MUSCULAR CONTRACTION 



61 



thing takes place as when we raise our own fore-arm. This 

 action is performed 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 



