THE SENSORI-MOTOK SYSTEM 



21 



One example may be given here, that of the bending of the 

 forearm on the upper arm. Four bones are included in this 

 mechanism — the scapula, humerus, radius, and ulna. When we 

 consider only the movement of the forearm on the upper by the 

 elbow- joint, the scapula and humerus are the fixed, and the radio- 

 ulna (considered now as one segment) is the movable part. 



TkncLoos 



oP -_ 

 origin 



-Shoulder blade 

 JJead of humerus 



-Jiuirierus 



J^/ex or muscle 

 (biceps) 



2^x ten so r muscle 

 ( Triceps) 



Tendons 



oF 



/nsertion 



-l/fncL 

 Jiaclius 



Fig. 5. — Diagram of the Muscles that Move the Foreakm. 



Two muscles, we see, are necessary for the movement — the 

 biceps, which is the bending (or flexor) muscle, and the triceps, 

 which is the straightening (or extensor) one. In each case the 

 muscle consists of the tendons of origin (those which are attached 

 to the fixed bone), the belly or contractile part, and the tendons of 

 insertion (which are attached to the movable bone). When the 

 radio- ulna is to be bent, or flexed, on the upper arm the biceps 

 contracts, or shortens (that is, it becomes thicker and shorter), and 

 simultaneously the triceps relaxes, or lengthens (that is, it 

 becomes thinner and longer). By the pull of the tendon of 

 insertion of the biceps the radio- ulna is bent on the upper arm, 

 and by the pull of the tendon of insertion of the triceps it is 

 drawn back again into a straight line with the humerus. It is 

 important to note that the two muscles are antagonistic ones, and 

 that the flexion of one of them is always associated with the 



