PHYSIOLOGY OF THE NERVOUS SYSTEM. 



7-20L 



Effects of the galvanic stimulus. The most 

 perfect and powerful physical stimulus of motor 

 nerves, and that which most nearly imitates the 

 natural mental stimulus, is the galvanic current. 

 That the nerve should be duly excited by the 

 galvanic current it is necessary that the current 

 should pass along its fibres for however short a 

 distance. If it pass across the fibre, and at right 

 angles to it, it will produce no effect upon the 

 muscles; but if it travel along it, even for the 

 twentieth or a smaller portion of an inch, it will 

 effectually excite the nerve and its muscles, 

 just as when the will stimulates it to action. 



The influence of the galvanic current upon 

 nerves is so remarkable that it deserves the 

 careful study of physiologists and of practition- 

 ers in medicine who often have recourse to the 

 galvanic stimulus with the hope of rousing the 

 dormant energies of nerves. It is to the Italian 

 school of Physiciens that we owe the highly 

 interesting series of facts which have been col- 

 lected upon the influence of the galvanic cur- 

 rent upon nerves, to Galvani, Valli, Volta, 

 Marianini, Nobili, and, although last not 

 least, to my distinguished friend, Professor 

 Matteucci, of Pisa, by whose well-devised ex- 

 periments and researches a flood of light has 

 been thrown upon this hitherto obscure and 

 difficult subject. 



I shall content myself here with briefly no- 

 ticing the points most deserving of attention as 

 bearing upon the laws of action of the nerves. 



1. When a galvanic current is passed for how- 

 ever short a distance along a nerve which 

 contains motor fibres, muscular contractions 

 will be excited at the moment of completing 

 as well at that of breaking the circuit, but not 

 while the current is passing. These phenomena 

 take place whatever be the direction in which 

 the current be passed, whether from the nervous 

 centre towards the periphery, (when the current 

 is distinguished as the direct current,) or from 

 the periphery towards the centre (when the 

 current is styled the inverse current). 



These effects may be produced in warm as 

 well as in cold-blooded animals. In the former, 

 however, the physical conditions necessary for 

 the display of the vital forces continue for so 



brief a period that cold-blooded animals should 

 be selected for the experiments. On this ac- 

 count, as well as because of their peculiar sus- 

 ceptibility to the galvanic current, frogs are ge- 

 nerally employed for this purpose. The most 

 striking way of exhibiting the influence of the 

 current, direct and inverse, upon the nerves is 

 illustrated by the annexed woodcut. It repre- 

 sents a frog prepared in the manner adopted by 

 Galvani. The integuments have been removed 

 from the lower extremities, which have been 

 separated from the trunk by the division of the 

 lumbar region of the spine. The lumbar nerves 

 are carefully raised from the muscles on which 

 they lie, but are suffered to retain their con- 

 nection with the spinal cord and with the 

 thighs. The pelvic bones, however, are re- 

 moved so as to admit of the more free separa- 

 tion of the extremities, as well as to isolate the 

 nerves more completely. Each leg is immersed 

 in a glass or cup of water, and the current is 

 made to pass through the limbs by immersing 

 each wire of the battery in the water of the 

 cups. It is obvious that in one limb the current 

 is direct, whilst in the other it is inverse. 



The advantage of this arrangement is that it 

 affords great facility in making and breaking the 

 current without bringing the conducting wire 

 of the battery into actual contact with either 

 limb. One wire may be left constantly in the 

 water, while the other can be alternately intro- 

 duced or removed from it as we wish to ob- 

 serve the effects of completing or of breaking 

 the current. 



2. If the current be allowed to pass for a 

 short time through the nerves of a frog, pre- 

 pared as before-mentioned, contractions will no 

 longer take place in both limbs at the same 

 time, but only in one upon completing the 

 circuit, in the other on breaking. And we 

 shall always find that the contractions occur on 

 making in the limb in which the current is direct, 

 on breaking in the limb in which the current is 

 inverse. I find it useful to adopt the following 

 formula to impress this fact upon the memory ; 

 MD, BI, making direct, breaking inverse. 



3. If the current continue to pass for some 

 time longer, these phenomena cease completely 



Fig. 398. 



Lower extremities of the prepared Frog. 



P, positive wire of the battery ; N, negative ditto. In the limb A the current will be inverse ; in B it will be direct. 



2 z 2 *** 



