ELECTRICAL CURRENTS OF THE NERVES. 587 



Under such circumstances the nerve may be considered to be divided into two 

 parts, an intrapolar and an extrapolar portion, the latter being again sub- 

 divided into that portion situated nearer the nervous centres than the current, 

 or central, and that situated between the current and the muscle, or distal, 

 of the current, the latter being sometimes also called the myopolar portion. 

 As the positive pole of an electrical current is termed the anode, and the 

 negative the cathode, Pfluger has applied the terms auelectrotouus and 

 cathelectrotouus to distinguish those conditions established in the portions 

 of nerve in the immediate vicinity of the poles of a constant battery. He 

 has shown that the intrapolar portion of nerve is divided into two zones 

 (Fig. 198) ; in one of these, situated at and near the Positive pole, the ex- 

 citability of the nerve is diminished, whilst in the other, situated at and near 

 the Negative pole, the excitability is increased. The extent of the portion 

 of nerve in which the excitability is lowered, the positive or anelectrotouic 



whilst the nega- 



zone, is small in proportion to the weakness of the current ; whilst 

 tive or cathelectronic zone, in which the excitability is exalted, is corre- 

 spondingly large; so that, with very weak currents, nearly the-whole of the 

 intrapolar portion of the nerve is in a state of exalted excitability, whilst 

 with strong currents the intrapolar portion of the nerve is almost wholly 

 in a state of depressed excitability. The same conditions of exalted or de- 

 pressed excitability also extend for a considerable distance beyond the poles 

 i. e., in the extrapolar portions of the nerves in both directions. The mode 

 of determining the variations in the excitability of the nerve at the two poles 

 may be understood with the aid of the diagram (Fig. 198). In Fig. 199, c c 



FIG. 198. 



ff 



A, the Positive, B, the Negative Pole of a constant current applied to a nerve, a, b, c, a curve, showing 

 the effects of a weak current ; d, e, f, of a stronger current; g, h, i, of a very strong current. The por- 

 tion of the curve below the line shows the degree and extent of the depression, the part above of the 

 exaltation of the excitability of the nerve. 



represents the poles of the constant or polarizing currents by means of which 

 the state of electrotouus is established, and R R those of the exciting current, 

 or that by means of which the excitability of the nerve is tested ; the latter 

 may be replaced by a mechanical stimulus or by a chemical stimulus. Eck- 

 hard employed a solution of common salt. Before throwing the nerve into 

 an electrotouic state by the application of the poles of the constant current, 

 a few trials are made to ascertain what is the weakest stimulus which, when 

 applied at d, will produce contraction in the muscle. When this is known, 

 the constant current is applied, and it will then be found that if the direc- 

 tion of this be as in Fig. 199, either a much weaker stimulus applied at d 

 will produce contraction in the muscle, or the same stimulus will produce a 

 much more vigorous contraction, in either case showing that the excitability 

 of the nerve has been increased. This is expressed by the curve b, e, g. If 

 an electrical stimulus is employed, the current should be passed in the direc- 

 tion indicated, as in that case it would tend to reduce the influence of the 

 cathode of c c, and by so much render the results obtained more striking. This 



