II 



'. .-. I - 



JfERVOUS 8TSTEM. 



11 



CS) th*4 *? ft VMraaafc, *) WCM 

 Ik* CfBBJM Of CM* M ID b* 



. , ,. 



: | - 



,1k -b, 



; *o thai all 



Of OB* and 



y 



Ijelim 

 may occur. _are 



of the eo<eJlod embryowVoeUs 

 other* throw oot a varying 

 'and *; ell create la pert, ooMeoted with 



.at a eohMqUOnt period. totnOreMC bjT 



Many > ii naH*alao 



at all ereette. I do aot know how othenria* to explain the 

 reoce ef two Mclci hi the nerve-ealU of young animal*. 

 " theeaU* DBBainliil by 



The 



ei-biee an nrfcjhMli oa tb* .pot, but their 

 in each a way that the central 



irtia.mil alway. precede the peripheral. With the exception of 

 Ike as.tjea.Hln rf UM Bervce. they arc developed from fusiform 

 ml I'll eeDa, which are nothing eUe than modification* of the 



ve calls of the embryo, and are conjoined into pale. 



, i_^j *&&, or fibre. 0001" ; -<H)OS"' broad. 



only of fibre* of thi* kind, and of th* 

 being gray or dull white, like the 

 aaaUy. in th* human embryo at the 

 they always aauim* a whiter colour, and the 

 llary aubstaoo. continue* to be more and more 

 Uf the three possible modes of development of 

 urn. one only in the precent state 

 that namely, aa to whether the 



la a atruetur* depoaited between th* membrane and 

 eate ef the embryonic and** ted fibre*; in which oaae the 

 ef the letter wouU become the axis fibre. But beside* this, 

 originate in what did not occur to Schwann, 

 atorpbosi* of the external portion of the 



- res; and the axia-fibre may b* only the 

 which he* not undergone a fatty meta- 



to ilslirmlas which of these two view* is 



shows only thia much, that the content* 

 tore* invariably, by degrees obtain dark 



it the aspect of a true dark-bordered 

 aped to the proper origin of 

 it can be proved that the fibres, 

 the. chaaje.de act altar in siaa, th* auppoaition I 

 ild still appear the more correct. 



of the termhMtwa* of the nerve*, which appear* 

 . jaseat mlHloii dinerrat from thoae exhibited 

 ay be readily traced in th* Uil. of the larva of the 

 tae. We there tod. s. U mentioned by Schwann, the 

 ftoaary fWaaaa** of the aerrea to b* pale branched fibres, measuring 

 O-ooi -_r*OT". which here aad there aaaewmoae, all finally termi- 

 Betiaaj m free nbril. ef the fiaeet kind, measuring 0-OOi-'" <r004.'" 

 There no diaaeulty hi aacejlat that Iheee fibre* aria* from the 

 ml mill ef faaKarai or elaiMU cells, for, in th. first pUos, such 

 eaU. he eeaa, in part still in dee* oppceiUaa with bat inde- 

 BeadaBt ef IkcBj ; pert asjcre er Ice* connected by mean* of their 



trt ; aad. asraodly. eaUaocW ooour at the diviaiona of the 



hMa, which ere there Mewhet dOated ; aad, at all cvente in young 

 lanw. wMa UMCB are asacqlils I the weU-kaowB angular vitelline 

 cnrpwsMa. wHh which, at ftrt, all th. cell, of the embryo arc filled. 



hastted to few abort tmaks cioeajy applied to the maeeuatr etntoture* 

 I* UM tofl; bet they are gradeatr/ developed hi Ike 



UM biter IheBMalvaa, almost la the ~" 

 oftheM larvw, oatte directly by deUoate 



Mnl ^BB^Ina of the fuactioas of the slam salary part* 

 been JsrftsJ. KolUher saah*. the following obeerva, 



tBnv4ii"BMal eaaf *^^ t^BBfi*>VHa> ' 



Ww^ that all ite diviaiaa* which preside 

 sac. la greater or lee* 



a gray eiibHaun In grealu or MM 

 UM awdte of UM piaal and cerebral 

 and brain ; while* UM name which 



ef th* gray eubeUao*. H nay 

 II* 1 1 Mil la it* structure a* 

 to thi* I would remark a. 

 with in aitaatioaa frotn 



be** Bvecccd. aa la the anterior bone of the apinal 

 UM ttra* of UM aatorior roote, la the medulU 

 ,et UM psaale ef erada ef Ike BMfar cerebral acrves. hi Ike 

 attaaal eahetoaee ef UM enwbeUMB, the pan. variolii. and orura 



ttaHHri 2 V^aftaA taatf BaHbttiaVC & aWB abvttat aal la^tl aaMkavlti f/as aavkftliam. 



