ii CHANGE OF FORM IX MUSCLE DURING ACTIVITY 81 



does not merely increase the excitability, but it also acts directly 

 as a discharging stimulus. Heidenhain showed for striated skeletal 

 muscle that not merely the mechanical yield of work (which is 

 essentially conditioned by the magnitude of contraction or height 

 of twitch) increases, but that, generally speaking, a larger proportion 

 of the potential energy stored up in the muscle is employed, i.e. 

 that exchange takes place between the greater part of the chemical 

 tensions ; and this has been confirmed by later experiments. But 

 when this occurs with one and the same strength of a given 

 stimulus, the cause must lie in a change of state in the muscle 

 itself, which might be termed increase of excitability. When we 

 find that, beyond a certain point, expansion, or tension, per se may 

 act on cardiac muscle as a permanent stimulus, inasmuch as with- 

 out the addition of any further stimulus it can discharge long 

 series of rhythmical contractions while in other cases an external 

 impact, a new stimulus, may also be required, which, however, 

 would be inadequate without the simultaneous extension of the 

 muscle it seems legitimate to refer the increase of excitability (of 

 which it is usual to speak in the latter case) to the presence of a 

 permanent condition of excitation, caused by the extension-stimulus, 

 but in itself inadequate to produce visible effects of stimulation. From 

 this point of view the state of increased excitability of living 

 matter would only gradually become distinguishable from the 

 state of excitation. Later on we shall encounter numerous facts 

 which are in favour of this theory. 



When a muscle is so heavily loaded that it is unable to raise 

 the suspended weight, the most powerful stimulus will fail to 

 produce any external visible alteration, while at the same time the 

 properties of the muscle are fundamentally altered. In the first 

 place, the elastic traction (tension) of the excited muscle is consider- 

 ably greater in its initial length (i.e. in its unexcited state) than it is 

 in the resting condition, for the true contraction first appears 

 in consequence of this altered state, since the dependent load is 

 overcome by the increase of elasticity due to excitation. By using 

 Tick's method it is possible to prevent a muscle from perceptibly 

 altering in length, and at the same time to show its actual tension 

 by a visible indication. This is most simply effected by attaching 

 the muscle to the short arm of a two-armed lever, while an 

 elastic spring confines the movements of the longer arm. If the 

 latter is drawn out beyond the point of insertion of the spring, 



G 



