56 THE CONTRACTILE TISSUES. 



nerve. Thus, if the trunk of a nerve be pinched, or subjected to sudden 

 heat, or dipped in certain chemical substances, or acted upon by various 

 galvanic currents, contractions are seen in the muscles to which branches of 

 the nerves are distributed. 



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 stimulus does not give rise in the nerve to any visible change of form ; 

 but that changes of some kind or other are set up and propagated along the 

 nerve down to the muscle is shown 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 L e., mani- 

 fests its irritability by 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 call these changes 

 thus propagated along a nerve " nervous impulses." 



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

 traction 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 of the many fine nerve-branches, which, as 

 we shall see, are abundant in the muscle, passing along 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 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 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 stimulating 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 bloodvessels), 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 the other hand, if the muscle be 

 allowed to remain in the body, and so be exposed to the action of the poison, 

 but the nerve be divided high up and the part connected with the muscle 

 gently lifted up before the urari is introduced into the system, so that no 

 blood flows to it. and so that it is protected from tl & influence of the 

 poison, stimulation of the nerve will be found to produce no contractions in 



