Section II. VR0PERT1B8 OF TBS, INVOLUNTARY MUSCLES. (Part 1)

Section II. VR0PERT1B8 OF TBS, INVOLUNTARY MUSCLES. ifm\i ti I III' I - < . ' .i I'lii I .H Having, in the 5rst paK of tliis chapter, pointed out the most important faculties of the muscular system in general, I shall in tliis place only notice some peculia- rities which arc exhibited in the involuntary organs. The greater number of the physical properties are more striking in their muscles of tlm, than in those of the pre- ceding class. Thus their fibres are nwre extensible, more cobe&ive, and more elastic ; and in virtue of the latter power, they possess a greater degree of tonic contrac- tion. |, The principal differences, howe%'er, relate to the vital properties, and particularly to the manner in which con- tractility i-s excited. In the state of health, the involuntary muscles are ge- 2 ii2 p '^ NKRV0U8 P0W8R NOT RSSBNTIAL nerally considered as Iwing insensible, and in the ordinaiy accvptatiou of that t«nr), this opinion is corroct ; for it has bi-en asccrt^iiiicd, timt if tlic heart he expoM^l during life in tlie buniau subject, it may be touched qlmosl with- out the consciousness of the indindual. But, as X harp had o(X^a.tion fre<]uently to remark, it by no mieatu fol- lows, because the mind does not percet%x tiie imprcsnoo made on auy organ, that therefore the organ is insen- sible. Oa tlte coutrary, it 13 known tlutt the iris, Iho heart, the fibrous coat of the stomach, &c. arc capable of rc<:ei«ng the impression of liglit. of blood, food, &c. which act a& stimulants to excite their contntctioa. The contractile power of the bolluw mu^es, which in essentially the same as that of the exlemiil muscles, is n-raarkable in consequence of the maimer in which it is excited. Volition, which is the nntural stimulus to the latter class of oi^ans, is never capable of determin- ing the contraction of the former ; thus, for example, we can at pleasure excite the action of tlie muscles of the arm, but no cfiTurt of the will cim produce any alteration in the movement of the heart, or in peristaltic contraction of the intestine.* Again, the brain nnd the nen'es which are cst^ential to the exercise of voluntary motion, are not necessary to the production of involuntary action ; for thus is manifested in those fuituses which have been bom alive without either brain or spinal cord ; and also, tn the Gise of tlie heart, aAer all communication betwi'cn it and the cacephalou has been cut off by the division o{ * Altliough the will Iwi usually m Idlle kflueno* on Ibe ulton of tim tnvoluntBry minrlo, yd Ihm tiR initancca of ■ contitry nttwo, So<m individukli bave (U pow«r of rumiauLaD, nfanhiia pnoM wqWfiDt m act of the ulll. Ailill monf Trmnrkiiblcraa! i* Kilted bj Dr. CbajroD oT U oflicui rMidiug at Botli, who luicl Diu pow^r o( ttttiniy ilonitng the cwilnc- UoM of the htui ; utd a niralUr naitipic b menlkined by fttlm. TO INVOLUNTARY MOTION. 469 its nerves. These facts prove that under ordinary cir- cumstances, neither volition nor the influence of the nervous system, is essential to the action of the heart, or to that of the other involuntary muscles. It may then be asked what determines these organs to contract. In answer to which query it may be stated, that they are excited by local and peculiar agents — the heart by the blood — the stomach by the food — and the iris by light.* * Notirithilftnding that in Ibe quiet ibte of the body, the involuntai; mo* tioiw proceed Independently of tlffi narvoui tyttnu, it ii necesaty to itpnl vbat »u italed in a preceding page, that tbey nwy be influenced by the panions of the mind, and alio by mecbanical and cbemical agenti applied to tlie brain and tpioal cord. If it be objected that Ifaia statement implfet a contradiction, we may reply with le Galloii aod Philip liial two bet* well Btoertaftted, bowevu inconsiitent they may aeem, do nol OTOrtuni each otb^, ^ulonly prove the imperfeclion of our knowledge. 470 NKEVOUS SYSTEM. CHAPTER ELEVENTH. OF THE NERVOUS SYSTEM. This system in the human species consists : I. Of a large mass called the hrain. 2. Of the spinal cord.* 3. Of numerous cords termed nerves. 