CENTRIPETAL AND CENTRIFUGAL FIBRES. 265 



FUNCTIONS OF NERVE FIBRES. 



That the function of nerve-tubes is to conduct impressions, is proved 

 by many different facts. On putting a ligature round a nerve, Functions of 

 or cutting it across, it no longer transmits the usual influ- nerve fibres - 

 ences. A more critical examination shows that impressions made on 

 the external extremities of a nerve are conveyed by it to the centres, 

 and the influences originating in the nervous centres are conducted along 

 such trunks to the parts to which they are distributed. This double 

 duty therefore implies that there are two classes of tubes, the centripetal 

 and centrifugal, though thus far no structural difference between them 

 has been detected. They can not of themselves either originate impres- 

 sions or motions, these in every instance arising from external or central 

 agency. The centrifugal fibres, when cut across, may show Centri etal * 

 no effect if the part still remaining attached to the nerve cen- and centrifu- 

 tre is irritated ; but if the other part connected with the pe- gal fibres ' 

 riphery be pressed upon or pinched, muscular contraction, that is, mo- 

 tion, results. If centripetal fibres be examined in like manner, the part 

 connected with the periphery being irritated, no result arises ; but if 

 the part connected with the centre be irritated, sensations, general or 

 special, as the case may be, are perceived. These several effects ensue 

 when the motor or sensory nerve is intact ; for, on irritating the one or 

 the other, motion or sensation, as the case may be, is produced. If the 

 whole trunk of a centripetal nerve be irritated, the mind refers the sen- 

 sation to all those parts to which the branches of that nerve are distrib- 

 uted ; if a part only, then the sensation is limited to those portions to 

 which the fibrils of that part go ; but, besides this, the mind also recog- 

 nizes the particular spot upon the trunk to which the irritation has been 

 applied. In like manner, when the entire trunk of a centrifugal nerve is 

 irritated, all the muscles which it supplies contract ; or, if only a part, 

 then those muscles which are supplied from that part. From the ana- 

 tomical fact that a nerve-tube does not anastomose with its neighbors, 

 the influences which it conveys are transmitted along it without any lat- 

 eral diffusion, and every fibre discharges one duty, and one Unity of func _ 

 alone. The centripetral can never assume the function of tion in the 

 the centrifugal ; and in the case of nerves of special sense, 

 there is the same restriction: the optic nerve can not transmit the im- 

 pressions of sounds, nor the auditory the vibrations of light ; the nerves 

 of common sensation are affected neither by one or the other, but they 

 are by variations of temperature. The velocity with which Rate of con _ 

 these influences pass along nerve fibres is indefinitely less ductitility in 

 than that with which electricity moves in a metal conductor. nerves - 

 Thus far, however, no satisfactory measure of it has been obtained. 



