Section II. VR0PERT1B8 OF TBS, INVOLUNTARY MUSCLES. (Part 2)

and relations of these important organs. The nerves of the cerebro-spinal axi.s do not com^, municate with each other across the median plane ; hut] there are exceptions in the optic nenxs, and ocouioii ally in the pathetic and auditory nerves, which ami united towards their origins ; and olsi> in the pneumOA] gastric ner'es, which are connected in their courve termination. On the contrary, the nervous cords whiel .,,. * Tliabody willbedcKribedintbCMCoiKlpMloriUiekiplar. « OTHRS COHML'NICATIONS. belong to the sympathetic, form free connexiotu across thu median pknc ; tbuc are remurkable ex&inples of this disposition in the cardiac und solar plexuses. The communications, of thu second kind, or those between tlie nervous masses of tlie name side of the body, are very numerous and complicated. Those of the encephalon have been imra'elled and traced witJi most cxiruurdinary skill and success, by Gall. This exctllenl anatomist han demonstrated the uninterrupted couree of the fibres of the spinal cord, and of tho medulla oblongata : he has most satisfactorily proved, that some of the fasciculi of the latter organ arc con- tinued throu(?h the substance t)f the cerebellum, and ultimMely to the laminie on its surface ; that others pass tlirough the pons varolii, the thalami nervorum optico- rum, and the corpora striata, till they at lengtli reach the convcJutions on the exterior of the cerebrum. There are many other connexions between the individual parts of the brain, but the coneidvratioii of tla-m foils within the province of descriptive anatomy. vliThe nerves belonging to the same class communicate together ; thus the dilferent motor nerves are connected witli ejich other, and tJie some may be said of the sensi- tive nerves. There are ako connexiomi between ner^-ea that exercise distinct functions. The spinal oeri'vs, wluch are compOAed of sentient and motor fibres, com- municate with each other ; as in the plexu.ses of the arm-pit and loins. The connexion between the two classes of nerves is even more apparent in the union of the ophthfdmic and superior maxillary divisions of the fiftli pair, which arc of the sentient order, with the branches of the portio dura of the seventh pair, which is a nerve of motion. Besides these lafgu connexions, iw , if.lK* IV RlPITiON OP tlMBOm't 'r-' it 18 thought, by many wialomistR, ibnt the fiIiimi.»nt»'of the iudivuiual nervous cords interlace with vach othor. There is some doubt coDCeming the natme Oftbt cuinmunication within the ph-xuses. Is there an actuttl tntermixtun: of the nervoiui mattvr twlonging to the rfiA ferent fibres ? ' We learn from Monro, that " in the plexttwe, thi fibres of the different trunlu are intermixed^ luid that o'ery nerve under the plexus consists of fibres of all the nerves, which are tied together above its origin from the plexus." Mr. Bell, who has paid great attention to this aubjecti states, that the several motor and sentient filaments re- tain their respective ofhces from one extremity to the other, and that they are as distinct from each other hi their whole course and distribution, as they are at their origins in the spinal cord. Occaaionidly. however, two filaments of n different kind are cumhiiicd m> a.s to be- stow a double power on the nerve thus constituted. „ This distinguished physiologist thinks farther, that in a plexus the motor nerves interchange branch^ with each other, by which means tlie actions of the iiumerOMii muRcles supplied by these ncr'e6, are combined ho as produce the varied motions of the limbs. We may conclude, from these observationa, although in a plexus there is a free connexion iK'twMl' the nervous branches, they principally lelate to thotit <iA tlht motor character ; and that where two brnnrhrs rlif- ferently endowe<l do communicate, each atiU retaint> its proper and distinct oflice. " Tlie last communication that requires to be noticed htP one which has e<msiderable inflncnw on the phenomenW' of the nervous functions ; I allude to the eonneiciim%" J CINBKlTIOUfI AND MEDULLARY BDS3TANCBS. 481 wlu<^ exist between the nerves of the cerebrospinal axis 'wjd ttkosc of the great syrapflthetic. Tliese are usually ACCotnplbJivd by means of ganglions ; and it is in this manner that the cerebral and »piniil nerves oommunicate with the ncn-es of the heart, of the in- te.itines, &c. In some instances the union it established by plexuA, M where the branches of the eighth pair join with the cardiac anl soUr plexuses. 