MOVEMENTS.] 



PHYSIOLOGY. 



VI 



you give the string, and that is brought to bear on 

 your lever at the point where the string is fastened to 

 the radius, ue, nearer the fulcrum than the point 

 where the weight is applied ; and you know that when 

 you have the fulcrum at one end and the power 

 ^between the fulcrum and the weight, you have a lever 

 )f the third order. 



Now, in order to make the thing a little more like 

 rhat takes place in your own arm, instead of boring 

 fa hole through the humerus, let the string glide in a 

 [groove which you will see at the top of the humerus, 

 and fasten the end of it to the shoulder-blade or any- 

 thing you like above the humerus, and let the string 

 [be jusflong enough to let the arm be quite straight- 

 jned out, but no longer, so that when the arm is 

 rtraigbt the string is just about tight, or at least not 

 loose. 



Now shorten the string by pinching it up into a. 

 [loop. Whenever you do this you will bend the fore- 

 arm on the arm. Suppose you used a string which 

 [you had not to pinch up, but which, when you pleased, 

 [you could make to shorten itself. Every time 

 |it shortened itself it would pull the fore- 

 farm up and would bend the arm — and every 

 time it slackened again, the arm would fall 

 back into the straight position. 



In your arm there is not a string, but a body, placed 

 I very much as our string is placed, and which has the 

 power of shortening itself when required. Every time 

 it shortens itself it bends the arm, and when it has 

 done shortening and lengthens again, the arm falls back 

 into its straight position. This body which thus can 

 shorten and lengthen itself is called a muscle. 



