MODE OF ACTION OF NERVES. 231 



the excitability, and so arresting its motorial effect on the muscles, as 

 is the case in the motor nerves. When the limb is separate, and the 

 sensibility is moderate, pain is excited through a sensory nerve, only 

 at the end of a direct or centrifugal current, and at the commencement 

 of an inverse or centripetal current, results which are the opposite of 

 those observed as regards the muscular contractions (Pfliiger) ; lastly, 

 a constant current does not inhibit the sensorial conducting power of 

 a sensory nerve. 



The experiments which establish the numerous foregoing facts, are 

 amongst the most difficult and refined in physiological science, and 

 clearly prove an intimate relation between the electrical state and the 

 physiological action of the nerves; but at present, the true relation- 

 ship between these two conditions can only be matter of conjecture. 

 It has already been mentioned that, in stimulating a motor nerve, a 

 constant current produces no muscular contraction, so long as it is of 

 uniform strength, or uninterrupted; but that a variation in the inten- 

 sity of the current, or an interruption of the current, will produce 

 contraction. Now, the result of applying any stimulus, whether me- 

 chanical, chemical, or electrical, to a nerve, is, as we have seen, to 

 cause a diminution of the normal, or usual nerve current; and such a 

 diminution or variation in the current, it is allowable to suppose, may 

 determine the muscular contraction. Again, when a nerve is excited 

 by an electrical current, and thrown into an electrotonic state, we 

 have an alteration in the static current, and, besides this, a new one 

 established in its stead; but contractions only take place at the com- 

 mencement and end of the electrotonic state, not during its continu- 

 ance that is to say, at the interruption of the current. If, therefore, 

 by any process taking place in the nerve cells of the gray matter, 

 such momentary electrotonic states may be produced, contractions 

 would necessarily occur when it was established, or when it was inter- 

 rupted. Such an explanation of the action of the nerve substance is, 

 however, at present hypothetical. The possibility of some internal 

 change, of an electrical character, in a living tissue, influencing the 

 excitability of a nerve, and so making the muscles supplied by that 

 nerve to contract, is proved by the interesting experiment of causing 

 the muscles of the so-called rheoscopic limb of a frog, to contract, by 

 means of the stimulus of the altered electrical currents of the muscles 

 of another frog's limb. The rheoscopic frog's limb consists of the 

 hinder limb of a frog, denuded of its skin, and cut off just above the 

 knee, but having the whole length of the sciatic nerve preserved un- 

 injured, and still connected with the detached leg. Such a limb, 

 sometimes also called galvanoscopic, serves, when insulated by being 

 laid on a piece of glass, for the various experiments on the motor 

 properties of the nerve ; for the muscles contract when the nerve is 

 pinched, scratched, irritated with saline solutions or acids, or when it 

 is electrified, in which latter case the current must be made to pass, 

 not directly across the nerve, but along some appreciable portion of 

 its length. But the experiment to which allusion is made above, con- 

 sists in laying the projecting portion of the sciatic nerve of course 

 uninjured upon the muscles of another frog's leg, and stimulating 



