SEC. 1. THE PHENOMENA OF MUSCLE AND NERVE. 



Muscular and Nervous Irritability. 



40. The skeletal muscles of a frog, the brain and spinal 

 cord of which have been destroyed, do not exhibit any spontaneous 

 movements or contractions, even though the nerves be otherwise 

 quite intact. Left undisturbed the whole body may decompose 

 without any contraction of any of the skeletal muscles having 

 been witnessed. Neither the skeletal muscles nor the nerves 

 distributed to them possess any power of automatic action. 



If however a muscle be laid bare and be more or less violently 

 disturbed, if for instance it be pinched, or touched with a hot wire, 

 or brought into contact with certain chemical substances, or sub- 

 jected to the action of galvanic currents, it will move, that is con- 

 tract, whenever it is thus disturbed. Though not exhibiting any 

 spontaneous activity, the muscle is (and continues for some time after 

 the general death of the animal to be) irritable. Though it remains 

 quite quiescent when left untouched, its powers are then dormant 

 only, not absent. These require to be roused or ' stimulated ' by 

 some change or disturbance in order that they may manifest 

 themselves. The substances or agents which are thus able to 

 evoke the activity of an irritable muscle are spoken of as stimuli. 



But to produce a contraction in a muscle the stimulus need 

 not be applied directly to the muscle ; it may be applied indirectly 

 by means of the 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, contrac- 

 tions are seen in the muscles to which branches of the nerve 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 



