34 SCIENCE PRIMERS. [§ iii. 



cord cut across or injured in your neck, you might 

 still live, but you would be paralysed. You might 

 will to bend the arm, but you could not do it. You 

 would know you were willing^ you would feel you 

 were making an effort, but the effort would be unvail- 

 ing. The spinal cord is part of the bridge between 

 the will and the muscle. 



When you bend your arm, then, this is what takes 

 place. By the exercise of your will a some- 

 thing is started in your brain. That some- 

 thing — we will not stop now to ask what that 

 something is — passes from your brain to the 

 spinal cord, leaves the spinal cord and travels 

 along certain' nerves, picking its way among 

 the intricate bundles of delicate nervous 

 threads which run from the upper part of 

 the spinal cord to the arm until it reaches 

 the biceps muscle. The muscle, directly that 

 '^something" comes to it along its nerves, 

 contracts, shortens, and grows thick ; it rises 

 up in the arm ; its lower tendon pulls at 

 the radius ; the radius with the ulna moves 

 on the fulcrum of the humerus at the elbow- 

 joint, and the arm is bent. 



You wish to leave off bending the arm. Your will 

 ceases to act. The something to which your will 

 had given rise dies away in the brain, dies away in 

 the spinal cord, dies away in the nerves, even in the 

 finest twigs. The muscle, no longer excited by that 

 something, ceases to contract, ceases to swell up, 

 ceases to pull at the radius, and the fore-arm by its 

 own weight falls into its former straightness, stretch- 

 ing, as it falls, the muscle to its natural length. 



