SPECIAL PHYSIOLOGY. 



going to a muscle, m, so as to produce the electrotonic condition above 

 and below it, the portion, <?, of the nerve, lying outside the excited 



Diagram Gr. 



Diagram G illustrates the phenomena of an electrotonus and cathelectrotonus; and the changes in the 

 physiological property of excitability of a nerve, accompanying those states, a is a portion of a nerve, sup- 

 posed to be distributed to the muscle, TO. B is a cell giving a constant current in the direction of the 

 arrows, and passing through, from the anode, or positive pole +, to the cathode, or negative pole , through 

 the part of the nerve a. The part of the nerve marked c, is said to be in the state of anelectrotonus, and 

 has its excitability diminished; the part marked fe, is said to be in a state of cathelectrotonus, and has its 

 excitability increased. In the portion of the nerve marked a, the excitability is heightened in the neigh- 

 borhood of the part b, and lowered in the part near c. Somewhere between b and c, is a point where no 

 change in the excitability occurs. 



part, which is next to the positive pole +, and therefore behind the 

 point of entrance of the current, has its excitability or its power of 

 conduction lessened or diminished, whilst the portion, b, of the nerve 

 next to the negative pole , and therefore in front of the point of 

 exit of the exciting current, shows an increase or heightening of its 

 physiological properties. In other words, of the two parts of the 

 nerve beyond the current, the part, c, next the entering current, has 

 its properties diminished, and the part, b, next to the point of exit of 

 the current, has its properties heightened. As the positive pole of a 

 galvanic circuit is called the anode, the electrical condition of the 

 nerve near that pole is called anelectrotonus, and manifests a lessening 

 of the excitability ; whilst, as the negative pole is called cathode, the 

 condition of the nerve at that part, is named cata electrotonus, or 

 cathelectrotonus, and exhibits a heightening of the nervous irritability. 

 Between the two poles, a, the nerve is also similarly affected ; the 

 part nearest b, has its excitability exalted, and the part nearest c, has 

 it lowered. Somewhere in this piece of nerve, is a point where no 

 change occurs. The position of this point moves nearer to the nega- 

 tive pole of that current, as the exciting current is made stronger, 

 until the whole piece of nerve, a, has its excitability diminished; 

 whilst, as the current is made weaker, this point travels towards the 

 positive pole of the current, until the excitability of the whole piece, 

 , is heightened. It has further been proved that the excitability or 

 conductility of an electrotonized nerve, is not only lessened or height- 

 ened in intensity, but that it responds to stimuli generally, more 

 slowly than in the healthy nerve. Moreover, it appears that a con- 

 stant exciting current of a certain strength, arrests the power of a 

 motor nerve to produce muscular contraction by, as it were, holding 

 in check the effect of other motorial stimuli upon the nerve. This 

 action is called inhibition, and the current producing it, the inhibiting 

 current. Lastly, as we shall immediately more particularly describe, 

 an excited motor nerve produces contractions in a muscle, not during 



