No. 466. LONDON, SATURDAY, AUGUST 4. [1831-32. LECTURES ON VETERINARY MEDICINE, DELIVERED IN THE UNIVERSITY OF LONDON, BY MR. YOUATT. LECTURE XXXVIII. A SKETCH OF THE NERVOUS SYSTEM-THE ORIGINS AND FUNCTIONS OF THE NERVES OF PECULIAR SENSATION, THE OLFAC- TORY, OPTIC, AND AUDITORY ; AND ALSO OF THE NERVES OF PURE MOTION. THE THIRD, FOURTH, SIXTH, AND TWELFTH PAIRS. I I HAVE observed in a former lecture, that the cerebral and the spinal nerves pre- sent themselves symmetrically, that they are arranged in pairs, one on each side of the mesian line, that runs from the anterior portion of the brain to the cauda equina. I will first enumerate them in the order in which they present themselves at the base of the brain; rapidly describing their origin, destination, and function. We shall then, perhaps, be able to class them ac. cording to their respective functions, and thus, perhaps, get rid of a little of the mystery which has been supposed to encircle the nervous system. Having done this, I shall enter at some length into the consideration of the office of each, and that will be the time for me to endeavour to justify my departure from the nomencla- ture and physiological exposition hitherto adopted by veterinary writers. Description of the Cerebral Nerves accord- ing to their Situation at the Base of the Brain.—Commencing anteriorly, we find, 1. The Olfactory nerve arising from the corpora striata and callosum ; passing through the cribriform plate of the ethmoid bone, and distributed on the membrane of the nose—a nerve of peculiar sensation, and its minute ramifications, affected only by the odoriferous particles of bodies. 2. The Optic nerve arising from the tha- lamus nervi optici, and connected by medul- lary striae with the corpora quadrigemina,— pursuing its course to the back of the eye, piercing the sclerotic coat, expanding over the choroid coat, and forming the retina ; a nerve of peculiar sensation, and affected only by the particles of light. 3. The Motor Oculi, from the middle of the crus cerebri to all the muscles of the eye, except the superior oblique and the abductor-a nerve of voluntary motion. 4. The Patheticus, from a little sulcus on the crura cerebri at the base of the testes to the superior oblique muscle,-a nerve of voluntary motion. 5. The Triaeminits, evidently proceeding from two roots : the inferior one from the crus cerebri—the superior one from the crus cerebelli. The superior one forming or passing through a ganglion-the inferior one unconnected with the ganglion, but uniting with the other branch immediately beyond the ganglion, and both contributing to form one compound nerve distributed over the face,-a nerve of common sensation and of voluntary motion. 6. The Abducens, from a transverse sul cus between the pons varolii and moto , root of the eighth pair (the portio dura of the seventh), and going partly to the retractor, but principally to the abductor muscle of the eye-a nerve of voluntary motion. 7. The Voluntary and Organic illotor Nerve of the Face.—The portio dura, springing evidently from two roots; the first to 00 traced from the central column of the in- ferior surface of the medulla oblonrata, and the other from the head of the lateral column, or corpus olivare, if it might ba so termed in the quadruped ;-united two- gether as they escape from the brain, and forming a compound nerve, the voluntary motor nerve of the face, and directing also those motions, partly toluntary and partly in- voluntary, which are connected with the re- spiratory system of organic life. 8. The Auditory (the portio mollis of the seventh), traced from the floor,of the fourth ventricle ; passing down the side of the medulla oblongata ; entering into the same sheath with the eighth (the portio dura) ;
LONDON, SATURDAY, AUGUST 4. [1831-32.
