August 27, 1886.] 



SCIENCE. 



197 



sort of vibratory action, as sound is transmitted 

 through a stretched wire. As this theory does 

 not involve the using up of any material, but sim- 

 ply the transferring of motion, it may be called 

 ' the kinetic hypothesis.' 



Inasmuch as the discharging hypothesis involves 

 the destruction of organic material, we may, if 

 this theory be correct, reasonably expect to find 

 in the active nerve-fibre evidences of chemical de- 

 composition and of heat production. Moi'eover, 

 if the organic substances are used faster than they 

 are replaced,' or their products of decomposition 

 removed, as they would naturally be under con- 

 stant stimulation, we may expect to observe a 

 diminution of nerve-action during the continuance 

 of the stimulation ; in other words, we shall have 

 the phenomena of fatigue. On the kinetic hypoth- 

 esis, on the other hand, we may expect to find 

 an entire absence of chemical decomposition and 

 fatigue, and, if the moving particles are endowed 

 with perfect elasticity, an absence also of heat 

 production. 



The only functional chemical change of nerves 

 for the existence of which an experimental proof 

 has been offered, is the change in the reaction 

 with test-paper. Just as the normally alkaline 

 tissue of muscles becomes neutral or acid in 

 activity, so, according to Funke^ and Ranke,^ do 

 nerve-fibres and the w^hite substance of the spinal 

 cord change in activity from an alkaline to an 

 acid reaction. Liebreich^ and Heidenhain,* on 

 the other hand, experimenting with a slightly 

 different method, failed to get any evidence of 

 the acidification of nerves in connection with 

 functional activity. The phenomenon must in- 

 deed be a delicate one, since Ranke himself urges 

 that the question should be decided by experi- 

 ments on the spinal cord, and should not depend 

 upon the ' doubtful results of tests apjalied to the 

 nerve-trunks.' Now, since the cord contains 

 gTay as well as white substance, and as the gray 

 substance, according to Ranke hiujself, becomes 

 more acid than the white in functional activity, 

 it is clear that an acid reaction of the white sub- 

 stance of the spinal cord may depend upon an acid 

 formed in the gray and passing by diffusion into 

 the white substance. This possibility, which is 

 indeed admitted by Ranke, seems to deprive the 

 experiments on the spinal cord of what little value 

 they possessed as evidence of the production of 

 acid in connection with the activity of nerve- 

 fibres. 



The other chemical changes which have occa- 



1 Arch. anat. undphys., 1859, 835. 



2 Centralbl. med. wiss., 1868, 769 ; 1869, 97. 



3 Tagebl. naturf. vers. Frankfurt, 1867, 73. 



* Studien, iv. 248 ; Centralbl. med. wiss., 1868, 633. 



sionally been asserted to occur in active nerves, 

 rest on still weaker experimental evidence, and it 

 is therefore clear that chemical investigation gives 

 us but little reason for maintaining a discharging, 

 in opposition to a kinetic, theory of nerve action. 



The first experiments to test the heat-produc- 

 tion of active nerves were those of Helmholtz,^ 

 who, after studying the analogous phenomenon in 

 muscles, extended his investigations to nei-ve- 

 fibres. He failed, however, when all sources of 

 error were carfuUy avoided, to obtain any evi- 

 dence of heat-production in connection with 

 nervous activity, though his apparatus was capa- 

 ble of registering a change of temperature of 

 0.002° C. Similar negative results were obtained 

 by Heidenhain.^ On the other hand, Valentine,^ 

 Oehl,* and Schiff^ maintained that nerve-fibres 

 really are warmed by the passage of the nerve 

 impulse. It seems, then, that the results of 

 thermometric investigations speak no more posi- 

 tively than those of chemical research in favor 

 of a discharging rather than a kinetic theory of 

 nerve action. 



The evidence of the activity of a nerve may be 

 either direct or indirect. The direct evidence 

 consists in the occurrence of that change of the 

 electrical condition known as the * negative varia- 

 tion,' of Du Bois Reymond, or the ' action-cur- 

 rent,' of Hermann. The latter writer quotes the 

 former as authority for the statement that this 

 phenomenon becomes less intense in successive 

 repetitions of the experiment, and regards this as 

 evidence of the exhaustion of the nerve-fibre. 

 Unfortunately, Hermann does not refer to the 

 exact passage which contains this statement, and 

 an examination of the chapter on the negative 

 variation of nerves, in Du Bois Reymond's ' Un- 

 tersuchungen,' fails to show any systematic study 

 of the effects of fatigue on this i^henomenon. 



The indnect evidence of the activity of a nerve 

 consists in the effect which it produces upon the 

 central and peripheral organs with which it is 

 connected. Of these effects, the contraction of a 

 muscle is the one which is most conveniently ob- 

 served, but the fact that a muscle is more readily 

 exhausted than a nerve, renders it impossible to 

 study the fatigue of nerves in this way without 

 some special modification of the experiment. 



Bernstein '^ was the first to employ the muscular 

 contraction in experiments on the exhaustion of 

 nerves. This observer finally reached the conclu- 

 sion that a nerve may be exhausted by 5' - 15' 

 tetanic stimulation. The experiments of Bern- 



1 Archiv. anat. und phys., 1848, 158. 



2 Studien, iv. 250. s Moleschott ' Undersuch.' ix. 225. 

 * Gaz. med. Paris, 1886, 225. s pflUger's archiv., iv. 230. 

 ^ Pfiilger'^s archiv., xv. 298. 



