70 PHYSIOLOGY FOR BEGINNERS CHAP. 



in the muscle till they ultimately send a single nerve fibre to 

 each individual muscle fibre. 



Turn the frog on its belly. Separate the muscles on the back 

 of the thigh ; you will find a nerve running down the thigh to the 

 leg, a white strand giving off a few branches as it goes. It is a 

 little on the inner side of the middle of the back of the thigh. 

 Pinch the nerve, the muscles below the spot will contract ; this 

 will be especially clear with the large muscle forming the calf. 

 The nerve may be similarly excited by a hot wire, a drop of acid, 

 or by electricity, and the muscles below will contract, that is to 

 say, provided that you have not injured the nerve below the place 

 at which you are exciting it. If the nerve going to a muscle 

 is pricked, pinched, or otherwise stimulated as it is called, 

 the muscle gives a contraction, which may be a sudden twitch 

 and soon over, or if the stimulation is continued, may last some 

 little time. No visible change is produced in the nerve, but 

 some influence passing from particle to particle along it, travels 

 quickly from the place of stimulation down to the muscle. 

 This change which runs along the nerve is called a nervous 

 impulse. The skeletal muscles performing the movements of 

 the body are only put into action when they receive impulses 

 coming along nerves going to them. The nerves which supply 

 muscles and put them into action are called motor nerves. 

 The nerves, as we have seen, proceed from the brain or the 

 spinal cord. Nerves transmit impulses along their substance 

 but do not originate them, and the impulses which the motor 

 nerves carry when movements of the body are performed have 

 their origin in the brain or in the spinal cord. 



The Mechanism of Movement 



In performing movements, one part of the body is moved 

 towards, from, or round another part kept at rest. In a large 

 number of movements those involving a joint the bone 

 of the moving part acts as a lever, turning about that part 

 of itself which is within the joint concerned. A lever is 

 a bar which can be moved about a fixed point in its length. 

 This fixed point is called the fulcrum. In a crowbar turning over 

 a wedge (as in I. Fig. 34), the fulcrum is at the wedge on the 

 ground, and much nearer to the weight to be raised than to 



