulate the blood 

 since when they expand and the air rushes into them, the 

 blood as well is drawn toward the cavity of the chest, 

 and when they contract, the blood tends to leave this 

 ^egion. 



173. Does the elasticity of the arterial wall furnish 



FIG. 94. Vein 

 fact, laid open to 

 show Two 

 Valves. 



FIG. 95. Valve 

 in Vein dis- 

 tended with 

 Blood. 



