30 SCIENCE PRIMERS, [§ in. 



it was? Evidently the same thing that happened 

 when yc u pinched up and shortened the string which, 

 if you look back you will see, we supposed to be 

 placed very much as the biceps with its tendons is 

 placed. The radius and ulna would be pulled 

 up, the fore-arm would be bent on the arm. 



Now tendons have no power of shortening them- 

 selves, but muscular substance does possess this re* 

 markable power of suddenly shortening itself. Under 

 certain circumstances each soft muscular fibre of 

 which the muscle is made will suddenly become 

 shorter, and thus the whole muscle becomes shorter, 

 and so pulls its two tendinous ends closer together, 

 and if one end be fastened to something fixed, and 

 the other to something moveable, the moveable thing 

 will be moved. 



This way that a muscle or a muscular fibre has 

 of suddenly shortening itself is called a muscular 

 contraction. All muscled, all muscular 

 fibres, have the power of contracting. Now 

 a mass of substance like the biceps might grow 

 shorter in two ways. It might squeeze itself together 

 and become smaller altogether, it might squeeze itself 

 as you would squeeze a sponge into a smaller bulk. 

 Or it might change ks form and not its bulk, becoming 

 thicker as it became shorter, just as you might by 

 pressing the two ends together squeeze a long thin 

 roll of soft wax into a short thick one. It might get 

 shorter in either of these two ways, but it does actually 

 do so in the latter way ; it gets thicker at the same time 

 that it gets shorter, and gets nearly as much thicker 

 as it gets shorter. And that is why, when you put 

 your hand on the arm which is being bent, you feel 



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