MUSCULAR CONTRACTION 



61 



when we raise our own fore-arm. This action is per- 

 formed by means of the biceps muscle which arises from 

 the scapula and is inserted into the fore-arm. When 

 the latter is raised we feel a lump rise on the front of 

 the upper arm due to the thickening of the biceps. 



FIG. 17. Diagram of apparatus for demonstrating the contraction of the gastro- 



cnemius muscle. 



A, upright, bearing two adjustable horizontal arms. To the upper of these (B) is 

 fixed by a clamp the femur (fe), having the gastrocnemius (gstr) in connection 

 with it. To the lower arm (C) is fixed a light lever (L) movable in a vertical 

 plane, and having the tendon of insertion of the muscle attached to it by a thread. 

 The dotted lines show the form of the gastrocnemius and the position of the lever 

 during contraction of the muscle, sc. nv. the sciatic nerve. 



This shortening and thickening of the muscle is 

 termed a contraction. Do not fail to notice that this 

 word is used in a special sense. When we say that a 

 red-hot bar of iron contracts on cooling, we mean that 

 it becomes smaller in all dimensions undergoes an 

 actual decrease in bulk. But in muscular contraction 

 there is no alteration in bulk : the decrease in length is 



