LATENCY OF THE MECHANICAL RESPONSE. 381 



in which two submaxiinal stimuli acting practically simultaneously, produce 

 the effect of a single maximal one, from those in which the first of two 

 excitations following one another with an interval between them prepares for 

 its successor, as in the so-called " staircase " phenomenon, first described by 

 Bowditch in the heart. 1 When, after having cut off the apex of the ventricle 

 of the frog's heart, single induction currents are led through it at intervals of 

 from five to ten seconds, by electrodes at a very short distance from one 

 another applied to its surface, the heights of the curves show that the second 

 beat is stronger than the first, the third than the second, and so on, the 

 increments diminishing progressively until the heights of the curves become 

 uniform. In 1875 it was observed by Tiegel- that a similar augmentation 

 takes place in skeletal muscles. It is best seen when a curarised gastrocnemius, 

 in which the circulation is carefully maintained, is excited for a long time at 

 regular intervals, in the same way as in Kronecker's experiments on exhaustion. 

 In this case the augmentation of effect when the stimulation is maximal, may go 

 on for some three hundred excitations, after which the heights of the successive 

 curves begin to diminish. When stimuli which are not quite maximal are 

 used, the increase is maintained longer, and was observed by Tiegel to be 

 accompanied by a remarkable degree of vascular congestion in the muscle. 



Period of latency of the mechanical response. — About half a 

 century ago Helmholtz discovered that a considerable interval of time 

 intervenes between the excitation of a muscle by an instantaneous 

 stimulus, and the beginning of the response. He designated this interval 

 the period of latent stimulation, and from his measurements of its 

 duration — employing methods which have served all subsequent 

 observers as models — concluded that it was about one-hundredth of a 

 second. Although Helmholtz clearly indicated that the delay was 

 influenced by the strength of the stimulus and by the excitability of the 

 muscle, his estimate was regarded for many years in physiology as if it 

 had the authority of a physical law, which could be applied without 

 reserve, not only to the whole organ, but to each of its constituent 

 structural elements. It was not until 1879 that Gad 3 showed that about 

 half of this delay was due to mechanical conditions which had been 

 previously overlooked ; for even when the load was very inconsiderable, 

 he found that there was loss of time from inertia, which could be much 

 diminished by interposing an elastic band between the weight and the 

 lever. He further observed that when a parallel-fibred muscle is 

 excited by an induction current led through its lower half, the con- 

 traction is not only delayed, but may even be preceded by a slight 

 elongation. This he rightly attributed to the extension of the unexcited 

 part, which takes place at the moment the excited part begins to 

 shorten ; and he pointed out that the yielding either of muscular parts 

 not excited or of non-muscular parts is a second cause of delay. 



A few years later similar conclusions were arrived at by Tigerstedt, 4 both 

 as regards the true chiration of the delay, and the reason why, when observed 

 in the ordinary way, it appears to be so long. His method of measurement 

 consisted in recording the time after excitation at which a very delicately 

 adjusted contact is broken by the contraction. He found that the gastro- 

 cnemius, when excited directly by an ascending over-maximal break induction 



1 Arb. a. d. physiol. Anst. zu Lcipzi;/, Bd. i. S. 156. " Ibid.. 1875, S. 37. 



3 "Ueber das Latenzstadium des Muskelelementes, " Arch. f. Physiol., Leipzig, 1870, 

 S. 250. 



4 Arch. f. Physiol., Leipzig, 1895, S. 111. 



