GENERAL PHYSIOLOGY OF THE NERVOUS SYSTEM. 203 



they are attached to special things. The special physiology 

 of various nerves will hereafter be considered in connection 

 with the working of various mechanisms in the Body. 



The Nature of a Nervous Impulse. Since between 

 sense-organs and sensory centres, and these latter and the 

 muscles, nervous impulses are the only means of communi- 

 cation, it is through them that we arrive at our opinions con- 

 cerning the external universe and through them that we are 

 able to act upon it; their ultimate nature is therefore a 

 matter of great interest, but one about which we unfortu- 

 nately know very little. We cannot well imagine it any- 

 thing but a mode of motion of the molecules of the nerve- 

 fibres, but beyond this hypothesis we cannot go far. In 

 many points tl^e phenomena presented by nerve-fibres as 

 transmitters of disturbances are like the phenomena of wires 

 as transmitters of electricity, and when the phenomena of cur- 

 rent electricity were first observed there was a great ten- 

 dency, explaining one unknown by another, to consider ner- 

 vous impulses merely as electrical currents. The increase of 

 our knowledge concerning both nerves and electric currents, 

 however/has made such an hypothesis almost, if not quite, 

 untenable. In the first place nerve-fibres are extremely bad 

 conductors of electricity so bad that it is impossible to sup- 

 pose them used in the Body for that purpose; and in the 

 second place, merely physical continuity of a nerve-fibre, 

 such as would not interfere with the passage of an electric 

 current, will not suffice for the transmission of a nervous im- 

 pulse. For instance if a damp string be tied around a nerve, 

 or if it be cut and its two moist ends placed in contact, no 

 nervous impulse will be transmitted across the constricted or 

 divided point although an electrical current would pass 

 readily. An electrical shock may be used like many other 

 stimuli to upset the equilibrium of the nerve-molecules and 

 start a nervous impulse, which then travels along the fibre, 

 but is just as different from the stimulus exciting it as a 

 muscular contraction is from the stimulus which calls it 

 forth. 



Careful study of the action-current give's, perhaps, some 

 information regarding the nature of nervous impulses. That 

 local negativity which causes the current begin& at the stimu- 

 lated point of a nerve at the same time as the nervous impulse 

 and travels along the nerve at the same rate. Hence we con- 



