TOTOU8 AT AT KM. 



NERVOUS SYSTEM. 



tw* 



tb* point* oo whkh they 

 ^ a r****Uoh Bight 

 with which UM tonga* 



from tfc* aoMney with which UM tonga* 

 ifc* obsMVotora oj UM >t&*vli*vtt portion* of 

 , aad Anna UM treat *har* whiehth* MM* 



IB UM BM*th.aad from the great ahar* 



hatts of UM lager* oaa dfattagsia double issiirseaViai from M 2th to 

 Mth es* BB iswh apart ; UM palm at about half on inch, and other port* 



of UM stta *t various greater rtM 



bi^afUD^. of tb*iw*r arm. the thigh. and MOM other part., 



noat. Applied to the akin at th* 

 the thigh. and MOM other part., 



.(MiftUiga* 

 yoUMraf it 

 yb.t 



of tiaUmg. or itching. ar burning heat, 

 ; U>* pain of di***M in dinWwit 



peculiar to 



than the skin i* ever the seat of the 

 *t; the muscles 

 it tissues is as varied 

 also ar* in**ooible to 

 .i.j other similar tissoes 



L Certain pwli 

 ; UM frrndiKM a 



sw "! or bmt, or in any way injured without exciting any 

 III Mil, abas UM7 an pulkd or twisted, or rabbeted to any other 

 of tho** mechanical influence* which it is *tmr omoe in the *"iii*l 

 Moam to rmu*. Otbar parts again, a* no* of UM inUrnal organs, 

 UM boo-, and oartilagea, ar* wwoaibl* during health, bat in diaiaM 

 era and peculiar pate. For aU the** differences however no 

 i* y*t known. It u kitown (hat certain nerves (the motor) 



of th* sympathetic system) ore, if evrr. only ooca- 

 of ianistlons ; that others (the nerves of the 

 convey only the impression* of thos* agent* for the 

 ' osrtoin organs ar* peculiarly adapted; and that 

 of conveying impressions of contact, 

 what illffsuBBes of structure or arrangement 

 sble opinion can at present 



M**jy of UM |kii!jni illustrative of th* mod* ofoondnction of 

 UM Borrow laJasnee in UM asnsitiv* nerves are repeated in the motor 

 ilMSBla. with this diaWeaos. that whii* in th. sensitive nerve, im- 

 ed from th* circumference to th* centre, or 

 of the nervous filament in any tissue, to ite 

 UM brain or spinal chord; in th* motor nerves the 

 always paes in th* opposite direction, that is, from the 

 to UM asuiiBiBJiBiui. or from UM brain or spinal chord to th* 

 When tb* motor filament* of a 



are irritated in any part of ite coune. no pain i* felt, but th* 

 to which it is distributed contract; when a similar nerve U cut 

 mated from conveying the nervous influence). 

 th* Uamente that pas* off from it, beyond 

 distributed, loo* th* power of being acted 

 >ny stimulus, 

 reUin their 

 nre which is 

 irriuud, no perceptible 



-__ ~_~.; fcfth* end of the other portion, which i* now asperated 

 CTMB UM hrala, be irritetod. UM muMUs to which it* filamente p 



jrefaeMtiaaityaf aervoo. mbetaoce, OB the absenosofany 

 L""" ***** Bdja*eat BlaBMnte, by wlich one might 

 tsonV* of another, and on the Maenss* of agent, whatever 



MM oa*> of Mjory or otlMr 



Irritated, may be mad* of th* motor 

 As ia by for tb* greater number of 

 IBBMBU are contained within the same 

 described separately, will, in 

 Boh aervca. b* combined. 



AM whM UM MTM of any limb U irritated, auddrn pain U felt and 

 WII.B* oMvohioa* **MM; when rimiW oerre I* di'idod, th* 

 g**?*** < ""'"! *> oUola; if th* upper part of the 

 d.d.wTe(thMrti,,, ililwiUith.liramtb.lrritat.dalBi* 



l. ' 



**> of the lhh tai* jltm, h*j M H 



fthe Mlre f ti* ulurao. wbi 

 **"^^"^*^"* 



UM -.TOI.U.UT, 

 . *" ' * 



along UM 

 toSW 



* it can be tnwmiUed at 

 "'"'or* called Tolun- 



^ totM>dto f >h M{u ' t 

 to which tLy paa. are 



whh th* mind, Mept in the 



rt 



far rvea OB* who 

 **aaf**B>ass 



_ -| a, 



' 



T 4 * th. motor ^ 

 -Uawlo. of the 



ly UM posMoa aad the mod* of 

 L*i4Hf4 osrteia oaMs, order the act of 

 a otrtab) Uad or strength of motion 



i* obliged to exert ooinoldonUy all th* muscles that can minister to 

 that motion. Thus there ar* two muscles, at least, that bend the 

 for* arm, and no person can make one of them act while the other 

 remains inactive; th* will can only determine certain effects, but it 

 cannot determine the muscle* by which they shall be accomplished. 