VETERINARY MEDICINE,DELIVERED IN THE
UNIVERSITY OF LONDON,BY
A SKETCH OF THE NERVOUS SYSTEM-THE
ORIGINS AND FUNCTIONS OF THE NERVES
OF PECULIAR SENSATION, THE OLFAC-
TORY, OPTIC, AND AUDITORY ; AND ALSOOF THE NERVES OF PURE MOTION. THE
THIRD, FOURTH, SIXTH, AND TWELFTHPAIRS. I
I HAVE observed in a former lecture, thatthe cerebral and the spinal nerves pre-sent themselves symmetrically, that theyare arranged in pairs, one on each side ofthe mesian line, that runs from the anteriorportion of the brain to the cauda equina. Iwill first enumerate them in the order inwhich they present themselves at the baseof the brain; rapidly describing their
origin, destination, and function. We shallthen, perhaps, be able to class them ac.
cording to their respective functions, andthus, perhaps, get rid of a little of the
mystery which has been supposed to
encircle the nervous system. Having donethis, I shall enter at some length into theconsideration of the office of each, and thatwill be the time for me to endeavour to
justify my departure from the nomencla-ture and physiological exposition hithertoadopted by veterinary writers.
Description of the Cerebral Nerves accord-ing to their Situation at the Base of theBrain.—Commencing anteriorly, we find,
1. The Olfactory nerve arising from thecorpora striata and callosum ; passing throughthe cribriform plate of the ethmoid bone,and distributed on the membrane of thenose—a nerve of peculiar sensation, and itsminute ramifications, affected only by theodoriferous particles of bodies.
2. The Optic nerve arising from the tha-lamus nervi optici, and connected by medul-lary striae with the corpora quadrigemina,—pursuing its course to the back of the eye,piercing the sclerotic coat, expanding overthe choroid coat, and forming the retina ; anerve of peculiar sensation, and affected onlyby the particles of light.
3. The Motor Oculi, from the middle ofthe crus cerebri to all the muscles of the
eye, except the superior oblique and theabductor-a nerve of voluntary motion.
4. The Patheticus, from a little sulcus onthe crura cerebri at the base of the testesto the superior oblique muscle,-a nerve ofvoluntary motion.
5. The Triaeminits, evidently proceedingfrom two roots : the inferior one from thecrus cerebri—the superior one from thecrus cerebelli. The superior one formingor passing through a ganglion-the inferiorone unconnected with the ganglion, butuniting with the other branch immediatelybeyond the ganglion, and both contributingto form one compound nerve distributedover the face,-a nerve of common sensationand of voluntary motion.
6. The Abducens, from a transverse sulcus between the pons varolii and moto ,root of the eighth pair (the portio dura of theseventh), and going partly to the retractor,but principally to the abductor muscle ofthe eye-a nerve of voluntary motion.
7. The Voluntary and Organic illotor Nerveof the Face.—The portio dura, springingevidently from two roots; the first to 00traced from the central column of the in-ferior surface of the medulla oblonrata,and the other from the head of the lateralcolumn, or corpus olivare, if it might baso termed in the quadruped ;-united two-
gether as they escape from the brain, andforming a compound nerve, the voluntarymotor nerve of the face, and directing alsothose motions, partly toluntary and partly in-voluntary, which are connected with the re-spiratory system of organic life.
8. The Auditory (the portio mollis of theseventh), traced from the floor,of the fourthventricle ; passing down the side of themedulla oblongata ; entering into the samesheath with the eighth (the portio dura) ;
escaping through the foramen auditoriuminternum, and spreading its pulpy ramifi-cations over the internal chambers of theear, - a nerve of peculiar sensation, andaffected onlv bv the vibrations of the air.
9. The Glosso-Pharyngeus, arising fromthe lateral column of the medulla oblon-gata by several little filaments, partiallyramifying over the constrictor muscles ofthe pharynx, but chiefly expended in thebase and substance of the tongue. An or-
ganic TIIoto!’ nerve associating the action ofthe muscles of the pharynx and tongue,with the discharge of the function of re-spiration and deglutition.;5pllULIUII and .
