42-i NERVOUS IRRITABILITY 



1. The stimulus applied to a nerve, whether sensitive or motor, pro- 

 duces the same effect throughout its entire length. 



In the natural condition of the parts, the impressions made upon the 

 external integument, when they give rise to sensation, are transmitted 

 by the sensitive nerve through its whole course to the nervous centre. 

 There it is perceived as a sensation; and the sensation thus produced is 

 referred by the individual, not to the brain or to any part of the nerve, 

 but to the integument where the nerve originated. If an irritation be 

 applied to the nerve in the middle of its course, the sensation is still 

 perceived as if it came from the same portion of the integument and had 

 travelled through the same distance as before. It is well known that 

 after amputation of a limb in the human subject, if the severed extremity 

 of one of the nerves happen to be compressed or otherwise irritated by 

 the tissues of the cicatrix, or if it be the seat of inflammatory action, 

 many different sensations of pain, movement, heat, or cold, are excited, 

 which are always referred by the individual to the amputated portion 

 of the limb ; and patients often assert that they can feel the separated 

 parts as distinctly as if they were still attached to the body. The im- 

 pression conveyed through the remaining portion of the nerve is the 

 same as if the whole of it were still in existence. 



The action of the motor nerves is of a similar kind. A voluntary 

 stimulus, which originates in the brain, passes through the entire length 

 of a motor nerve to reach the muscles and excite their contraction. 

 But if the nerve be divided at any intermediate point, and a galvanic 

 stimulus applied to the peripheral portion, a contraction follows in the 

 same muscles as before. In each case, the physiological effect is pro- 

 duced at the extremity of the nerve fibres ; and it seems to make no 

 essential difference in its character from how great a distance it has 

 been transmitted. 



So far as yet known, therefore, the nerve fibre, whether sensitive or 

 motor, when its irritability is excited, may be thrown at once into a 

 condition of activity throughout its entire length; the whole nerve 

 assuming a state of polarity, analogous to that of a magnetized bar, in 

 which the visible phenomena of attraction or repulsion are manifested 

 only at its extremities, although the whole substance of the bar partici- 

 pates in its magnetic molecular action. When the exciting stimulus, as 

 in the sensitive nerves, is naturally applied at the peripheral extremity, 

 it must necessarily be communicated from without inward ; and when 

 it commences at the inner extremity, as in a motor nerve, it must move 

 in a direction from within outward. But nothing thus far shows that 

 it may not be capable of moving in either of these two directions in the 

 same nervous fibre. The following experiments show that this is in 

 reality the case, so far as regards the sensitive nerves. 



2. Sensitive impressions may pass, in the fibres of a sensitive nerve, 

 either from without inward or from within outward. 



This of course never takes place in the natural condition of the parts ; 

 but its possibility has been demonstrated, in the experiments of Paul 