5ll ■' Sbction II. I ' OBOANIZATION. •triTuE dttfurcnt parts of the ncrrous sx'stem ars formed of two Kubcstancea. which arc diatinguished by ChPir' colour, situiiliun, and composition. One of these mate*' rials in diKtinguLshed by the name of the cinrritiotu or eoriiml mbjitance; but Uilli these temnii have been injtii' diciously chosen ; in the first piiire, because this matter varies in its appearance from nearly a white shade to a red, yellowish, or even black colour; and in the second pUc«, because it is occn^^ionally situated in tihe interior;' aod, consequently, cannot form a cortex or bark. The' second sutislancc hns been called the m«<hllaiy matter; ft term equally objectionable an the preceding ; as it has been proved to consist of lihres, it would be moat ap^ propriate lo call it the tihrous substance. ''-' The variations iu the colour of tlie grey matter have' probably g;iven ri^ic to the contradictions oli^erved in au- thors, as to which of the two substances first apfteurs in the deve1o|)ment of the nervous aystem. <tall contends tbjit the grey pulpy substance u formed prior to the white i that it a always found in those places where the ' 2i 482 IS OKEY MATTER FIKST FOBMBD? white fibres become more numerous ; and losUy. that it receives r (Treat number of bltMnl-^'essels. From these circ\unstajic«8, Drs. Gatl and Spuncheim suppose that the pulpy matter is the matrix of the nenres, and that it notmiihes the fibrous substance. An opinion, opposed to this, has been advanced by 8e'eral most acute ana- tomiKt£ ; thm, it is stated by Ticdemann, tbut the fibrous matter of the spiaal cord is first formed ; he admits, how- ever, that the grey substance is accumulated in great quantity in those parts of the cord from which the laige nen-es proceed, and that it auginents the nervous eiier^.* The priority of the appearance of the grey matter is also denied by Serre-s:+ but he states, stibscqueolly, that a grejHsh fluid constitutes the primitive state of tbe spinal cord. We have already oUuded to the fact discovered by Cuvicr, of the nervotui svstem of the a-sterise being entirely composed of white matter ; and it is also stated by Desmoulins, that in fishes, and in many reptiles, tbe spinal cord does not exhibit any grey matter. In answer to these objections, it has been obser^-ed. that the preceding anatomists have paid more attention to the colour of the so called jfrey substance, than to ilt essential properties of pulpiness and vajicularit)* ; and that the important point to be determined is, whether the exi-stence of a gelatinous and gre\ish subsliuice im a necessary antecedent to the fonnntion of the wliiu; fibres. The researches that have been made conccnung the development of the brain and spinal cord in the cmhr^-o of tbe human subject and of animals, prove, (hnt these or:gaas are formed in the first instance of a homogencoiu ■ Sir E. HometilwhM prat ItnpnrlaDfe to ibr pty tuUlaiMe of tlwbnis, r«gnli»{c It ai ihc Mai of memory. Phil. Triw. 1821. t Anat Compartt du Ottmu. Preface, p. 37. DBSCRIPTtOX OP OBEY SUHHTA.NCB. 483 reddish irhitc matter, in which it is impossible to dis- tinguish hctwcci) the cortical and medullary substances ; but that at a later period the two substancts become evident. The grey substance is soft and apparently without fibres ; when it i» divided, its surface presents a number of red points and striee. The opinions of anatomists are not decided with respect to its intimate structure. Some hat'c supposed that it was formed of small follicles ; wliiltit others regard it as a tissue of blood-vcsRels ; it has been proved, however, by fine injections, that there is a pecuUar substance distinct trom the vessels, although the latter are very numerous. The grey matter becomes sofl, and swells a litUe by being placed in water; it also loses a great part of its colour ; it is hardened and rendered whitish by the action of acids, alcohol, and corrosive sublimate ; it is also said to become fibrous. This substance is ne^'er met with in an isolated state ; it is always in connexion with the fibrous matter. It li found in large quantity on the external surface of the cerebrum and cerebellum ; in the striated bodies, in the opdc thalami, and in the quadrigemina] bodies ; also, in the interior of the cerebellum, of the crura cerebri, of