524 MUSCLE-NERVE PHYSIOLOGY 



point of stimulation of the second one corresponds to the crest of the con- 

 traction of the first, a second curve, figure 330, will occur, which will commence 

 near the highest point of the first and will rise nearly as much higher, so that 

 the sum of the height of the two curves almost exactly equals twice the height 

 of the first. This phenomenon is called summation. If a third and fourth 

 shock be passed, a similar effect will ensue, and curves one above the other 

 will be traced, the third being slightly lower than the second, and the fourth 

 than the third. If a continuous series of shocks occur, however, the lever 

 after a time ceases to rise any further, and the contraction, which has reached 



FIG. 330. Tracing of a Double Muscle Curve. To be read from left to right. While 

 the muscle was engaged in the first contraction (whose complete course, had nothing 

 intervened, is indicated by the dotted line), a second induction shock was thrown in, at 

 such a time that the second contraction began just as the first was beginning to decline. 

 The second curve is seen to start from the first, as does the first from the base line. (M. 

 Foster.) 



its maximum, is maintained. The condition which ensues is called Tetanus. 

 A tetanus is really a summation of contractions, but unless the stimuli become 

 very rapid indeed, the muscle will still be in a condition of vibratory contrac- 

 tion and not of unvarying contraction. 



If the shocks, however, be repeated at very short intervals, varying, in the 

 frog, from eighteen to thirty per second, the muscle contracts to its utmost 

 at once and continues at its maximum contraction for some time. The 

 lever rises almost perpendicularly and then describes a straight line, figure 

 331, c. The rate of stimulation required increases with the rapidity of the 

 simple contraction. If the stimuli are not so rapid the line of maximum con- 

 traction becomes wavy, indicating a tendency of the muscle to relax during 

 the intervals between the stimuli, figure 331, b. As the muscle becomes 

 fatigued, a less rapid rate of stimulation is required to produce a complete 

 tetanus, owing to the prolongation of the relaxation period in such a muscle. 

 The height of the contraction, however, is lessened. This condition of pro- 

 longed relaxation is known as contracture. 



Co-ordinated Muscular Contractions. In the human body the skel- 

 etal muscles contract only on stimulation through their motor nerves; i.e., 

 under the influence of nerve impulses that have their origin in the central 



