THE EFFECT OF TWO SUCCESSIVE STIMULI. 565 



four impulses in the muscle were effective, so that they really represented short 

 tetanic contractions. 



Pathological. In the presence of secondary degeneration of the spinal cord 

 following apoplexy, of atrophic muscles associated with ankylosed extremities, of 

 muscular atrophy, of progressive ataxia, and of paralysis agitans of long standing, 

 the latent period is increased. On the other hand, it is diminished in the presence 

 of the contractures attending senile chorea and spastic tabes. The entire curve 

 appears to be lengthened in cases of icterus and diabetes. In cases of cerebral 

 hemiplegia in the stage of contracture the muscular contraction resembles the 

 veratrin-curve, as it does likewise in cases of spastic spinal paralysis and amyo- 

 trophic lateral sclerosis. In cases of pseudohypertrophy of the muscles, the as- 

 cending limb is short and the descending limb greatly lengthened. In the presence 

 of muscular atrophy following cerebral hemiplegia and tabes, the height of the 

 curve is reduced, ascent and descent take place gradually, and the contraction of 

 the atrophic muscle resembles that of a fatigued muscle. In cases of chorea the 

 curve is short. The reaction of degeneration is described on p. 669. According 

 to Goldscheider contraction takes place sluggishly also in conjunction with affec- 

 tions of the nerves, without any change in the irritability of the muscles them- 

 selves. In rare cases the observation has been made in man that spontaneous 

 motor stimuli give rise to prolonged muscular contractions, followed by after- 

 contractions (Thomsen's disease). The muscle-fibers of such patients are broad, 

 the nuclei increased in number, and the fibrils hypertrophied ; it has been sug- 

 gested that the white fibers are wanting. Fr. Schultze and others have observed 

 a peculiar muscular undulation. 



If two momentary shocks be applied successively to the muscle in 

 such a way that each would alone have induced a maximal contraction, 

 that is the greatest possible contraction, the effect will vary in accordance 

 with the time that elapses between the two shocks. If the second shock 

 be applied after the muscle has already become relaxed from the contrac- 

 tion of the first stimulus, then a second maximal contraction simply 

 results. If, however, the muscle is still in a phase of contraction or re- 

 laxation from the influence of the first stimulus, the second shock gives 

 rise to a new maximal contraction from the phase of contraction existing 

 at that time. If, finally, the second shock follows so quickly upon the 

 first that both occur during the period of latent stimulation, only one 

 maximal contraction results. 



If both stimuli are only of moderate strength, not sufficient to induce 

 maximal contraction, a summation of the effects of both takes place. 

 At whatever stage of contraction the muscle may be as a result of the 

 first stimulus (Fig. 195, 7, 6), the second shock will have an effect (b c) as 

 if the phase of contraction brought about by the first shock were the 

 natural passive form of the muscle. Thus, under favorable conditions, 

 the contraction may be even twice as large as that induced by the first 

 stimulus alone. The most favorable condition is the application of the 

 second stimulus ^V second after the first. The effects of both are also 

 produced if the second shock is applied within the period of latent stimu- 

 lation. 



The second contraction of a summated contraction reaches its height in a 

 shorter period of time than the first contraction alone would have done. The 

 time for b c (Fig. 195, /) is, thus, shorter than that for a b. 



If a series of shocks be applied to the muscle in rather rapid succession, 

 the muscle will have no time to relax in the intervals. It, therefore, 

 in accordance with the rapidity with which the stimuli follow one another, 

 remains in a state of continuous, shock-like, tremulous contraction that 

 is designated tetanus. The condition of tetanus, or rigid spasm, is, thus, 

 not a state of continuous, uniform contraction, but a discontinuous 



