442 MUSCLE PHENOMENA. 



IRRITABILITY; CONTRACTILITY; ELECTRIC PHENOMENA 



OF MUSCLE. 



. Irritability is the property possessed by living tissues by virtue 

 of which they respond to certain external agents called irritants 

 or stimuli. A stimulus, therefore, is an agent which is capable of 

 producing in living tissues certain changes by which is manifested 

 the fact that they are living, the character of these changes varying 

 according to the tissue which is the subject of the stimulation. 

 When this change consists in one of form, it is contractility. 

 Thus, simple protoplasm, as in the ameba, will, when touched, 

 draw in the processes or pseudopodia which it had previously put 

 out (p. 24). Here the touch was the stimulus which caused the 

 irritability of the protoplasm to manifest itself by contraction. 

 Or if a muscle is stimulated by an electric current, it shortens, 

 thus manifesting its irritability by contraction. In both these 

 instances the response to the stimulus is a change of form. If, 

 however, a current of electricity is passed through a nerve, the 

 closest inspection fails to reveal any change in the nerve itself: 

 it neither moves its position nor in any wise changes its form ; 

 and yet a nerve is irritable i. e., has the property of responding 

 to a stimulus. If it is a motor nerve that is, one distributed to 

 a muscle when it is stimulated its contractility will be mani- 

 fested by a contraction of that muscle ; or if it is a secretory 

 nerve that is, one supplying a gland its irritability will be 

 manifested by an increased activity of the gland. 



It is not, however, essential that in order to manifest contrac- 

 tility muscles should be stimulated through the motor nerve which 

 is distributed to them, as a stimulus applied directly to the muscle 

 itself will cause the muscle to contract. That muscular tissue pos- 

 sesses irritability independently of the nerves distributed to it was 

 for a long time in dispute, but is now conceded by all authorities, 

 the proof which was furnished by Claude Bernard's experiment 

 being incontrovertible. This consists in destroying the brain of a 

 frog by pithing it i. e., passing a blunt needle into the cranial 

 cavity and moving it about. This destroys consciousness ; but 

 the circulation of blood continues. The left sciatic nerve is then 

 dissected out, and a ligature passed beneath it, and all the tissues 

 of the thigh excepting the nerve are tightly tied : thus is cut off 

 the blood-supply to all the parts below the ligature ; but the nerve 

 is at the same time uninjured. Under the skin of the back a few 

 drops of a 2 per cent, solution of curare are then injected. If after 

 some time, about half an hour, the nerve is stimulated, there will 

 be no contraction of the muscles supplied by it ; but if the stimulus 

 is applied directly to the muscles, they will respond. Experiment 

 shows that curare poisons the motor end-plates, so that although 

 the nerve carries the current to this end-organ, its influence can 



