72 THE CONTRACTILE TISSUES. 



in extent and character according to the condition of the muscle, the strength 

 of the induction-shock, the load which the muscle is bearing, and various 

 attendant circumstances, but the three phases may vary independently. The 

 latent period may be longer or shorter, the shortening may take a longer or 

 shorter time to reach the same height, and especially the relaxation may be 

 slow or rapid, complete or imperfect. Even when the same strength of 

 induction-shock is used the contraction may be short and sharp or very long 

 drawn out, so that the curves described on a recording surface travelling at 

 the same rate in the two cases appear very different ; and under certain cir- 

 cumstances, as when a muscle is fatigued, the relaxation, more particularly 

 the last part of it, may be so slow, that it may be several seconds before the 

 muscle really regains its original length. 



Hence, if we say that the duration of a simple muscular contraction of the 

 gastrocnemius of a frog under ordinary circumstances is about y 1 -^ second, of 

 which y^ is taken up by the latent period, T f by the contraction, and y^ 

 by the relaxation, these must be taken as " round numbers," stated so as to 

 be easily remembered. The duration of each phase as well as of the whole 

 contraction varies in different animals, in different muscles of the same 

 animal, and in the same muscle under different conditions. 



The muscle-curve which we have been discussing is a curve of changes in 

 the length only of the muscle ; but if the muscle, instead of being suspended, 

 were laid flat on a glass plate and a lever laid over its belly, we should find, 

 upon sending an induction-shock into the nerve, that the lever was raised, 

 showing that the muscle during the contraction became thicker. And, if we 

 took a graphic record of the movements of the lever, we should obtain a 

 curve very similar to the one just discussed ; after a latent period the lever 

 would rise, showing that the muscle was getting thicker, and afterward would 

 fall, showing that the muscle was becoming thin again. In other words, in 

 contraction the lessening of the muscle lengthwise is accompanied by an 

 increase crosswise ; indeed, as we shall see later on, the muscle in contracting 

 is not diminished in bulk at all (or only to an exceedingly small extent, 

 about y^TTir f its total bulk), but makes up for its diminution in length by 

 increasing in its other diameters. 



48. A single induction shock is, as we have said, the most convenient 

 form of stimulus for producing a simple muscular contraction, but this may 

 also be obtained by other stimuli, provided that these are sufficiently sudden 

 and short in their action, as, for instance, by a prick of, or a sharp blow on, 

 the nerve or muscle. For the production of a single simple muscular con- 

 traction the changes in the nerve leading to the muscle must be of such a 

 kind as to constitute what may be called a single nervous impulse, and any 

 stimulus which will evoke a single nervous impulse only may be used to 

 produce a simple muscular contraction. 



As a rule, however, most stimuli, other than single induction-shocks, tend 

 to produce in a nerve several nervous impulses, and, as we shall see, the 

 nervous impulses which issue from the central nervous system, and so pass 

 along nerves to muscles, are, as a rule, not single and simple, but complex. 

 Hence, as a matter of fact, a simple muscular contraction is within the living 

 body a comparatively rare event (at least as far -as the skeletal muscles are 

 concerned), and cannot easily be produced outside the body otherwise than 

 by a single induction-shock. " The ordinary form of muscular contraction is 

 not a simple muscular contraction, but the more complex form known as a 

 tetanic contraction, to the study of which we must now turn. 