4. Of certain enlargements or knots, which are distinguished by the Dame of ganglions. These bodies are Che essential and exclusive organs of sensibility ; a property which produces most important and diversified restilts in the animal economy. The ancients, under the same name, confounded the nerves with the ligaments, the tendons, and even with the vessels. Although some discoveries were made previous to his time, yet Galen was the first anatomist who satisfactorily distinguished the nerves from the tendons and ligaments, by giving to the latter separate and characteristic appellations. This celebrated man also ascertained that the nerves are medullary in the interior, and membranous on the exterior, and in this manner he established their connexion with the spinal cord and the brain. He further observed, in opposition to the existing opinions, that the spinal marrow was * The brftLQ and Ihespinat cord are deacribed byinan;^ uiKtoiiiiMiof Dk preamt ilay, uudcr Ihe term of tnccphalom other vrriten restrict theme of Ibia word in tcconUnce wiili its derivation, and apply it only to those pari* of the tjntem that are lodged within the head. DEVBLOPUINT.IN ANIMALS. subservient to the brain ; and he is likewise regarded by many authorities, as the discoverer of the ganglions. Subsequent to the time of Galen, the science of anatomy was nearly stationary, and continued so during many centuries, so that it was not until the revival of letters that any progress was made in the lniowle<lgc of llie nervous system. At this period the structure of the body was cidtivatcd with great success ; although, in consequence of tlie too indiscriminate deference which was paid to the 0[)inions of the ancients, many errors were revived ; and these being tenaciously adhered to by succeeding anatomists, produced an injurious influence which has extended even to the present day. Within the last few years, however, a surprising progress lias been made in this bronf h of science. By the assistance of com-^ parativc anatomy, the real nature of many piurts of tlie nervous system, and the relations existing between them, have been satisfactorily explained ; whilst by obser^'a- tions on man, and experiments on the inferior animals, their functions have been greatly elucidated. , The origin and successive development of this system, have been traced from the most simple to tlie most complicated animals ; and this invcsligittion has proved that there are no organs which exhibit in their forma- tion, so perfect a gradation from simple to compound u those under cotuideration. (^, The lowest animob, the infusory animalcula;, appear ,tf> possess no nen'ous system, the 6rst traces of it being perceived in some of tlie polypi, as the hydra, under the form of microscopical particles which are disseminated thmugti the substance of their bodies. In all the Other classes of tJie invcrtebrated animals, tlie system may be described as consisting es»eutiully of swellings m'- DKYSLOfMRNT IN ANIMALS. organt^iuns wliich vary in -magnitude, and of oordaofii □CT'es connected with them. The gangliu And tilomeDts.. are first dUtm)rui)ihed in the nuliota, lutd vspt'cuitly iu tlie UBtefiae ; iu each of tliese animals the central orgnn- eoDsUls of a ring of white nervous matu-r* !>urroand-> > tng tlte iDoutb, or orifice of the fttumach, uid givfa^^j off delicate ibrcudfi, which arc diBtribuled iu a radiatio^r maimer tu tltcir f)ufl and contractile sutjslatw:e. •> In the articulated animals the nervoua system ceu*'! aists of. two cords, passing nearly in a parallel dirot-tion from one to the other extn-mily of the body ; on c8cU,i cord a small gauglion or nudule i& formed opf>o»itc U> \ each of the segments into which tlte animul is di'ided. ,1 Anatomists are not agreed on the nature of the flln- mcntH which proceerl from these ganglions ; &oTae m- t garding them as being analogous to the nvrvea of tiiftii sympathetic s'stem, whilst others compare them to tboMt of the spinid cord. The latter theorj- is supported by Mr. Bell, who obserx'es. that the ner-e8 of tlie leedi, or, ■worm, bestow sensibility, and also regulate and combing all the voluntary motions of these animals-f Now, 93' the same pro])crties in the vcrtcbrata are connected with the spinal cord, and with the nerves arising from it, wo, may conclude that these are tJie parts wliich correspond] with the nervous sv'stem of the invertebrated animals.} In some of the moUusca there is a distinct rudiment of n brain, which is enclosed in an imiierfecl and cur-, tilaginous cranium ; this is an evident indication of a * Cuvtcr «M lb* ftni who uuitaiiita lluil Uic iwrvow tyUen of ilie Mar- fiih IB cctupoKd or while aaUa wttlioul aixy uilautiUwc of ibc i^ny mIv ■Uncv. t PhihDS. Tiuui. I8?9, p. 309. BipMhlon oOhe Ktrm, p. 41. ) ftU<Fu'Oullinuuli1i)uolijg]r, p. 