146 THE MUSCLE CELLS 



(3) " The absolute amount of heat liberated in relaxation increases as 

 the duration of the stimulus increases, up to a certain limit, after which it 

 remains more or less constant." 



(4) The heat developed during relaxation is derived from the mechanical 

 potential energy liberated on excitation. 



A close analogy is drawn by these workers, between the energy changes 

 of muscle and of an accumulator. 



(a) The energy of an accumulator- stored electrical energy is liberated 

 by the application of a " trigger," i.e. by completing the circuit between 

 positive and negative poles. In the same way, the potential energy of 

 muscle is liberated by the application of a stimulus. The chemical changes 

 that accompany the evolution of energy from an accumulator do not 

 involve oxidations just as the initial processes of contraction in muscle 

 do not. 



(&) The amount of energy used in making electrical contact, e.g. de- 

 pressing key, is very small and bears no relation to the amount of energy 

 so liberated from the accumulator, as is the amount of energy required to 

 stimulate a muscle fibre. 



(c) If a suitable machine is in circuit, the energy set free from the accumu- 

 lator will manifest itself as " work " ; if not, it will be converted into heat. 



(d) The moment the key is pressed the current starts in the electrical 

 machine say an electric motor. It passes round the coils of the arma- 

 tures, and their soft iron cores develop attractive powers. The current, 

 however, does not immediately rise to its full value owing to the self 

 induction of the coils, nor do the soft iron cores become magnetised in- 

 stantaneously under the influence of the magnetic field of the coil. Hence, 

 the attraction exerted by the fixed poles on the armatures increases gradually 

 up to a certain limit and then remains constant. These phenomena are 

 similar to the development of mechanical response in a muscle. 



(e) As long as the circulation is complete and the accumulator inex- 

 hausted, the motor will rotate. Roughly speaking, the mechanical response 

 and the length of stimulation are synchronous. 



(/) If the current is left on too long, the accumulator becomes polarised, 

 the current falls off and the magnetic field consequently decreases. After 

 a period of rest, the electromotive force of the storage element rises to its 

 full value and the magnetic field is restored. This may be compared to 

 muscular fatigue. 



(g) On breaking the electrical circuit the magnetic field of the armature 

 diminishes rapidly, though not instantaneously, to zero, and the motor 

 slows down and stops. This process is analogous to relaxation. . 



(h) The current passing through the coils of the armatures and trans- 

 forming them into magnets, endows them with (magnetic) potential 

 energy which, as we have just seen, is, after a brief latent period converted 

 into the kinetic energy of rotation. If the load on the motor is so great 

 that revolution is prevented, then although tension is developed, no work 

 is done. In the same way, muscle develops elastic potential energy on 

 excitation which may result in the performance of external mechanical 

 work if the muscle be allowed to shorten, but in an isometric contraction, 

 shortening is prevented and no " work " is done. " The work actually 

 performed depends in both cases on the conditions of loading." 



(i) Heat production. 



(i) Developmental. The electrical energy sent through the armature 



