360 THE PROPERTIES OF STRIPED MUSCLE. 



In attempting to form a conception of the change which a muscle 

 undergoes in response to a single excitation, in its transition from the 

 unexcited to the contracted state, we derive assistance from the pro- 

 visional hypothesis, that it is capable of assuming two forms — a longer 

 and a shorter — and that in each of them it is in elastic equilibrium. 

 By this expression is meant that the internal elastic forces which are 

 in operation in the structure of the organ are in each position so 

 balanced, that if the organ is put out of shape by an external force, 

 its equilibrium length is restored from the moment that this force 

 ceases to act. In the case of a muscle arranged to contract under 

 isotonic conditions in response to an instantaneous stimulus, we 

 determine approximately the first of these positions, (a) when, 

 without exciting the muscle, we employ an extremely light load ; and 

 the second (b) when, under similar mechanical conditions, we stimulate 

 maximally ; the excitatory change of form is a transition from a to b. 

 The change, as it is observed by the graphic method, is found to occupy 

 some time (in the muscles of the frog at ordinary temperatures about 

 T fo- of a second), but what is recorded is the result of an infinite 

 number of constituent changes, which do not occur simultaneously in 

 all parts. The transition from the unexcited to the excited state must 

 be supposed to last in each part for a less time than the whole 

 period of shortening of the muscle ; so that, between its accomplishment 

 and the attainment by the muscle of the new form, we may think of it 

 as striving towards a new state of elastic equilibrium. 1 



If we could consider the state of elastic equilibrium of a momen- 

 tarily excited muscle, designated b, to be constant under the same 

 physiological conditions, it would follow that the tension of a muscle at 

 any moment during its period of shortening would always depend on its 

 relative length, provided that the time lapsed since excitation remained 

 the same, i.e., that if in a muscle actually contracting in response to an 

 instantaneous stimulus, the tension were measured at a given moment 

 (say 4, 5, 6 or 7-hundredths of a second after excitation), it would 

 always be the same in a muscle of the same length, by whatever 

 means it had been brought to this length. Unless it can be shown that 

 this is so, the provisional assumption set forth above must be accepted, 

 with the reservation, not that a muscle in contracting is actuated by 

 other than elastic forces, but that these forces or some of them are not 

 constant. 



The question whether or not a muscle of a certain length, at a certain 

 temperature and at a certain time after excitation, will always, so long as it is 

 in a normal condition, have the same tension, presents itself in relation to the 

 following experiment : 2 — 



A series of isotonic curves having been drawn under the conditions described 

 in the preceding paragraph, an additional curve is drawn with a load of 

 10 grms., with this difference, that to the light lever H H' (Fig. 190), previously 

 used, a heavy equilibrated oscillating bar is attached, so that the one cannot 

 move without the other. For this purpose a wooden bar is used, which 

 rotates in the same vertical plane as H H'. It carries at either end a sliding 

 weight of lead. The two weights, which are equal, can be so adjusted that 

 the centre of gravity of lever and weight is in the axis. The writing point 

 follows the course indicated by the broken line in Fig. 193, showing that the 



1 Weber, Wagner's " Handworterbuch, " Bd. iii. Abtli. 2, S. 110. 



2 Fick, op. cit., S. 120. 



