PHYSIOLOGY OF THE NERVOUS SYSTEM. 



720i 



nerves, of which one has been satisfactorily 

 proved to be sentient, the other motor, the 

 former being generally the larger, and having 

 the peculiar feature of a ganglion being formed 

 upon it. 



There is scarcely 'a nerve in the body, which, 

 in strictness, ought not to be regarded as a com- 

 pound one ; the physiological character of each 

 nerve must depend on the endowment of the ma- 

 jority of its fibres, and the nerve will be called 

 sensitive or motor, according to the predomi- 

 nance of motor or sensitive fibres in it. For 

 example, the facial nerve, or port io dura of the 

 seventh pair, is called motor, because it is almost 

 wholly composed of motor fibres ; but it 

 contains, besides, in very much smaller 

 number, some sensitive filaments which it 

 derives from anastomoses with neighbouring 

 nerves. The third, fourth, and sixth nerves 

 are of similar constitution to the facial. In the 

 ramifications of the fifth nerve, on the other 

 hand, the filaments of sensation are predomi- 

 nant; those of motion being much fewer, and 

 confined to the ramifications of its inferior 

 maxillary, division. 



There is no difference between a motor, and 

 a sensitive nerve as regards structure. Ehrenberg, 

 indeed, endeavoured to establish that the vari- 

 cose character of the fibre belonged to nerves 

 of special sense; but subsequent observation 

 showed this to be incorrect. We can attribute 

 the difference of endowment of the fibres to 

 no other cause, but to the nature of their peri- 

 pheral and central connections. The same 

 nervous force is propagated by the fibres of 

 each kind, but whether that force is to excite 

 motion or sensation must depend on the connec- 

 tion of the fibres with muscles in the one case, 

 and with the centre of sensation in the other. 



The terms afferent and efferent have been 

 used in expressing the function of different 

 fibres, and they are convenient terms to a cer- 

 tain extent. But the use of them tends to con- 

 vey erroneous ideas respecting the change which 

 takes place in a nerve when stimulated, as if 

 that change took place only in one direction. 

 It is true that, in a motor nerve, the stimulus 

 ordinarily acts from the centre, and the nervous 

 force is propagated peripheral! ; and on the 

 other hand, in the sentient nerve, the stimulus 

 is usually applied at the periphery, and the 

 nervous force proceeds centrad. It is the place 

 at which the stimulus is applied which usually 

 determines the direction in which the nervous 

 force travels. But there are no good grounds 

 for supposing that the molecular change con- 

 sequent upon the stimulation of a nerve is 

 limited to that part of the nerve-fibre which 

 is included between the point stimulated, and 

 the centre or the muscles, where the effect 

 of the stimulation appears; on the contrary, 

 it is not improbable that, at whatever point 

 the stimulus be applied, the whole length 

 of the nerve-fibre participates in the change. 

 This is not unlikely in the case of molor 

 nerves. For a continued or violent irritation of 

 a motor nerve, in some part of its course, caus- 

 ing spasm or convulsive movement of the 

 muscles it supplies, may be propagated along 



VOL. III. 



its whole length to the centre, and may there 

 give rise to irritation of neighbouring fibres, 

 whether motor or sensitive, exciting more con- 

 vulsion and pain. The phenomena of many 

 cases of epilepsy, in which the fit begins with 

 irritation of a few muscles, may be referred to 

 in illustration of this position.* And it is also 

 very probable as regards sensitive nerves. If 

 the ulnar nerve be irritated when it passes be- 

 hind the internal condyle, a sensation of tingling 

 is excited, which is referred to the sentient 

 surface of the ring and little fingers ; and if 

 the irritation is kept 'up, the skin of those fingers 

 becomes tender to the touch, its sensibility 

 being very much exalted. This fact cannot be 

 explained unless upon the supposition that 

 the molecular change in the nerve-fibres, pro- 

 duced by the irritation, extended to the peri- 

 phery as well as to the centre, exalting the 

 excitability of their distal extremities. 



It is a highly interesting physiological fact, 

 which has an important practical bearing, that 

 at whatever part of their course sentient nerve- 

 fibres be irritated, the same sensation will be 

 produced, whether the seat of the irritation be 

 the centre, the periphery, or the middle of their 

 course, provided only the same fibres are irri- 

 tated in the same degree. Thus it frequently 

 happens that sensations are referred to the ex- 

 tremities of a nerve when the existing irritation 

 is situated at its point of implantation in the 

 centre. The sensation of tingling or formica- 

 tion, in the hand or foot, arm or leg, is fre- 

 quently an indication of cerebral or spinal 

 disease; but the practitioner should not forget 

 that precisely the same sensation may be caused 

 by an irritation taking place in the course of the 

 nerve. I have frequent occasion to estimate 

 the importance of this fact in the treatment of 

 cases of Sciatica. This disease generally con- 

 sists in an irritated state of the nerve in some 

 part of its course by a gouty matter, and it may 

 be treated with the best effects by blisters ap- 

 plied over the nerve. As, however, the morbid 

 impregnation may have taken place at any part 

 of the course of the nerve, it is a very useful 

 practice, when a single application fails, to apply 

 the blisters over different parts in succession, 

 instead of confining the vesication to one region. 



This law of action of sensitive nerves gives 

 the clue to the explanation of the extraordinary 

 but well-attested fact, that persons who have 

 suffered amputation will continue to feel a con- 

 sciousness of the presence of the amputated 

 limb, immediately after, and often for a long 

 time, or even always, after its removal. I have 

 met with two cases, in one of which the arm, 

 in the other the leg, was amputated so long 

 before as forty years ; yet each person declared 

 that he had the sensation of his fingers or toes 

 as distinctly as before the operation. And not 

 only does the consciousness above referred to 

 exist, but likewise, when the principal nerve 

 of the limb is irritated, the patient complains of 

 pains or tingling, which he refers to the fingers 



* I am aware that these phenomena admit of 

 another explanation, but there is no reason why 

 they might not likewise originate, in many cases, in 

 irritation of a few motor fibres. 



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