CHAP, ii.] THE CONTRACTILE TISSUES. 8S 



conditions the maximum contraction may be very considerable: the 

 shortening in tetanus may amount to three-fifths of the total 

 length of the muscle. 



The amount of contraction then depends on the strength of the 

 stimulus, whatever be the stimulus ; but this holds good within 

 certain limits only ; to this point however we shall return later on. 



51. If, having ascertained in a perfectly fresh muscle-nerve 

 preparation the amount of contraction produced by this and that 

 strength of stimulus, we leave the preparation by itself for some 

 time, say for a few hours, and then repeat the observations, we 

 shall find that stronger stimuli, stronger shocks for instance, are 

 required to produce the same amount of contraction as before; 

 that is to say, the irritability of the preparation, the power to 

 respond to stimuli, has in the meanwhile diminished. After a 

 further interval we should find the irritability still further dimin- 

 ished : even very strong shocks would be unable to evoke con- 

 tractions as large as those previously caused by weak shocks. At 

 last we should find that no shocks, no stimuli, however strong, were 

 able to produce any visible contraction whatever. The amount 

 of contraction in fact evoked by a stimulus depends not only on 

 the strength of the stimulus but also on the degree of irritability 

 of the muscle-nerve preparation. 



Immediately upon removal from the body, the preparation pos- 

 sesses a certain amount of irritability, not differing very materially 

 from that which the muscle and nerve possess while within and 

 forming an integral part of the body ; but after removal from the 

 body the preparation loses irritability, the rate of loss being 

 dependent on a variety of circumstances; and this goes on until, 

 since no stimulus which we can apply will give rise to a contraction, 

 we say the irritability has wholly disappeared. 



We might take this disappearance of irritability as marking the 

 death of the preparation, but it is followed sooner or later by a 

 curious change in the muscle, which is called rigor mortis, and 

 which we shall study presently ; and it is convenient to regard this 

 rigor mortis as marking the death of the muscle. 



The irritable muscle then, when stimulated either directly, the 

 stimulus being applied to itself, or indirectly, the stimulus being 

 applied to its nerve, responds to the stimulus by a change of form 

 which is essentially a shortening and thickening. By the short- 

 ening (and thickening) the muscle in contracting is able to do 

 work, to move the parts to which it is attached; it thus sets free 

 energy. We have now to study more in detail how this energy is 

 set free and the laws which regulate its expenditure. 



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