EXCITA TOR Y ELECTROMOTIVE CHANGES OF NER VE. 525 



to that under the contact on the cross section. The same result is 

 obtained if the distal portion of the nerve is excited by chemical 

 means, 1 or by a series of mechanical stimuli. 2 Changes of a similar 

 kind also occur in an efferent nerve left attached to the intact spinal 

 cord whenever a prolonged reflex discharge from the central nervous 

 system is evoked. The efferent medullated nerves of other animals, 

 including those of mammalia, give corresponding effects. In the 

 monkey, cat, rabbit, etc., an excitatory negative variation may be 

 observed in the central end of the divided sciatic, when the nerve, 

 the spinal cord, or the cerebral cortex is roused by either electrical, 

 mechanical, or chemical stimulation. 3 



The negative variation may be observed equally well in afferent 

 nerves ; thus, on dividing a lumbar posterior root in the mammal, and 

 connecting the portion peripheral to the section with a galvanometer 

 circuit by means of contacts on its surface and cross section, the negat- 

 ive diminution of the resting nerve change is observed when distal 

 portions of the sciatic nerve are stimulated by induced currents. So, 

 too, the cross sectional difference in the lower fragment of the divided 

 spinal cord shows a marked diminution when either a posterior root or 

 the sciatic nerve is excited, and since, in the latter case, such diminu- 

 tion is abolished by section of all the posterior roots of the nerve, it is 

 clearly due to the excitation of afferent nerves only. 



Just as the negative variation in an efferent nerve can be obtained 

 in consequence of the reflex discharge from the central nervous 

 system, so, too, a similar variation has been obtained in some afferent 

 nerves by the natural stimulus of the peripheral nerve-ending. 

 In the frog, du Bois-Eeymond observed the effect in the upper 

 portion of a divided sciatic when the skin was acted on by acids, etc. 4 

 S. Fuchs has observed the variation in the peripheral end of the 

 trigeminus of the Torpedo divided near its cranial origin, when the 

 skin containing Lorenzini's ampullae and Savi's vesicles was pressed. 5 

 Finally, Beck observed electromotive changes in the occipital lobe of 

 the rabbit when the retina was stimulated by light, 6 and similar effects 

 have been found to occur in other portions of the cerebral cortex in 

 response to excitation of other afferent nerves. 7 



There can therefore be little doubt that the negative variation is 

 the electrical expression of excitatory changes in the nerves. Further 

 proofs of this are furnished by the following characteristics of the 

 electrical change. It is diminished or abolished by such agents 

 as ether, which lower the functional attributes of excitability and 

 conductivity. It is present during the period of excitation, and dis- 

 appears when, on cessation of the stimulus, the other indirect signs 

 of functional activity, such as the muscle responses, subside. It is best 

 produced by such stimuli as are found pre-eminently adapted by their 

 character to evoke the functional activity of the nerve, as judged by 



1 Griitzner, Arch. f. d. ges. Physiol., Bonn, 1878, Bd. xvii. ; 1881, Bd. xxv.; Kiihne, 

 Unsersuch. a. d. physiol. Inst. d. Univ. Heidelberg, 1881, Bd. iii. 



2 Hering, " Lotos," Frag, 1888, Bd. ix. ; Steinach, Arch. f. d. ges. Physiol., Bonn, Bd. 

 vi. S. 516. 



3 Gotch and Horsley, Phil. Trans., London, 1891, vol. clxxxii. B. 



4 E. du Bois-Reymond, "Untersuchungen," Bd. ii. S. 473, loc. cit. 



5 Fuchs, Arch./, d. ges. Physiol., Bonn, 1895, Bd. lix. S. 468. 



fi Beck, Centralbl.f. Physiol., Leipzig u. Wien, 1890, Bd. iv. S. 16. 

 7 Caton, Brit. Med. Assoc. Meeting, London, 1875 ; Fleisch. v. Marlow, Centralbl. /. 

 Physiol., Leipzig u. Wien, 1890, Bd. iv. S. 18 ; Danilewsky, ibid., 1891, Bd. v. S. 1. 