10. The Par Vagum of our anatomists-the pneumo-gastric nerve of the French,but, more properly, the cerebro-visceral or-ganic motor nerve. It arises from the la-teral column of the medulla oblongata, bynumerous filaments, a little posteriorly tothe glosso-pharyngeus, and escapes fromthe cranium in the same sheath with it.After distributing several branches to thepharynx, the larynx, and the oesophagus, itgives large and singularly involved plexusesto the heart and the lungs ; and having en-tered the abdomen, its ramifications on thestomach are numerous and complicated.Some plexuses go to the liver, and an im.portant branch, uniting with the grand or-ganic nerve of the abdomen, helps to formthe similunar ganglion,—an organic motor’
11. Included in the same sheath withthe glosso-pharyngeal and the cerebro-visceral, is a singular nerve, which anato-mists have agreed to call the spinal acces-sory nerve. It is formed by the union ofseveral minute filaments derived from thelateral column of the spinal chord, some ofwhich may be traced as low as the fifthcer-vical nerve. Having, in the manner I shallhereafter describe, reached the cerebro.-vis,ceral and glosso--pharyngeal, it neither iden-tifiesitself with them,nor communicates withthe substance of the brain, but turns roundand escapes in the same sheath with thosenerves, and, soon separating from them, isdistributed over some of the muscles of theneck and shoulder, associating with them inthe involuntary discharge of the function ofrespiration, and therefore being an organicmotor nerve.
12. Still posteriorly, on the medulla ob-longata, but rising from the central columnof its inferior surface, we find the tingualesescaping through the anterior condyloidforamina, running between the pterigoideus,and the larynx, penetrating into the sub-stance of the tongue, and giving motion tothat organ. It is a voluntai-y moto)’ nerve.
Here ends our brief catalogue of the cere-bral nerves ; but our subject would not becomplete if I did not glance at the origins
of the nerves along the course of the spinalchord. At the very point where the me-dulla oblongata terminates in the spinalchord, we find the first pair, the sub-occipital,and we may take it as a specimen of all the
I rest. Numerous little filaments arise, fromthe longitudinal sulcus between the centraland lateral columns on the inferior surfaceof the chord, but which are derived from, ormaybe traced to, the central column ; theyapproximate and unite, and form a nervouschord. Other filaments spring from the cor-responding sulcus on the superior surface,but they are not so numerous: and theyare larger and bolder in their origin. Theyalso approximate and unite, and form a
nervous chord ; but at the point of union,and still within the dura mater, these latterform a ganglion,—a. little swelling or knot,- immediately beyond which, the chordshrinks again to nearly its former bulk;
and then, and still within the dura mater,for at the rnninpnt. of escaping frnm it tha
two chords unite and form a perfect but com.pound nerve, of which I shall say no moreat present, than that it combines the pro.perties it derived from these central co-
lumns on the inferior and superior surfacesof the spinal chord, and is a nerve of sen-sation and voluntary motion. It is dis-tributed to the neighbouring parts, and itanastomoses with those above and below,for important purposes, which will be thesubject of future consideration. Given out ateach articulation of the oerv)ealbones,s9we proceed down the neck we find one ofthese compound nerves ; and towards thelower part of the neck, we observe fihunentsfrom the fourth, fifth, and sixth cervicalnerves, uniting to form a singular nerve,which takes its course down the neck, with-out anastomosis, and traverses the chest,and ramifies upon the diaphragm. This
phrenic nerve is the motor nerve of the dia-phragm,—the grand agent in respiration,and, therefore, we have no difficulty in de-nominating it ara organic motor nerve. I
shall have to show, that it is not improba-; ble, that although the filaments which cùm.
pose it are derived from the cervical nerves,iyet they may actually have their origin fromf that portion of the spinal chord which seemsc to be devoted to the nurnoses of organic life.
