MUSCULAR CONTRACTION 3 



the mechanical response rises to a maximum ; and (iii) a period 

 in which the mechanical response falls off, i.e. the muscle 

 relaxes. If a sufficient interval elapses between the completion 

 of relaxation and the application of a second stimulus, the 

 contraction curve traced out by the response to the latter will 

 be identical with the first. If, however, we record the response 

 to a succession of stimuli of equal strength sent in successively 

 at equal intervals very soon after relaxation is complete, the 

 period of relaxation becomes progressively more prolonged, and 

 the height to which the lever rises at each contraction gradually 

 falls off. Thus we must add to the above a fourth phase, the 

 recovery period, during which the muscle is restored to its 

 original condition. When a second stimulus is applied to a 

 muscle before it has completed the process of relaxation, the 

 contraction due to the second starts from the level at which the 

 previous one happens to be when the new one comes into 

 operation ; if the process is repeated a summation results so 

 that the height of the combined contraction is greater than that 

 of a single twitch. But each stimulus produces less increase 

 than its predecessor, a Hmit being soon attained when further 

 succession of stimuli only permits a maximal level to be main- 

 tained. This maximum depends partly on the frequency with 

 which the stimuH are sent in, i.e. how far the contraction due 

 to one stimulus has progressed before that due to the next 

 starts. Prolonged contractions of this kind correspond to the 

 dehberate and sustained movements of everyday life, and are 

 described by the term tetanus. 



In seeking for light on the way in which the muscular 

 mechanism works it will be best to consider separately the 

 energy changes which occur when a muscle is excited. There 

 are, in addition to the mechanical response itself (which can 

 only occur if the muscle is permitted to shorten), chemical, 

 thermal, and electrical phenomena which appear whenever the 

 muscle is excited with an adequate stimulus. Since we are 

 ultimately concerned to explain the origin of the mechanical 

 response, it is of the utmost importance to have in the first 

 place some means of treating the mechanical energy of con- 

 traction with quantitative accuracy. Though this might at 