the pons varolii, of the medulla oblongata, and of the me- dulla spinalis; it is likewise observed in the ganglions. The nerves arc generally supposed to consist of the white substance ; hut it is said by Monro,* that thoy are of a cineritious colour, and Gall states that the grey substance accompanies the nen'es in their coume.t ■ Ob*. OD th« Kf nroui STMrai. p. 3S. t Rtthmbis lur Ic Sj'Minx! X«rvnix, pp. 66, SI. 2 I 3 484. MBOVLLABT OS F11IS008 HATTER. ' "TKc vAite of Tnp3uIlRfv^riervT>u« suhstnrice is mmb- tnlly fibrous. In mRny parts of the srstera the filnvs -w so distinct thnt thc>' are visible without any artificial preparation ; they are ulso readily seen by scraping any ^MiTtion of the brain. In order, howwer, to Hemonatrate the librvs satisfactorily, it ii necessary to harden lliem by ft prolonged maceration in alcohol, or by the action of diluted nitric or muriatic acid ; the same effect may also he produced by placinf^ a portion of brain in a solution of the oxymuriate of merairj', or for a few minutes in boiling; oil. This fibrou.s substance has long been known to flnatomists ; but it is only in the preseot day that the existcncre of fibres in all parts of it has bceo generally admitted. The size of the nervous fibres ap. pears to vary in the brain, in tlie spinal cord, and in the nerves ; their consistency is also liable to i-ariation. The reevarches of modem nnntumists, and pnrticiUurly thoM ' of Gall, have ascertained the important fact of the con- tinuity of the fibres in oil parts of the nen-ous syBtem. The white substance actpiires a yellowish colour and a homy appearance when it has been dried ; if it be cut in very thin slices it becomes semi-transparent ; it rognins its colour and opacity when it is placed in witter. The minute nrocturc of the grey and fibrous sul stances has been carefiilly examined, with the asfrint of the microscope, but without leading to any very Mt- lisfiictory results. The greater number of those who have undertaken observations of this kind have cotj- eluded, thnt the ncn'ous matter, whetlier pulpy or fibrous, is composed of minute globules, which arc connected together by a viECid and tntnsperent substance. 'ITic observations of Mr. Bauer have commanded] i MICROSCOPICAL OB9BBVATI0KS. 485 gKoi attention in this country, and on the Continent. in conMtjucace of the skill with wliidi they were con- ductvd, und the cxcdk-Qce of the instrunientit employed. According to the lutest of these researches, tlw nervous fiuhi^Lance is composed of globules, vrbicli arc disposed „jn lines, so as bo give the brain its fibrous appeanLOCo; their diameter 'aries from j~jj to j^ of an inch, the pre- dominant size being ^ ; they arc larger and more nu- , mcrous in the medullar)' than in tlic grey matter, which presents hut Urx distinct fibres.* Subsequent to the ^iovestigatioDS of Mr. Uaucr, some observations Iiave been I ipublished in Paris, )>y M. Milne Edwards. They ugrec , ,with the preceding account, except that the globules ore stJited to be of tlie same size in the brain and nerves,, in the four classes of vertebratcd animals. Dr. Hodgkiu and Mr. Lister hnvc not been able to , detect any globular texture in the nerves, or even in the fbrain ; in the latter organ they perceived a multitude of very small piuliclct), irr^ulur in their form and eizv, , Bud which they think arc more dependent on the disintc- .igration than on the organization of tlie nervous sub- ., stance.. They consider that Dr. Edwanls was deceived by the imperfection of his instruments. In examining thin slices of tlie brain 1 have perceived a great number of rounded corpuscles ; but, us ttu»e particles <lid not exhibit a fibrous arrangement, i( .is , doubtful if they ought not to be referred to the cause ., suggested by Dr. Hmlgkio. In the nerves, fibres lure I ■ ThoD pwliclu, Mcordiug to Sa E. Hoiu«, aro coaoceltil logeUw bjF a tnnipaMDt Dd gclilinoiu tulutatioe, wliidi vana in quantiiy in the diMtenl p.iiti of tliv biahi. Ilu imaeeirta thai tUi flatd kO « BOM laipamit part in (, thu up«niionio(U)ciicii'oui tj^em, aud iliutevcaUwcoiBDtunicBtianaf Mn- NlioD and voliUou daptudi, more or Iw^ tqxm iL Piil. Trant. l9i,p.3B. 4B6 BLOOD-VESSELS AND LYMPHATICS. i distinct which pass in a longitudinal nod rather undu- lating manner. I could not distinguish any globulL-s in the fihrcs, although a few were seen irregularly ioter- spenod between them ; it yttm iropOHsible