The dorsal, lumbar, and sacral nerves,have the same structure, and discharge the
same function. They are all nerves ot sen-sation and voluntary motion; and those otthe extremities, derived from plexuses ofthe cervical and dorsal, and the lumbar andthe sacral, possess the same character.The Ganglial lrerve.-There is one nervewanting to complete our list. It belongsneither to the cerebral nor the spinal sys-tem, and seems in its function to be inde-pendent of bath. At the base of the cra-
ninfrt, and in front of the atlas, I find & 11pyriform reddish body, which gradually rcontracts and terminates in a nerve. I in- I
quite not now into the origin of this gan- IIglion, but a superncial glance at the nervetoils ift6 that it is performing some impor-tant office. It is connecting itself with thecerebro-spinal, and with every cervical
nerve, but more particularly it is formingcomplicated plexuses on every neighbour-ing blood-vessel. I trace it particularly onboth the external and the internal carotid,and I follow it in the subdivisions of thesevessels, until, from the minuteness of thevessel and the pulpiness of the nerve, iteludes my sight. Hereafter I shull haveto trace the course of the nerve in the tho-rax, forming, with the cerebro visceral, aplexus, or rather an investing membranearound every vessel of the heart and lungs ;and then having reached the abdomen, and,in the semilunar gaf)glion combined its owninfluence with that of the cerebro-visceraland the phrenic, it becomes the sun or cen-tre of organic nervous power, diffusing itsradiations over every artery and absorbent,and gland and ganglion—everything con-nected with secretion, nutrition, and life-itself the very principle of life and action-the soul of the organic system. It wastermed, before its character and powerwere suspected, the sympathetic nerve, be-cause it seemed to connect the whole sys-tem together; it is denominated by othersthe ganglial nerve, from its supposed origineither in the superior cervical, or the semi-lunar ganglion; but it would mora pro-perly be designated the great organic nerve—the secretory, nutritive, chemical, while thecerebro -visceral is the motor organic nerve-the power which presides over, and effectsthe changes in, that fluid which the motornerve keeps in circulation. But we are not
yet quite prepared for this.My object was to give you a kind of
bird’s-eye view of the nervous system, thatwe might perhaps be enabled so to class itsdifferent portions, and trace its connexions,as to remove much of the mystery that haserroneously been supposed to hang over thesubiect.
Divisions of the Nervous System.-One di.vision is plain and palpable—the animaland the organic—that which is concernedin the conveyance of impressions to thebrain, and volitions to the muscles, and thatwhich is connectell with respiration, circu-lation, digestion, and life itself. The sixfirst of the cerebral, the eighth (portiomollis), and the twelfth (the linguales),and all the spinal nerves are those of animallife. All the others except the seventh(portio dura) are nerves of organic life.Both the animal and the organic nerves ad-mit of a simple subdivision into those of
pi6re sènsation, concerned alone in tranSe
mitting impressions to the brain as the
first, second, and seventh ; or pure 1iwtion
conveying the volitions of the mind ; as thethird, fourth, sixth, and twelfth (linguales)- =-or, in accomplishing both purposes, as thefifth pair within the cranium and all the spi-nal nerves. The division of the organicnerves is not so evident; we have, however,plainly enough the simple motor. organicnerve in the ninth (the glosso-pharyngeus),the eleventh (the spinal accessory), andthe phrenic—and we shall cautiously in.quire whether we do not find it also in thetenth (the par vagum), and the chemicaland vital one in the ganglial nerve ; while,plainly enough, the seventh pair occupiesa a neutral ground, or belongs both to animaland organic life-it is both the votxtntary andthe organic motor nerve of the face. Whenwe have made ourselves masters of this
theory of the origins and functions of thenerves, we shall not have much difficultyin unravelling many of their anastomoses,and comprehending the wise and benevo.lent purposes that are accomplished bythem.We will commence with the animal
nerves, and, first of all, with those of pureor peculiar sensation.
The Olfactory, or Firat Pair cf Nerves.—The nerve which first presents itself at thebase of the brain, if we commence ante..
riorly, is the olfactory nerve. It arises partlyfrom the corpus striatum, - but a medullarytractus, terminating in it, and evidentlygiving one origin to it, may be traced tothe base of the corpus callosuin.