'J7I>. - IH TOB VBRTSBKATA. 478 proper nervous centre ft)r regulating the organs of sen- sation and of motion. The nervous system of the vertebrated animals con- sists of a central mass, which is composed of a body called spinal cord, and of an enlargement which is added to its superior extremity, under the name of brain. The former structure gives attachment to a number of nerves, regularly disposed in pairs on the two sides of the median plane of the body, each of which possesses, at its central extremity, a swelling or ganglion. Lastly, there exists on tiie fore part of the vertebral column, two gan^onic cotds, named the p-eat gympathetie nerves. These component parts exhibit many varieties in the fouf dasses of the vertebrated animals, which' have been traced wift great mionteness by several modem anato- nusts. • ThoN wbo take an iataant ia this impoftaot branch of compnatira BDatoniy, will find ample details in the writings of Sttm, TiultQumD, Spuor. belin and Desmoulins. 474 CONNEXION BBTHVEN THB BEAIN ■■■■■■! PART FIRST. Section t. OF THE' NBRVODS SYSTEM IN OENERAI,.' . The different parts which compose this sjstein, al- though endoTced with distinct powers, are united with. esch other in such a manner that they constitute a coQr, nected Vhole.. Anatomists, from the time of Galen downwards, have very generally regarded the brain aa the sole origin and centre of this system ; and in accord- ance with this idea, the spinal cord and the nerves werc^ described as prolongations of the cerebral organ, and «ven the great sympathetic was viewed in the same light. The researches of comparative anatomy have proved the frrorof this opinion; for it is now well known that the nerves and the spinal cord are formed in the inferior animals and in the embryo of the higher classes, anterior to the brain. Numerous examples are also recorded of acephalous monsters in which, although the brain was deficient, the nerves and the spinal cord were perfectly formed. Many physiologists have more recently supposed that the nervous system possesses more than one centre. Thus Dr. J. Johnstone, towards the close of the last century, pointed out the distinction between the nenTs of voluntary and involuntary parts ; the former being derived from the brain and spinal cord, and the latter <iAJin AND'Tire NlBVMJ'jfiriC- 478 from the ganglions.* Tim doctrine was subsequently embraced by Bichut, wh(> states that the nervous system ought to be (lixided into two parts, essentially distinct from each other, and having for their principid centres, one the brain and its dependi-Dcics, und tht> other the ganglions. Some modern writers have supposed that there are several nervous centres ; such is the opinion of Cuvicr and De lUainville; the latter considers that the nen-ous system ought to be divided into as many parts as there are principal functions. Drs. Gall and Spurzheim contend that the nervous system is not an unit, but that it consistB of many essentially different parts, which have their own individual ori^pns, and which ere in mutual communi- cation. -'"Tlie numerous experiments which have been perform^ ed on living animals, prove that particular parts of the nervous system are allotted to the exercise of pnrticidiir functions ; so that no individuM organ can be considerud as the source whence all the other structures are deri^-edv because each part of the sj-stem may have, in a certain degree, an Independent existence. In the higlier clauses of animals, however, and especially in man, the brain is so largely developed thai it has a most striking in- fluence CD the other nervous masses, and the whole of those are so intimately connected that tliey cannot be regarded in an insulated manner. ^'"Although the investigations of modern physiologists .')»