to decide whethtr these globules consisted of nervous mattfcr, or of some animal fluid. The nervous system receives a very ample supply of arterial blood, which is indispensable to tlie exercise of its functions. The great quantity of blood sent to . the brain has attracted the notice of anatomists, and attempts have been made to a&certain its proportion to that which dradates in the other parts of the body. }laller calculated that one fifth of the blood entering the aorta, is carried to the head. This estimate is thought by Monro to exceed the truth i he concludes,, however, that one tenth part circulates within the head, which he says is nearly four times more tlian is dis- tributed in the other divisions of the aortic system, as the weight of the enccphaion is about one-fortieth part of the weight of the whole body. The veins of the brain and spinal cord exhibit several peculiarities, which relate, to their structure, and to the manner of their arrangement. Notliing is satisfactorily known of the lymphatic vessels in the nervous system ; the brain has been minutely examined with Uie hope of discoix-ring them, but without success ; a few trunks only have been detected in the dura mater and pia mater. We may conclude, however, from analog)' and the effects, of disease, that there are in the encephalon as elsewhere, appropriate agents of absorption. Some lymphatics are occasionally perceptible on the surface of the largest nerves. The different organs belonging to the nervous system DURA MATKK, TUNICA ARACHNOIDE9. are enveloped in ccrt&in membrfmes which 'ary in thetr structure mid nuinbur. Tho brain and the spinal cord posscHj threo distinct coverings; the dura mater, the tunica arachnoides, and the pia mater ; the nerves, on the contrary, haw only one tunic, which has been ap- propriately named the nenrilema. The dura nuiter is a strong and fibrous membrane. whidt constttutei^ the Internal periosteum of the skull ; it forms certain partitions which divide the hemispheres of the cerebrum and of the cerebellum, and thus prevent their compression in the various movements of tJie head. The vertebral portion of the dura mater only looKcly aur«> rounds the spinal cord, in order to admit of those exten- sive and complicated motions which are performed be- tween the vertebne. The tunica aracltttoitlfjif is a serous membrane which lines the inner surface of the preceding structure, and also covers the brain and spinal cord. The arachnoid membrane secretes a serous tluid, which has been generally considered by pb'siologtsts to exist in a stale of vapour durini^ health. It has, however, been dis- covered by M. Ma^ndie, that an a(]ueous fluid is always present in living and healthy animals on the surface of the brain and spinal cord, and also in the ventricles of tlie former. It appears to have an impor- tant relation to these organs, for the sudden loss of it occasions dulness and immobility ; in the course of four and twenty hours the water being reproduced, the stupor was observed to disappear. The most important membrane, and, indeed, the only one that is essential to the nervous structure, is ■ See p. 356. t Ste p. IM. 4B8 MurrHEURlLEHA. that which li called pia mata; whtre it in connt-ciwi with the cncephalon and npinal cord, and nextriiaraaj when it covers the nen'es. Man}- anatomists regard tlie pia mater of th« brain as being distinct from the vuscular envelope of the spinal cord, and both, as different textures from the mcmbnuic of the ner%'e». It appenrs to me, however, that thisic coverings are merely portions of the same stnicture, mo- dified in their density and vascular!^' according to the exposure, mobility, or sizo of the organs with wtiicb' they are connected. It h well known that the ment* branc of the spinal cord and the ncurilcma of the crfiniiu nen-es art continuous with the pia mater of the base of the bruin, and that, in a similar manner, the curering of the vertebral neires is prolonged from that tielonging to the medulla spinalis. The difference in the appear- ance of these membranes is not in itself a sutficicnl rea. son to deny the identity of their organization ; for there are many parts thjit exhibit more strildng diversitie«, which undoubtedly belong to the same dttss of organs ; er. ffr. the peritoneum of the loins, and the arachnoid tunic of the ventricles. The inycsting membrane sends various processes be- tween the component parts of the