Its Character as a Nerve of peculiar sensa-tion.—Whether we trace its origin to thecorpus striatum or the corpus callosum, itis derived from the superior portion of themedullary matter at the base of the brain-that which would be a continuation of thesuperior surface of the spinal chord if sofar prolonged. If we follow either root tothe corpus callosum, the olfactory nerve isevidently referable to the centre of thatsuperior surface, and there is nothing in-consistent with the same derivation, whenwe attribute it to the corpus striatum. Itproceeds in a manner bodily from the sub-stance of the brain-it is an apparent pro-longatior of the medullary matter of the brain.
Comparative- anatomists should not de-scribe it as a mere mamillary process-" the ashen-coloured termination of thebrain abutting upon the ethmoid bone."Although the nervous chord is not sepa-rated from the brain to so great an extentas that of the optic, or even the auditorynerve, and not nearly so much as we find itin the human subject, yet it has decidedlyleft the brain before it reaches the cribri-form plate. Its course is short, and it has lat
a single anastomosis. I would beg you topay attention to these pecuiiarities, these 1characteristics (as I shall, by-and-by, be ’justified in calling them) of a nerve of pe-
citliar sensation. Its minute ramificationsI have said are affected only by the odorife-rous particles of bodies.
Size of the Nerves.—Observe the develop-ment of this nerve, bearing a beautiful pro-portion to the necessities of the animal.First, compare it with the same nerve inman—in him simply connected with plea-sure-in the brute with life itself. In abrain not more than half the size of that ofthe human being, namely, that of the horse,the olfactory nerve is four times as largeas in man. Compare its bulk in our differentdomesticated animals. In the horse it is Blarge, for in a state of nature it affords hisonly warning against poisonous plants-but,almost all over the world, he has become ourstabled servant, comparatively rarely sentto collect his own nutriment amidst the
herbage of the field, and having the greaterpart of his food provided for him. The oxis oftener driven to shift for himself, or, ifworked by day, he is usually turned- out atnight, and needs a somewhat acuter senseof smell. Observe, that although his brainis but little more than half the size of thatof the horse, the olfactory nerve is nearly aslarge. In the sheep and swine it is com-
paratively as large as in the ox, or evermore so ; and in the dog you cannot fail oremarking its still greater comparative bulk
Peculiarities about them.—The olfactoryis, next to the fifth, the largest of the cere-bral nerves, and it is the softest in its tex-ture, nevertheless its fibrous structure hasbeen demonstrated, and it is the excessivelooseness and thinness of the neurilemathat gave it its pulpy appearance. It has a
singular cavity in it, the use of which I amunable to state. That cavity is a prolongation of the anterior cornu of the lateral ven-
tricle ; it terminates however in a blincpouch before the nerve reaches the ethmoicbone, and there is consequently no channeby which, as was formerly supposed, an]fluid can be conveyed from the brain int<the nose.
Termination.—In company with, and un- Ider the real mamillary process, it abutsupon the cribriform or thin perforated plateof the ethmoid bone, and sends its pulpyfibres through it, which are visibly spreadover the upper part of the septum and supe-rior turbinated bone, and which probablyextend over the whole of the nasal cavity.
The Olfactory Nerves in Birds.-In our
occasional patient, the feathered biped, theolfactory nerve comes nearer to a mamillaryprocess, for it arises from the very point ofthe anterior termination of the brain, andseems to be a mere continuation of it.
Having left the cranium it enters into abony canal, and so passes through the orbitwhich interposes ; and it is finally distri.buted, not on the ethmoid bone, for .that isin a manner wanting in the bird, but on thepituitary membrane covering the superiorconchœ of the nose.
The Optic, or Second Pair of Nerves.—Onthe crura cerebri, when they first appearat the base of the brain, there are seen twoprominent medullary chords, winding theirway over the crura in a direction inwardlyand anteriorly. They first emerge from
’ under the hemispheres, opposite to the cor.pus albicans, and they meet each other im-
s mediately anterior to the infundibulum andthe pituitary gland.
