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CIRCULATION.] PHYSIOLOGY, »i»f 



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Wh?at makes it move ? 



Suppose you had a long thin muscle, fastened at 

 one end to something firm, and with a weight hanging ' 

 at the other end. You know that every time the 

 muscle contracted it would pull on the weight and 

 draw it up. But suppose, instead of hanging a weight 

 on to the muscle, you wrapped the muscle round a 

 bladder full of water. What would happen then each 

 time the muscle contracted ? Why, evidently it would 

 squeeze the bladder, and if there were a hole in the 

 bladder some of the water would be squeezed out. 

 That is just what takes place in the heart. You have 

 already learnt that the heart is muscuj^. Each cavity 

 of the heart, each auricle, and each ventricle is, so to 

 speak, a thin bag with a number of muscles wrapped 

 round it. In an ordinary muscle of the body, the 

 bundles of fibres of which the muscle is made up are |j 

 placed carefully and regularly side by side. You can Ij 

 see this very well in a round of boiled beef, which is 

 little more than a mass of great muscles running in 

 different directions. You know that if you try to cut 

 a thin slice right across the round, at one part your 

 carving-knife will go " with the grain " of the meat, ]{ 

 i.e. you will cut the fibres lengthways ; at another part 

 it will go " against the grain," />. you will cut the 

 fibres crossways. In both parts, the bundles of fibres 

 will run very regularly. But in the heart the bundles 

 are interlaced with each other in a very wonderful 

 fashion, so that it is very difficult to make out the 

 grain. They are so arranged in order that the mus- 

 cular fibres may squeeze all parts of each bag at the 

 same time. ' ■ 



Each cavity of the heart, then, auricle or ventricle, 



