232 1NNERVATION. [CHAP. IX. 



are no good grounds for supposing that the molecular change conse- 

 quent upon the stimulation of a nerve is limited to that part of 

 the nerve 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 motor nerves. 

 For a continued or violent irritation of a motor nerve in some part 

 of its course, causing spasm or convulsive movement of the muscles 

 it supplies, maybe propagated along its whole leDgth to the centre, 

 and may there give rise to irritation of neighbouring fibres, whether 

 motor or sensitive, exciting more convulsion and pain. The pheno- 

 mena of many cases of epilepsy, in which the fit begins with irrita- 

 tion of a few muscles, may be referred to in illustration of this posi- 

 tion. And it is equally probable as regards sensitive nerves. If the 

 ulnar nerve be irritated where it passes behind 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 be kept 

 up, the skin of those fingers becomes tender to the touch, its sen- 

 bility being very much exalted. This fact cannot be explained un- 

 less upon the supposition that the molecular change in the nerve 

 fibres, produced by the irritation, extends peripherad as well as 

 centrad, exalting the excitability of their distal extremities. 



At whatever part of their course sensitive fibres be irritated, the 

 same sensation will be produced, whether the seat of irritation be 

 the centre, the periphery, or the middle of their course, provided only 

 the same fibres are irritated in the same degree. Nothing is more cer- 

 tain than that an affection of the central extremity of the nerve-fibres 

 is sufficient to excite sensations precisely similar to those which the 

 excitation of the peripheral portion of the same fibres would produce. 

 Hence it is that a morbid irritation at the centre is frequently referred 

 to the periphery; and that the sensation of tingling or formication, 

 in the band or foot, leg or arm, becomes an indication of cerebral or 

 spinal disease. The remarkable fact, that persons who have suffered 

 amputation will continue to feel a consciousness of the presence of 

 the amputated limb long after its removal, derives some explanation 

 from this doctrine. Two cases have lately come before us, in one of 

 which the arm, in the other the leg, was amputated, so long before 

 as forty years ; yet each person had the sensation of his fingers or 

 toes as distinctly as immediately after the operation. And not 

 only is there, in such cases, the consciousness above referred to, 

 but likewise, when the principal nerve of the limb is irritated, the 



