78 TETANUS. [BOOK i. 



In other words, in contraction the lessening of the muscle length- 

 wise 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 T OIJO^ of 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 con- 

 traction, 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 sharp blow on, the nerve or muscle. For 

 the production of a single simple muscular contraction 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 compara- 

 tively rare event (at least as far as the skeletal muscles are 

 concerned), and cannot easily be produced outside the body other- 

 wise 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. 



Tetanic Contractions. 



49. If a single induction-shock be followed at a certain 

 interval by a second shock of the same strength, the first simple 

 contraction will be followed by a second simple contraction, both 

 contractions being separate and distinct ; and if the shocks be 

 repeated a series of rhythmically recurring separate simple con- 

 tractions may be obtained. If however the interval between two 

 shocks made short, if for instance it be made only just long enough 

 to allow the first contraction to have passed its maximum before 

 the latent period of the second is over, the curves of the two 

 contractions will bear some such relation to each other as that 

 shewn in Fig. 13. It will be observed that the second curve is 

 almost in all respects like the first except that it starts, so to 

 speak, from the first curve instead of from the base-line. The 

 second nervous impulse has acted on the already contracted 

 muscle, and made it contract again just as it would have done if 

 there had been no first impulse and the muscle had been at rest. 

 The two contractions are added together and the lever is raised 



