THE MECHANICAL RESPONSE. 361 



free end of the muscle, instead of at once ascending rapidly, as in the curve 

 previously drawn with the light lever from the same starting point, rises very 

 slowly at first, afterwards, i.e. at a period of about T ^„ of a second later, 

 as rapidly as when unhampered by having to set in motion an inert 

 equilibrated mass. It will be noticed that this curve, which may be 

 called an " inertia curve," crosses each of the previous ones twice. Each 

 point of intersection indicates that the muscle has on two occasions been 

 of the same length, at the same time after excitation. On each of the first 

 occasions the tension of the muscle is known, for each occurred in the course 

 of an isotonic curve of known tension. If the hypothesis above stated is 

 correct, the tension on the two occasions must in each case have been the 

 same. Consequently if, on the base line a b (Fig. 193a), we set off the lifts, 

 i.e. the heights of the free end of the muscle corresponding to the inter- 

 sections, and draw from the points thus marked ordinates, each representing 

 the tension as indicated by the isotonic curve at the points of intersection, 

 then a line joining the free ends of these ordinates (broken line in the figure) 

 exhibits the relation of tension to length during the period that the lever 

 was acted on by the contracting muscle. The space included between the 

 broken line and the base line expresses, in gramme-millimetres, the work which 

 would be done on the lever and oscillating bar during the same period, if the 

 tension were a function of the length. The bar is made to do external work 

 (in lifting a weight) which can be measured. 1 The amount of this work, i.e. 

 of the actual work done by the muscle when acting on the lever, can therefore 

 be compared with the amount estimated from the diagram. In numerous 

 experiments, made by Professor Fick, it was found that the actual work 

 done exceeds — in some instances considerably — the amount estimated. It 

 therefore appears that a muscle when resisted in the way above described, 

 acquires a tension during its period of effort, which is greater than it possesses 

 under what seem to be precisely the same physiological conditions, and 

 is extended to the same length by a weight which it is lifting without any 

 further resistance. In other words, the effort made by a muscle at any given 

 moment after an instantaneous excitation, does not depend exclusively on its 

 length, but is also affected by the mechanical conditions under which it 

 shortens. The question whether this " reaction " of a muscle to resistance is 

 physiological or physical will be considered later. 



Considering that a muscle in the discharge of its normal function is 

 always resisted, it is of more immediate importance to inquire how it 

 deals with resistance, than to investigate, as we have done in the 

 preceding pages, its behaviour when allowed to contract so as to lift a 

 weight attached to its free end. At a very early period in the progress 

 of experimental physiology, Schwann, one of the founders of the science, 

 enunciated the fundamental principle that, on the whole, under the same 

 physiological conditions, the tension of a contracting muscle varies with 

 its length. For this purpose he used a very simple method, now called 

 the method of "after-loading." He attached the upwardly directed 

 tendon of the gastrocnemius of the frog to one end of a balance, of 

 which the other end supported a weight. The beam at the muscle 

 end was prevented from rising by a stop, so that the muscle, so long 

 as it was relaxed, was not stretched by the weight. The experiment 

 consisted in exciting the muscle, and observing what weight was just 

 insufficient to hold the beam against the stop — in other words, what 



1 This is effected by hanging the weight 011 a pulley on the axis of the equilibrated bar, 

 in such a way that the weight begins to be lifted at the moment that the shortening of the 

 muscle is completed. The height to which the weight is carried is recorded graphically. 

 From the record, the work done on the equilibrated bar can be estimated. 



