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38 MUSCULAR CONTRACTION. [BOOK i. 



The nerve like the muscle is irritable, it is thrown into a state 

 of activity by a stimulus ; but unlike the muscle it does not itself 

 contract. The changes set up in the nerve by the stimulus are not 

 visible changes of form; but that changes of some kind or other 

 are set up and propagated along the nerve down to the muscle is 

 shewn by the fact that the muscle contracts when a part of the 

 nerve at some distance from itself is stimulated. Both nerve 

 'and muscle are irritable, but only the muscle is contractile, i. e. 

 manifests its irritability by a contraction. The nerve manifests its 

 irritability by transmitting along itself, without any visible alteration 

 :of form, certain molecular changes set up by the stimulus. We shall 

 icall these changes thus propagated along a nerve, 'nervous impulses.' 



We have stated above that the muscle is irritable in the sense 

 that it may be thrown into contractions by stimuli applied directly 

 to itself. But it might fairly be urged that the contractions so pro- 

 duced are in reality due to the fact that, although the stimulus is 

 apparently applied directly to the muscle, it is, after all, the fine 

 nerve-branches, so abundant in the muscle, which are actually 

 stimulated. The following facts however go far to prove that 

 the muscular fibres themselves are capable of being directly stimu- 

 lated without the intervention of any nerves. When a frog (or other 

 animal) is poisoned with urari, the nerves may be subjected to 

 the strongest stimuli without causing any contractions in the 

 muscles to which they are distributed ; yet even ordinary stimuli 

 applied directly to the muscle readily cause contractions. If before 

 introducing the urari into the system, a ligature be passed under- 

 neath the sciatic nerve in one leg, for instance the right, and drawn 

 tightly round the whole leg to the exclusion of the nerve, it is 

 evident that the urari when injected into the back of the animal, 

 will gain access to the right sciatic nerve above the ligature, but 

 not below, while it will have free access to the whole left sciatic. If, 

 as soon as the urari has taken effect, the two sciatic nerves be 

 stimulated, no movement of the left leg will be produced by stimu- 

 lating the left sciatic, whereas strong contractions of the muscles of 

 the right leg below the ligature will follow stimulation of the right 

 sciatic, whether the nerve be stimulated above or below the ligature. 

 Now since the upper parts of both sciatics are equally exposed to 

 the action of the poison, it is clear that the failure of the left nerve 

 to cause contraction is not attributable to any change having taken 

 place in the upper portion of the nerve, else why should not the 

 right, which has in its upper portion been equally exposed to the 

 action of the poison, also fail ? Evidently the poison acts on some 

 parts of the nerve lower down. If a single muscle be removed from 

 the circulation (by ligaturing its blood-vessels), previous to the 

 poisoning with urari, that muscle will contract when any part of the 

 nerve going to it is stimulated, though no other muscle in the body 

 will contract when its nerve is stimulated. Here the whole nerve 

 right down to the muscle has been exposed to the action of the 