a* la the aatorter haraa ef the apiaal chord, the eoqwra netiformM, 

 aa^^yaijijiMinii There dees act however. u>pear to be any 



eeoaitire cr motor funotiooa ; for, in the ganglia of the oerebro apinal 

 nerrw and of the rympathetio, and in the optic thalami, both aorta 

 of fibrea aria* in one place from amall, and in another from large 

 cella. It etema, therefore, aa in the oaee of the nerve-fibre*, that 

 there are Urge and email motor oelU, H well aa aenaitivc cella of 

 variou* ^|t^-^-^~-. a fact which U confirmed by comparative anatomy, 

 a* the Urge bipolar oaUt in fiabc* are manifeeUy eenaitive. No naential 

 diflerenoe can be pointed out between aenaitive and motor oella, 

 whether the Utter be of uniform or of different iz*, and iu particular 

 the variation! exiatinc between anoh cella are not greater than thoae 

 between UM motor oeD* in difforeot localities. Even the cella in the 

 cortical eubetance of the brain, to which phyaiologiaU aaiigu the 

 mental manifeetationa, with our pracent mean* of research, exhibit 

 no perceptible peculiaritiea. The nerve-cell* however may be divided 

 into thoce which are in direct connection with nerve-fibres, and those 

 which arc not thu* connected, but independent The former, of 

 ooune, are to be especially regarded a* aenaitive and motor; with 

 reaped to the latter, anatomy to aome extent afford* no information, 

 iuumuch a* that they preeent no proceaee* a* in the sympathetic 

 ganglia, and in come situation* in the brain. A* regard* thoae furnished 

 with prooaaaea, particularly the many-rayed cell*, which in many 

 situations undoubtedly are not prolonged into nerve-fibre*, it might 

 be considered certain that they both larger and smaller, by mean* 

 of their prooeaiea which fulfil the function* of nerves, and whether 

 the Utter anastomose or not bring different regions of the central 

 organ* into mutual connection, and participate in the reflex phe- 

 nomena, the sympathies, and other modes of association of the 

 function*. Cells of this kind exirt in the spinal chord and brain every- 

 where in very large quantities, but not in the ganglia ; although it is 

 not from this intended to imply that no reflex action* are performed 

 in thoae bodirc. 



" Beipecting the nerve-fibre*, anatomy i* not in a position to point 

 out any difference in them, between the sensitive and motor nerves ; 

 a circumstance however which physiologically can afford no reason 

 to ascribe identical functions to them. As regards the variou* size* 

 of the nerve-fibre*, the numerous change* in diameter undergone in 

 their courae by all the cerebro-spinal nerves, very obviously indicate 

 that these proportions have no relation to the function* of the fibres 

 in general Nevertheless, I do not look upon these relations of sire 

 a* altogether of little consequence, and in particular does the attenua- 

 tion of the fibre*, where they extend through gray substance, appear 

 to me to be important, a* also their diminution at their origins and 

 termination*. It i* however difficult to perceive the physiological 

 import of theae fact*. Were it the case that in the nerve-fibre* the 

 axis-cylinder alone waa the conducting, and the medullary sheath an 

 iuaulaling substance, and could it be proved that the medullary sheath* 

 were wanting in the attenuated portions, the peculiar activity of the 

 nerve-fibres in these situations (the transverse conduction in the spinal 

 chord, the acutenee* of sensibility at the terminations, Ac.) would be 

 satisfactorily explained. It U well known that auch a notion ha* 

 already been entertained by variou* writer*, and its conception has 

 usually proceeded upon the idea that a close alliance or identity 

 exists between electricity and the nervous force, and the medullary 

 heath abounding in fatty matter ha* from this point of view been 

 regarded aa an inaulator. But first, it is anything but demonstrated, that 

 the nerve* pceasca no other active force but electricity ; and secondly, 

 there is nothing to indicate an absence of the medullary sheath, and a 

 free condition of the axis-fibres in many peripheral extremitie* of the 

 nerree (tkin, muscles), and in those portion* of the central organ* 

 (spinal chord) in which a transverse conduction i* evident The question 

 alway* remain*, whether the medullary sheath, although not alto- 

 gether, yet at all event* partially, may not insulate more or leas, 

 according to Ha thlnlmsea. Since however this membrane is wanting 

 not only in many terminations of nerve*, where an insulating con- 

 ducting (acuity might not be required, but alao in other situations, as 

 in th* /nrrrfedra/a, and the nerve* of ftfreetyso* generally, a* well a* 

 in th* proocae** of the nerve-celts which certainly act a* nerve*, in 

 the central organ* of the higher animal*, and in the finest nerve-fibre* 

 in thoae situations (brain), the notion that auch i* its effect in the 

 dark-bordered nerve* Ion* all ground of support. It would seem to 

 me, the* the medullary sheath represent* nothing more than a pro- 

 tective soft envelop, for the tender central fibre." 



Although the imanselon at a nervous lyitem b a distinguishing. 

 feature of the higbtr animala, th* atuctural trace, of it* existence are 

 not found in a Urge eerie* of truly animal beings. It would appear 

 la the** caeca that the function of both the nervous and muscular 

 system* are carried on by the aim* tissue* Nor do theae functions 

 in the lower animal* distinguish them from plant*, for we find the 

 protoplasm in the interior of the oella of plant* endowed with the 

 same cower as the tieMMC of the polyp or jelly-fish. The pocseesion 

 therefore of a wrvous or muscular system, or the performance of 

 taliatluui cuppoeed to be dependent on these organ*, is not a charac- 

 ... . . ,.,... . 



It U in the higher Xe*U*tf*> that we nod the fint definite indication* 

 of thenirtrac* of a connected nervou* tyttem. It i* probable that 

 each txM. in all the Joa^qOa. although the softneei of their tiatue. 

 lenders it dimcull of detection. AooordSg to Ehreoberg, two nervous 

 may be detected in the Mtimta; one running along the margin 



