32 



SCIENCE PRIMERS. 



l§ HI. 



how the arm is bent. The biceps muscle contracts 

 and shortens, tries to bring its two tendinous ends 

 together. The upper tendons, being fastened to the 

 fixed shoulder-blade, cannot move; but the lower 

 tendon is fixed to the radius \ the radius, with the 

 ulna to which it is fastened, readily moves up and 

 down on the elbow-joint — the shape of bones in the 

 joint and all the arrangements of the joint, as we have 

 seen, readily permitting this. When the muscle, then, 

 pulls on its lower tendon, its pulls on the radius at 

 the point where the tendon is fastened on to the bone. 

 The radius thus pulled on forms with the ulna a 

 lever of the third order, working on the end of the 

 humerus as a fulcrum ; and thus as the tendon is 

 pulled the fore-arm is bent 



17, But now comes the question. What makes the 

 muscle shorten or contract? You willed to move 

 your arm, and moved it, as we have seen, by making 

 the biceps contract ; but how did your will make 

 the biceps contract ? 



If you could examine your arm as you did the leg 

 01 the rabbit, you would find running into your 

 biceps muscle, one or more of those soft white threads 

 or cords, which you have already learnt to recognize 

 as nerves. 



These nerves seem to grow in.^ and be lost in the 

 biceps muscle. We need not follow them any 

 further in that direction, but if we were to trace them 

 in the other direction, up the arm, we should find 

 that they soon meet with other similar nerves, and that 

 the several nerves joining together form stouter and 

 thicker nerve-cords. These again join others, and 

 so we should proceed until we came to quite stoutish 



