THE NERVOUS SYSTEM AND TEE INTELLECT. 



267 



almost completely elude the touch of experiment : 

 what means were there of seeking them in the 

 depths of the skull where they lie hid, without 

 endangering life ? How could an instrument be 

 applied to that soft pulp, that the slightest con- 

 tact crushes and kills ? How could a bundle of 

 filaments be isolated, to learn where the tubes it 

 is composed of go, and whence they come ? On 

 the other hand, in a nerve all this is plain ; the 

 filaments are all parallel, and their destination or 

 origin may easily be learned ; they are in a man- 

 ner isolated among the organs, and readily reached 

 by the scalpel, while the slight operation of bar- 

 ing them does not even disturb their action. 



Galen, finding that the nerves on the one 

 hand spread out into the muscles, and on the 

 other distribute themselves to the skin, the 

 tongue, the eye, had understood that they are 

 the principle of motion and of feeling alike ; that 

 they convey impressions from without to the 

 brain, the seat of the intelligent soul, and bring 

 back from it the stimulus causing muscular con- 

 traction. Assuredly, all this was not very dis- 

 tinct in the mind of the Greek physician, but we 

 cannot refrain from admiring him when we find 

 Descartes and Haller, twelve hundred years later, 

 taking up the science at the very point at which 

 Galen had left it. 



At all times the doctrines of philosophers 

 upon life have been more or less modified by 

 reigning theories in physics. It is an almost in- 

 evitable influence, and one which biology has 

 never shaken off. We shall find modern dis- 

 coveries as to the permanence and transforma- 

 tion of force appealed to at present in explana- 

 tion of the action of the nerves. So, too, the 

 great discussion between the partisans of the 

 emission theory and those of the undulatory, in 

 explanation of the phenomena of light, had its 

 echo in the physiology of the nerves. Some pre- 

 ferred the idea of simple vibration ; others, Des- 

 cartes among them, maintained a system more 

 nearly approaching emission; that of "animal 

 spirits " flowing along the tubes of the nerves, 

 lifting in their course " little flaps of skin," true 

 valves, as a liquid would do. Haller thought 

 himself compelled to refute this coarse physiolo- 

 gical materialism, into which even Galen had not 

 fallen ; but in Haller's time optics were neglected, 

 and electricity was in fashion ; there must, there- 

 fore, be a nervous fluid, as there was an electric 

 and a magnetic fluid. But the analogy in this 

 case was almost justified. Nervous phenomena 

 present a striking resemblance to electric ones in 

 several respects, so that to account for the for- 



mer the best course almost always is to call in 

 the latter ; but the resemblance is wholly exter- 

 nal, and there is no indentity as to the nature 

 peculiar to their essence. But it must not be 

 supposed that physiologists see nothing in the 

 brain and the nervous system but a sort of phys- 

 ical instrument ; they have been charged with 

 heresies enough of that kind to give them the 

 right of guarding against accusation in advance. 

 They cite the example of electricity to account 

 for nervous phenomena, precisely as the physicist 

 himself summons the positive theory of vibra- 

 tions in ponderable bodies to explain the phe- 

 nomena of light by the supposed vibrations of a 

 conjectured ether. 



A discovery has lately been made by German 

 physiologists which greatly simplifies the study 

 of the function of nerve-tubes by dispelling an 

 earlier prevalent illusion. When a gland secretes, 

 we see the liquid flow out ; when a muscle acts, 

 we see it contract. Our senses directly judge 

 that these organs are at work. There is nothing 

 like this in the nerves, and we guess at the mys- 

 terious effluvium that courses through them only 

 by its effects produced outside of them, and, as 

 these are in the form sometimes of sensation and 

 sometimes of motion, the two kinds of nerves 

 were supposed to possess different qualities. Cer- 

 tain poisons even were believed to have been dis- 

 covered — in new proof of this difference of qual- 

 ities — such as curare, which killed the motor 

 tubes, and left the sensitive ones alive. Schiff 

 and Du Bois-Reymond (whose coarse invective 

 does not interfere with our regard for his learn- 

 ing) have proved by exceedingly delicate experi- 

 ments that all nerve-filaments are, in truth, con- 

 ductors indifferently like, the electric wires join- 

 ing different instruments in a laboratory. The 

 motor nerves are those whose fluid acts on a 

 muscle, and the sensitive ones those whose fluid, 

 exactly like that of the others, reaches our in- 

 ward sense. If the effects differ, that is only 

 because of the different nature of the organs 

 affected, just as the electric current seems to 

 change its nature according to its action, mag- 

 netizing an iron bar here, producing a spark 

 there, or contracting a muscle, as if it were at 

 once life, magnet, and light. Yet, notwithstand- 

 ing effects so different, the nature of the current 

 in the wire has not varied. 



Though this discovery may seem to have lit- 

 tle concern with psychological inquiries, yet its 

 importance is considerable, because it at once 

 reduces by half the toil of studying the functions 

 of the brain. The part played by that mass of 



