CHAP. IL] THE CONTRACTILE TISSUES. 57 



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 irrita- 

 bility by transmitting along itself, without any visible alteration 

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

 shall call these changes thus propagated along a nerve, 'nervous 

 impulses.' 



41. We have stated above that the muscle may be thrown 

 into contractions by stimuli applied directly to itself. But it 

 might fairly be urged that the contractions so produced are in 

 reality due to the fact that the stimulus, although apparently 

 applied directly to the muscle, is, after all, brought to bear on some 

 or other of the many fine nerve-branches, which as we shall see are 

 abundant in the muscle, passing among and between the muscular 

 fibres in which they finally end. The following facts however go 

 far to prove that the muscular fibres themselves are capable of 

 being directly stimulated 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 underneath the sciatic nerve in one leg, for 

 instance the rigjit, 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 rest of the body, including 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 

 poison ; and yet it has lost none of its power over the muscle. On 



