T20K 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



or toes.* In such cases the central segments 

 of the amputated nerve-fibres remain ; if they 

 retain their healthy condition, they continue 

 to represent in the sensorium the various 

 points on the surface of the amputated limb, 

 and likewise the muscles which they were 

 destined to supply. If, however, the inte- 

 grity of the nerve-fibres has been impaired in 

 consequence of any morbid action which may 

 have followed the operation, then the sensation 

 exists imperfectly or not at all. 



It may be stated in connection with this 

 subject, and in confirmation of the view above 

 taken, that in many cases of complete paralysis 

 of a limb from cerebral disease, the patient, 

 although perfectly clear in his general mental 

 perceptions, is not conscious of the presence of 

 the paralysed member, and really feels as if it 

 did not exist. I have known instances in 

 which this unconsciousness has been so great 

 that the patient has actually mistaken the para- 

 lysed part for the limb of some other person 

 coming in contact with him, or for some en- 

 tirely foreign substance. One man fancied that 

 his paralysed arm was his wife's, and called to 

 her to take it away. In such cases the morbid 

 state of the brain prohibits the developement 

 of that affection of the centre of sensation upon 

 which the feeling of the connection of the limbs 

 depends. f 



The same law of action applies to nerves of 

 special, as to those of common sensation. 

 Thus, whilst ordinarily they propagate to the 

 centre impressions made at the periphery, we 

 find nevertheless that irritation of the nervous 

 trunk at any part of its course may give rise 

 to its peculiar sensation; and if the brain be 

 stimulated at the part in which the nerve is 

 implanted, similar sensations may be produced. 

 The phenomena of vision and hearing which 

 are excited in these ways are called "subjective;' 7 

 they are familiarly known to medical men as 

 not unfrequent precursors of more serious 

 symptoms of cerebral disease. Muscae voli- 

 tantes, ocular spectra, and tinnitus aurium, are 

 the most common instances of these pheno- 

 mena. Pressure on the eyeball, a galvanic 

 current passed through it or very near it, rota- 

 tion of the body, are capable of giving rise to 

 similar phenomena, by exciting the retina or 

 the central connections of the optic nerve, or by 

 disturbing the circulation of the blood in them. 

 A sense of giddiness, similar to that produced 



* Mliller records several instances in his Physio- 

 logy, vol. i. p. 746. (Eng. edition.) 



There is a man now in King's College Hospital 

 who suffered amputation at the upper third of the 

 arm, and whose entire scapula, with the shoulder 

 joint and great part of the clavicle, was removed by 

 Mr. Fergusson within the last two months. This 

 man still feels his fingers. 



t Valentin states that persons who are the sub- 

 jects of congenital imperfections, or absence of the 

 extremities, have nevertheless th'e internal sensa- 

 tions of such limbs in their perfect state. Accord- 

 ing to the view above taken this could not be, 

 unless the primitive nervous fibres are present 

 in their full number in the trunks of the nerves des- 

 tined for the limb. Repertorium fur Anat. und Phys. 

 1836, p. 330, and note to Baly's translation of 

 Miiller's Physiol. vol. i. p. 747. 



by the means last-named, is also a very com- 

 mon symptom of cerebral affection arising from 

 a disturbed circulation, or from the blood being- 

 deficient in one or more of its staminal princi- 

 ples, or vitiated by some morbid element. 



The stimuli of nerves. Nervous action is 

 ordinarily provoked by stimuli of two kinds, 

 mental and physical. Mental stimuli are those 

 resulting from the exercise of the will, or from 

 thought. Physical are due to some external 

 excitant; light, heat, sound, mechanical stimu- 

 lation, chemical substances, as acids or alkalis, 

 or electricity. 



In all voluntary movements an act of the 

 mind is the excitant of the nerve. Sensations 

 are caused generally by the influence of physi- 

 cal agents upon the peripheral extremities ol 

 nerves, which communicate with the sensorium 

 commune. The change thus produced in the 

 nerve gives rise, through the medium of this 

 communication, to a corresponding affection of 

 the mind. A mental stimulus, however, may 

 affect a nerve of sensation. Such stimulus 

 would originate in that part of the brain which 

 is the seat of the changes connected with the 

 intellectual actions, and affecting the centre of 

 sensation, would excite in certain sentient 

 nerves a change similar to that which a physical 

 stimulus applied to their peripheral extremities 

 is capable of producing. In this way the mind 

 is capable of exciting pain in any part. When 

 the attention has been long directed to any 

 particular situation, whether it has been pre- 

 viously the seat of pain or not, painful sensa- 

 tions may be excited there. Of this we have 

 many instances in practice. In the treatment 

 of cases of hysteria it is of great importance, 

 on this account, to direct the attention of the 

 patient as much as possible away from any 

 local affection. 



Motor nerves are never immediately excited 

 by a physical stimulus in the ordinary actions 

 of the body. A physical stimulus acts upon a 

 motor nerve always through a sensitive nerve ; 

 the actions thus produced are, commonly, called 

 reflex actions from the apparent reflexion of the 

 change excited by the afferent or sensitive nerve 

 in the nervous centre into the motor or efferent 

 nerve. This class of actions was first pointed 

 out and described by Prochaska, who viewed 

 them as consisting " in reflexione impressionum 

 sensoriarum in motorias." The contact of a 

 foreign substance, pressure, titillation, are the 

 ordinary physical means by which such actions 

 may be excited. As a good example of this 

 may be quoted the act of deglutition at the 

 isthmus faucium. 



Physical stimuli of other kinds, however, 

 may excite motor nerves. The pressure of a 

 morbid growth of any kind may irritate such 

 nerves and create spasm of the muscles they 

 supply. Any virulent fluid applied to a motor 

 nerve will irritate in a similar way- hot water 

 liquor potassae a mineral acid- a solution 

 of strychnine, &c. And for the same reason 

 certain morbid matters in the blood may irritate 

 nerves whether sensitive or motor, causing the 

 so-called neuralgic pain in the one case, and 

 cramp or spasm in the other. 



