SEC. 1. THE PHENOMENA OF MUSCLE AND NEKVE. 



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 subjected to the action of galvanic currents, it will move, that 

 is contract, 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 sub- 

 stances, or acted upon by various galvanic currents, contractions 

 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 



