76 ELECTRO-PHYSIOLOGY CHAP. 



as the latent period of muscular contraction, i.e. between the 

 moment of excitation and the estimated commencement of con- 

 traction, a great number of muscle elements must already have 

 been thrown into mechanical activity. Conclusions as to the 

 latency of muscle elements might with more justice be deduced 

 from the latent period of the expansion of a directly excited 

 point of the muscle, by which the state of activity of any 

 muscle particle can be followed as it develops. A muscle element 

 is much too small to produce any perceptible mechanical effect 

 by itself in contracting, so that the question of the magnitude of 

 its mechanical latent period cannot be solved by direct experi- 

 ment (Gad). It may be taken as proved by experiments, which 

 we shall discuss later, that there is no latency period in the 

 chemical changes in muscle substance, consequent upon excitation ; 

 whether there is any appreciable interval before mechanical energy 

 is locally developed, must be regarded as questionable. 



III. EFFECT OF LOADING (TENSION) UPON MAGNITUDE, DURA- 

 TION AND FORM OF MUSCULAR CONTRACTION 



It has been shown that the absolutely unloaded muscle (e.g. 

 swimming in mercury) retains its contracted form if no extend- 

 ing force is acting upon it. It is therefore impossible to obtain 

 a graphic record of the process of shortening and elongation in a 

 perfectly unstretched muscle. Some kind of lever, however 

 light, must rest upon it, and the movements of the lever are counter- 

 balanced by a traction (load) working against the shortening, 

 as soon as relaxation commences. This entails certain errors in 

 the curve of the contraction which particularly in the older 

 experiments, where inert masses were not eliminated have been 

 a great source of confusion. More especially in the descending 

 portion of the curve, secondary smaller waves, with no corre- 

 sponding active changes of form in the muscle, are produced by 

 intrinsic variations of the rapidly accelerated falling mass. 

 At a later period these fallacies were almost wholly avoided by 

 using the lightest possible lever, and choosing a suitable point of 

 attachment for the load (20). Where the tension of the muscle 

 remains approximately constant during the course of a contrac- 

 tion as is the case when it is attached to a long one-armed 

 lever, of the smallest possible bulk, while a weight close to the 



