638 EVOLUTION OF HEAT, LIGHT, AND ELECTRICITY. 



but if the points be not equidistant, then a slight deflection is produced, indicat- 

 ing that the parts nearer the middle are positive to those nearer the extremities. 

 It has not been found possible, owing to the small size of the nerve-trunks ex- 

 perimented on, to test in a similar manner the relative state of different points 

 of their transverse section ; but there can be little doubt, from the complete con- 

 formity which exists in other respects between the nervous and muscular cur- 

 rents, that the same law will be found to prevail in this as in the former case ; 

 namely, that the points nearer the surface are positive to those nearer the centre. 

 There is no difference between the motor and the sensory nerves in regard to 

 the direction of this current, the existence of which has been proved by M. du 

 Bois-Reymond, not only by the galvanometer, but also by the excitement of 

 contractions in the limb of the galvanoscopic frog. The "nervous current/' 

 like the muscular, must be considered as derived from the electromotive action 

 of the molecules of the nerve ; and, for the reasons already pointed out, the 

 intensity of the current in the immediate neighborhood of the molecules may 

 be infinitely greater 'than that which is shown by the galvanometer to exist in 

 the trunk of the nerve. 



672. We have now to follow M. du Bois-Reymond through his investigations 

 on the change in the condition of the " nervous current/' whilst the nerve is in 

 a state of functional activity, whether motorial or sensorial. For the examina- 

 tion of this, it is desirable to induce a state of continuous action in the nerve, 

 analogous to the tetanic contraction of muscle ; and this condition in the motor 

 nerve is manifestly that which induces tetanus in its muscle ; whilst in sensory 

 nerves it is that in which a violent sensation is uninterruptedly kept up. No 

 means of exciting such a state are so certain and simple as electric currents ; 

 but it is necessary in the first place to determine the modification which these 

 currents may themselves produce in the proper " nerv- 

 ous current." If a portion of nerve-trunk be so 

 placed (Fig. 173), that it touches one of the electrodes 

 by its transverse section (which may be designated T), 

 and the other by its surface or longitudinal section 

 (L), and a portion of its continuation be included in 

 a galvanic circuit, so that a current shall pass in the 

 direction zS9-p ? which is the same in its direction as 

 that between T^->L, then the intensity of the "nervous 

 current" T^->L, as indicated by the deflection of 

 the needle of the galvanometer, will be found to under- 

 go an increase ; whilst on the other hand, if the electric 

 current be passed in the contrary direction p^-*z, the 

 intensity of the " nervous current" T^-^L will de- 

 crease. The portion z^-*r of the nerve, which is 

 included in the electric circuit, is termed the excited 

 portion, and the current passed through it is the excit- 

 ing current ; on the other hand, the portion T^-*L in- 

 cluded between the electrodes of the galvanometer is the derived portion ; and 

 the altered condition of this part, which is produced by the extraneous current 

 (this current having been experimentally proved by M. du Bois-Reymond to 

 exert no direct influence on the galvanometer), is termed the electrotonic state 

 of the nerve. When the intensity of the "nervous current" is increased, the 

 nerve is said to be in the positive phase of this electrotonic state ; and when it is 

 diminished, the nerve is in the negative phase of that state. By a proper 

 arrangement, the same exciting current may be made to produce the positive 

 phase in one part of a nerve-trunk, and the negative phase in another. Thus if 

 the two extremities of a nerve (Fig. 170, p and c) be so connected with two 

 galvanometers, that both shall develope the "nervous current," and an inter- 