B Their Character as Nerves of a peculiarsensation.-The medullary chords of whichI have just spoken-the tractus optici,—canbe traced to the thalami nervorum opticorumin the centre of the brain ; they are, in fact,a continuation of the thalami; or the thalamicontract, and are prolonged into these me-dullary chords. I have also said that they
are connected by medullary striæ with the
,corpora quadrigemina, and particularlvwith the nates. Now the thalami and thenates evidently occupy that portion of thet base of the brain which would corresponda with the superior surface of the spinal chord;- and they occupy a central situation uponi that surface. The optic nerves are also af palpable prolongation of the medullary sub-:. stance of the brain. The course is somewhat
y longer than that of the olfactory nerve, but- through the whole of that course, they haten not a single anastomosis. They are alsos nerves of peculiar sensation, and their mi-e nute ramifications are affected only by the,a particles of light.’ Peculiarities of the Optic Nerves,—Their
Decussation.-l have shown you these me-
dullary chords, winding their way over thecrura, and meeting at the centre, and notonly meeting, but apparently decussating-crossing. There is a certain identifica-tion of medullary matter of the two nerves,to secure the more perfect discharge oftheir function-to assist experience in cor-recting the errors which would arise fromthe somewhat dissimilar impressions made,or the pictures (if we so dare to call them): painted on the two retina, and to ensureperfect and distinct vision. The impressiont made by the odoriferous particles of bodieson the two nostrils, or the two olfactory
nerves, are, in a state of health, preciselythe same ; the vibrations of the air, as they
r reach the tympanum of the ear on eithere side are the same ; but from the construc-y tion of the organ of sight, the picture onf each retina must differ, and that very
materially, with regard to a near object.
t. Look at any object first with one eye,
and then with the other, and you will un-derstand what I mean. This partial min- i
gling of medullary matter in the two inerves, was probably designed to preventor to remedy, the confusion of vision whichwould otherwise result. We have, how-ever, abundant proof that there is no actualdecussation. This brain was taken froma dog that had been blind in the right eyefor several years. Observe how the nerveon the right side is shrunk, and what a cu-rious semi-transparent hue it has assumed.Our principal patient, the horse, is sadlyexposed to inflammation of the eye, whichtoo often destroys the sight: but thexe isthis peculiarity about the disease, that whenblindness is produced in one eye, the other iis spared for a long time, and often for life.If we examine the brain of one of thesehorses, we uniformly find the nerve on theblind side shrunk ; it obeys the law whichgoverns every other part of the frame, andwhen it is no longer useful, it graduallywithers away. It has been long settled, Ibelieve, among human anatomists, that thereis no decussation of the nerves. The exami-nationofsome Russian criminals determinedthe matter. Theft was in some part of thatvast empire punished with the loss of aneye for the first offence, and death for thesecond. Some pirates who had suffered thefirst penalty, were afterwards convicted ofa second offence and executed, and thenerve on the blind side was shrunk in everyone of them. In birds, the optic nerves,which are exceedingly large, manifestlydecussate; and in fish, and particularly inthose with a bony skeleton, they cross eachother without mingling at all.
Their Fibrous Structure.-The fibrousstructure of the optic nerve is sufficientlyevident in all our quadruped patients, andthese fibres are so symmetrically arranged,as to leave a canal in the centre of thenerve, which had been observed by someold anatomists, but the function of whichhad been misunderstood. It contains thecentral artery of the retina, derived from theophthalmic artery, which penetrate throughthe dura mater after the nerves leave thecranium, and enter with the optic nerveinto the globe of the eye, for purposeswhich will be described when we treat ofvision. This canal also contains the cen-tral vein of the retina.
