122 Inside the Living Cell 



Royal Society, he said : 



'We are sensible of the most minute changes of muscular exer- 

 tion, by which we know the position of the body and limbs, when 

 there is no other means of knowledge open to us. If a rope-dancer 

 measures her steps by the eye, yet on the other hand a blind man can 

 balance his body. In study, walking and running, every effort of the 

 voluntary power, which gives motion to the body, is directed by a 

 sense of the condition of the muscles and without this sense they 

 could not regulate their actions.' 



He showed that the muscles are provided with sensory nerves, 

 which send out messages indicating their state of contraction and in 

 other ways also indicate what have been the results of the muscle 

 contractions. These messages reach the brain so that 'between the 

 brain and the muscles there is a circle of nerves; one nerve conveys 

 the influence from the brain to the muscle, and this gives the sense of 

 the condition of the muscle to the brain'. (Fig. 24.) 



This method of control is quite common in machines, which are 

 often provided with governors or speed controllers, arranged so that 

 the output of the machine controls the power which is supplied to it, 

 i.e. the input. A good example is the governor of a steam engine. 



r nerve 



FIG. 24. The nervous circle 



which closes a valve when the pressure exceeds a critical value. 



The flow of water through a pipe can be kept constant by making 

 the outflowing water partly close a valve in the inflow when the out- 

 flow speed is too great. Electric motors can easily be arranged with 

 speed controls which cut down the operating current when the speed 

 is too high. The general principle is known as 'feedback', i.e. some of 

 the output is used to control the input. 



The same principle is used in controlling the muscles. The im- 

 pulses received from the sensory nerves stimulate or inhibit the 

 impulses sent out by the motor nerves, and this controls whether 

 the muscle will be stimulated to contract or not stimulated. If the 

 sensory nerve is cut the muscle is as completely paralysed as if the 

 motor nerve is cut. In this way the muscle is kept in a constant state 

 of tension which is required to do the work it has to do. 



