358 THE PROPERTIES OF STRIPED MUSCLE. 



by equal increments from to 250 grms. The concavity of this curve 

 is always upwards ; had it been rectilinear, the area would have been 

 larger, but still inconsiderable as compared with the whole area c elf, 

 so that we arrive at this conclusion, namely, that the stretching of a muscle 

 before it is excited increases its efficiency in a degree out of all pro- 

 portion to the amount of extrinsic work required for the purpose, the 

 reason being that the amount of positive work done depends not on the 

 very inconsiderable force actually used to extend the muscle, but on the 

 negative work which would have been done had it been necessary to 

 extend the excited muscle to the same length. 



Response to a single instantaneous stimulus. — The twitch. — 

 The mechanical change which takes place in a muscle in response to 

 an instantaneous stimulation, is in all respects more complicated than 

 that which is determined by continuous excitation, for whereas in study- 

 ing the latter we have to compare a more or less persistent state which 

 lasts as long as its cause, with the still more permanent condition of the 

 muscle when inactive, we have in the former a process consisting of a 

 succession of states, to the whole of which the instantaneous exciting 

 cause is antecedent, and of which the duration and time-relations are 

 entirely dependent on the physiological endowments of the muscle. 



The knowledge which we possess of this process is gained by 

 observing it and recording its effects under the two conditions already 

 referred to — that in which the muscle is allowed to shorten without 

 hindrance {isotonic contraction), and that in which no diminution of its 

 length is allowed to take place. In the latter {isometric contraction) the 

 force with which it strives to shorten may be recorded with the aid of 

 suitable methods. 



The graphic record of the response to a single excitation under 



isotonic conditions. — For recording the diminution of length which a 



^^— . muscle undergoes when instantane- 



yy' \\ ously excited, Fick employed the 



/' \\ lever of the form shown in Fig. 



/ "A 190, HH'. The weight to be 



/ \ lifted hangs by its cord from the 



/ Y\ side of the axial pulley, the radius 



/ \^'~.^^' "^ of which is 4 mm. The muscle 



£. _r^_L IZllH^ acts on the lever at a distance of 



Fig. 192.— Isotonic curve drawn by a double 80 mm -» so tnat if tlie weight is 

 semi-membranosus preparation attached to 200 grms. the tension of the muscle 

 a lever similar to HH of Fig. 190. The when at rest is only 10 grms. If 

 dotted line shows the curve described when the rate of horizontal movement 

 the lever is made as light as possible. — rick. . ., -,. » , , 



oi the recording surface is about 



10 in. per second, the curve at ordinary temperatures has the form shown 

 in Fig. 192. Even when the lightest possible writing lever is used, its 

 movement is somewhat affected by its inertia ; so that the real form of the 

 curve is approximately that represented by the dotted line. 



It is a familiar fact, that when, in a succession of experiments on 

 the same muscle, the strength of the instantaneous stimulus, which at 

 the beginning is very inconsiderable, is increased step by step, there is 

 at first no response, that the muscle then shortens scarcely per- 

 ceptibly, but that, when this point has been gained, a very slight further 

 increase of the strength of the stimulus evokes a complete or, as it is 

 technically called, a maximal contraction. This term is used to signify 



L 



