40 CONTRACTILITY 



With each succeeding stimulus the height of contraction 

 continues to increase, but the increase becomes progressively- 

 diminished until a constant level is reached. As the 

 interval between the stimuh is diminished, the individual 

 curves become more completely fused until all distinction 

 between them is lost. This is known as tetanus. 



The question now arises whether a sustained voluntary 

 contraction is due to the reception by the muscle of a series 

 of interrupted stimuh from the central nervous system, or 

 to some kind of constant stimulus which we cannot imitate 

 experimentally. If non-polarisable electrodes be placed 

 on the forearm aitd connected with a string galvanometer, 

 on contraction of the flexor muscles the instrument will 

 show a response at the rate of about 50 per second. Volun- 

 tary contraction is therefore a form' of tetanus, and is due 

 to the arrival of frequently repeated stimuh from the 

 spinal cord. 



The Constant Current. — When a muscle is stimulated 

 with the constant current, a single contraction occurs at 

 make of the current and to a lesser extent at break. During 

 the passage of the current there is usually no contraction. 

 Change in the current, then, and not the current itself, is 

 the effective stimulus. We shall study this more closely 

 in the case of nerve. 



2. The Development of Tension 



Paradoxical as it may seem, shortening is not an essential 

 part of muscular contraction. When a muscle is made to 

 pull against a weak spring the tension of which is approxi- 

 mately constant whatever its length, the contraction is said 

 to be isotonic. Under these circumstances the muscle 

 undergoes its maximum shortening, and energy is hberated 

 in the form of work and heat. But if the muscle be made 

 to pull against a strong spring practically no shortening 

 will occur. Yet the muscle has undergone a profound 

 change of state^i^ has developed tensian. This is known 

 as an isometric contraction. The difference between the 



