PHYSIOLOGICAL PARADOXES. 



veins. Not because it runs down them the 

 valves prevent that but because in that posi- 

 tion they are filled from their smaller tribu- 

 taries so that they are distended to a larger 

 size by the stretching of their sides. This gives 

 the muscular and fibrous material of which 

 the sides are composed an opportunity of con- 

 tracting when the limb is raised to a natural 

 position, and driving out the excess of blood 

 against the now diminished pressure. 



Thus, all kinds of exercise, movement, and 

 massage contribute to assist the circulation 

 of blood so far as the veins are concerned, and 

 in this way help to promote the general health. 



The veins end in the heart, which may be 

 regarded as a specially enlarged part of the 

 circulation-tubes. As the improvement of the 

 circulation is beneficial to health, so its mainten- 

 ance is essential to life ; and in the heart are 

 provided the means of keeping it up, whether 

 the rest of the body be awake or asleep, in 

 activity or at rest. 



For this purpose it would not do to rely on 

 the chance compressions of exercise or outside 

 force. The means of regular and sufficient 

 compression must be provided on the spot. 

 Accordingly, in this part of the blood-vessels 

 the space capacity is enormously enlarged so 

 as to provide four chambers which answer the 

 same purpose as the cylinders in a pump. And 

 as we cannot havef pistons, the muscular part 



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