■■' . ■■■'Tha honour of tbi) didindion liM bwn gtnunJIy slinbuLed (b DichUi but Ike tiAi uo jmt claiii) t<> il, m Ihc cuay or D'. Joliatlonci conluining hn Opinioni coticttning iIh' itic of the ganglions "m [lubtliliei] m t till conn try, and intniUud iaio (h« Franch Unj;ut'<, i«v«nl yc»n bcfote tlie jiWly cvle- bnlod wuiki of HidiU mhAv tli«lr apptsniicc 476 DIVISION UF NEavOL'S SYSTRM. have shewn, that the ncrvoiut system consists qf im ag- gregation of many organs, which respectively exercise different Junctions, it is still very rlitfirult to make nnjr clussiticution of them that shall be free from defects. The most comprehcnsire arrsngemunt is thut which i* ftjunded on the ilivUion of the fuuctions into two grenf classes ; viz. the animal functions, or those nvhich are attended with conscionsnias ; and the organic functions, or those of which the individual is not conscious. The nervous oi^ns belonging to the first of these classes, consist of the spinal cord, the brain, and the aepnf connected witli them ; those of tlic second ore the ganglia of tlie great sympathetic, and the nerves nt- tacliCd to ttiem. It is neceesary to state that tltcrc are many exceptions to this division ; thus soi^e of the nen-es of the medulla oblongata fiimisli branches to the orgsiu) of the v^ietative fimclionti ; whilst the sympa* thetic sends filaments to the muscles of volition. | In proceeding with the consideration Of tlie»e two di- visions, 1 shall, in the first instance, dosi-rihc the pro- perties that they share in common, and afterwards |»inl out the peculiarities by which each is distinguished. > The nervous system exhibits in a striking mnnnvr the symmetrical form ; tliis di>positiou is obNcrvt-d in thr centnd and in the peripheral urgnns, but in u more petv foct degree in the former than in the lalttf : the aym- mctry is also more decided in the spinal cord Ltion is the brain, and in the interior of both these orgimiii than on the external surface ; thus the convolulionn of the cerehnmi and the layers of the cerebellum ore more sub> ject to deviations of form on the two sides of the liody than iJie more internal parts, 'ilte nerves pixKicvdi^ from the cnccphalon, the eerebrttl and spinal, tun all COMMUNICATIONS, symmetrical, with the exception of Ihc pncumo-gnstric, which are distributed to irregular orgaas. The part« which compose the system of the sympathetic nerves, are very irregular in their origin, and espfcially in their termination. With tlieso exccpdons, the resemblance of the liiteriii holws is ao great in altiKMit the whole eixtent of the nervous system, that it is often impossible to per- ceive the least difference between them, with respect to situation, form, and size. The groat nervous (nasst-s are ull placed deeply in the trunk of the body, »o a& to be defended from external violence ; some of tbem, as the brain and .spinal cord, are still further protected by being enclosed within strong osseous cavities. The principal nerve& are also removed from the surface, and are lodged in the most secure situations ; their terminal extremities being the only part& tliat are exposed to the contact of surrounding bodies. .1,,, ipOMMUNlCATIONS OF TUB NERVOUS SYSTEM. The various parts of this system are intimately con- nected with each other, and in this manner the mutual in- fluence or sympathy which is observed between their func- tions, is establifihed. The structures which form the nervoiTi communications, are the following : 1 . These which juc placed between the lateral halve-t of the sym> metrical organs. 2. Tliose which connect the ncn'ous masses of the same side of the body. 1 shall describe tiiese structiu'es in a connected and succinct manner, because I Itclieve their nature to be essentially tlic swuc in whatever part of Uic system they may be situated. Hi The tirst species of communication is principally met 478 CONNEXION BT COMU19SDR£«. vrith in the spinni con) and cncephalon, but U also er-~ iscs between some of the nerves, especially those of the sympathetic. ' The lateral columns of the spinal cord* are united by numerous fibres, which may be seen running tran*- venwily at the bottom of the anterior oiediim Hssure; and loagicudinally in the posterior fiuTOW. The lateral parts of the mcduUa oblongata are connected in a stmv> lar manner, except in the place where the decussation o^ the anterior pyramidal bodies occurs. This inter* crossing; has been denied by many anatomists ; there ts, howe-cr, no doubt of its existence, and uf its servioir to bring into cummunication several of the cerebral masses of one side of the body with an opposite portion of the spinal cord. The different bodies of the ccrebetium and of ihl cerebrum placed on the .sides of the median plane, am connected with eacii other by many tmnsverse tibrci^ which produce the commissures ; sei'sral of these, such as -the corpus ctUlosum, anterior and posterior com- missures, haw been lonj^ known, but we arc especially indebted to the researches of Gall, and of his pupil and collea^e, Spurzheim, for an acquaintance with the real nature