In all the phenomena hitherto considered, the mind takes a part ; 

 but in some circumstance* an impression pasta* along a sensitive nerve 

 to th* nervous centre, and although no sensation may have been pro- 

 duced, an influence is in return conveyed back from the centre through 

 a motor nerve, and motion is produced, either in the muscle* adjacent 

 to the part first impressed, or in those of soma other part of the 

 body. The phenomena of this clans are those of what is called the 

 Reflex Function ; a term which is derived from the idea that the im- 

 pression, passing centripeUlly, is reflected from the centra as soon aa 

 it arrive* there, and made to pass from it centrifugally. 



For the occurrence of these reflex actions, it is essential tli 

 nerves acted upon should retain their connection with the spinal 

 chord or with the brain. The spinal chord is sufficient for some of 

 these actions, and the power is therefore often spoken of as if it 

 belonged exclusively to it ; but it U evidently possessed by the whole 

 cerebro-spinal axis. Thus, if the hind leg of a reptile or ouy otlirr 

 animal be separated from the body, and the skin of any part of it be 

 irritated, no motion of ite muscles will ensue ; but if the leg be allowed 

 to retain ite connection with that part of tho spinal chord from which 

 ite nerve* come out, the some irritation of the skin will produce con- 

 vulsive contractions of the muscles. In the first case the impression 

 on the skin, if it passed along the sensitive nervous filamente at all, 

 was lost at their cut extremity ; in the second, it passed through them 

 to the spinal chord, and thence either it or tome other influence 

 returned immediately through the motor nerves to the muscles. 



On this property of Reflecting the impressions received from sensi- 

 tive to motor nerves, many of the muscular motions which were 

 formerly regarded as instinctive or sympathetic, and many others of 

 the most important phenomena, depend. Thus when light falls on 

 the retina, the impression received by the brain U instantly reflected 

 through the third nerve, and the iris, without any effort of the will, 

 contract* ; and if the light be very strong and sudden, the eyelids 

 involuntarily wince, as they do before any expected injury. In like 

 manner a stimulus applied to the nose excites involuntary sneezing 

 food, or any other substance in the fauces or pharynx, excites the 

 involuntary act of swallowing, and still more an irritation of the 

 larynx is reflected from the medulla oblongata, and excites in all the 

 respiratory muscles involuntary coughing. In all these cases sensation 

 coexist* with the reflection of tuo impression through the motor nerves; 

 but sensation is by no means essential to the phenomena; on the 

 contrary, in the moot marked examples the centripetal impression is 

 not perceived by the brain. Thus the sphincter muscles are constantly 

 maintained in a state of contraction under tho influence which posses 

 unfelt from the spinal chord ; and all the motions occurring in decap- 

 itated animal* (in which tho reflex actions ore more remarkable than 

 under any other circumstances) must probably be regarded as unac- 

 companied by sensation, since exactly similar phenomena occur in 

 persons suffering from some forms of paralysis, and who can never 

 feel the impression on the *kin upon which the involuntary motions 

 instantly follow. In the case of reflection from the optic nerve to tho 

 nerve* of the iris, and in most of those in which the sensitive nerves 

 pas* directly to the brain, sensation is produced with the reflex action ; 

 but in all th* cases where the centripetal nerve* pass to the spinal 

 chord, the reflex actions take place as well, or even more certainly, 

 after the removal of the brain. The spinal chord is essential for "a 

 reflex action in the nerves that pas out from it; the brain for a 

 imilsr action in ite own nerves: in the first case sensation is not 

 involved, in the second it generally is. 



Mr. Grainger (' Observations on the Spinal fchord') believes that the 

 impressions which produce reflex action*, and those which pass to and 

 from the brain in sensation and voluntary motion, are not conveyed 

 by the same art* of nervous fibres. He considers that there meet in 

 the spinal chord four set* of nervous filamente : one passing only to 

 th* exterior white matter, and then ascending to the brain, to which 

 they convey aensations; a second, corresponding to these, which pas* 

 from the brain along the exterior white matter of the chord, and thence 



the muscle*, for the conveyance of the influence of the will to them. 

 That* two este are the same as the commonly described sensitive and 

 motor filamenU ; th* others are analogous to these, but are supposed 



be subservient only to the reflex actions ; they pass to and from the 



'ilh the others, but instead of being continued up with tlinn 



brain along the exterior of the chord, penetrate into its nub- 



j ^V " . J 060 " 81 *** in it* central gray matter the recipient 



neater of the impressions which they oonvey. The observations 



tbe course of the fiUment* of the nerves, where they pass into tho 



spinal chord, upon which this very ingenious view U founded, have 



* yet Wn sufficiently confirmed to establish iu truth beyond 

 bt; but it still affords the most intelligible explanation of the 



r in which impressions, passing apparently through the same 



nt, in om* coses produce sensation, and in others none; and 



which, for ceruiu purposes, organs supplied with nerves from the 



Br*brXM|dnal axil ar* yet usually removed from the influence of the 



brain. 