The Reticulated Structure of the OpticNerve.—When, however, I speak of sym-metrical arrangement, do not misunderstandme; I do not mean to say, that the exactparallel direction of these fibres can always,or scarcely ever be traced. It would seemto be necessarv in such a nerve as this, ofpeculiar sensation, that there should be anintercommunication of nervous influenceand matter through its whole extent, other-
wise accident or disease might, oftener thanit does, cause strange confusion in the
impression conveyed; therefore there is acontinual exchange of minute branches inevery direction going forward between thefibrils, in some nerves through their wholeextent, in others more plainly to be tracedin certain parts, and occasionally carriedto such an extent, that the interior of thenerve presents only a complicated reticularappearance. It is so here ; although wecan trace the central canal plainly enough,the interchange of branches is so numerous,that the true parallel direction of the fibrescannot be distinguished.
Termination of’ the Optic Nerves.—Eachnerve, after having separated from its fel-low, pursues its course on its own side,escapes through the foramen opticum, andenters the cavity of the orbit; it there con-tinues its path obliquely, penetrates be-tween the muscles, and, particularly sur-rounded by the retractor muscle, reaches
. the inner and inferior and posterior part ofthe eye-ball, the coats of which it pene-. trates, and, entering the globe of the eye,t it expands over the choroid coat, and formst the retina.
Sheathed in Dura Mater .-The dura materquits the nerves as they emerge from thecranium, and they afterwards pursue theircourse surrounded and defended by theirown neurilema. The optic nerve is, how-ever, an exception to this rule, for the duramater continues to encircle it until itreaches the outer or sclerotic coat of the
eye. This, according to some, is for thepurpose of forming the sclerotic coat, whichcertainly bears considerable resemblance tothe dura mater; while the pia mater, ac-cording to the same anatomists, is expandedwithin the globe of the eye. 1%fore of thiswhen we treat of the structure of the eye ;at present I will only remark, that the dif-ference in situation, and length of course,between this and the other two nerves of
peculiar sensation, and the difference in; function too, will sufficiently account for thisi prolongation of the dura mater. It is ne- cessarv that the impression or intellie’ence
, conveyed by a nerve of peculiar sensation,should be distinct and pure. The olfactory,the moment it quits the cranium, ramifiesupon the membrane to which it is destined.We shall presently see, that the auditorynerve is defended by a bony canal until itarrives at the labyrinth of the ear. Nothingcan cause confusion in the impressionswhich are conveyed along these nerves, butthe optic nerve has to travel through theorbit before it reaches its point of destina-
tion, and is encircled by muscles of strongL and rapid action; this dense covering,therefore, is necessary to protect it fromthat compression, which might occasionally
interfere with the perfect discharge of itsfunction.
The Optic Aerve in Birds.-The structureof this nerve is singular in birds. Thefibrous conformation is evident ; but thesefibres are curiously collected into laminae inbirds of prey, and I think 1 have seen some-thing like it in some of the wilder birdswhich we have partially domesticated, as
the pheasant. The nerve cannot be dissect-ed, as in the quadruped, into a collection offibres, more or less numerous and compli-cated, but into a congeries of leaves; itseems like the folding of one continuousmembrane, and the retina is the expandingof the plaits. We can connect this, as weshall have to do many other peculiarities inthe eye of the bird, with the almost incon-ceivable acuteness of sight in birds of preyparticularly, and of which natural historyaffords us a thousand proofs.
The Auditory Nerve.—This nerve is the
eighth in order, reckoning anteriorly pos-teriorly. English anatomists have termedit the portio mollis of the seventh pair; Iwill venture, following the example of mostcontinental anatomists, and for reasons thatshall be stated hereafter, to call it simplythe auditory or eighth pair of nerves.
Character as a Nerve of peculiar Sensation. I—I find it on the floor of the fourth ven-tricle, which is not only a prolongationof, but may almost be said to be the supe-rior surface of, the spinal chord : I trace itfrom the slit of the calamus scriptorius,and therefore from the centre of that sur-face. Its pulpiness and transparency r ’en-der it now and then a little difficult to fol-low, and I mutilate or lose it in the dissec-tion, but you here plainly perceive it pass-ing down the side of the medulla oblongata,meeting the seventh nerve (the portio duraof the seventh), and then, in union with it,taking an outward direction, and enteringthe foramen auditorium internum. Its coursethere is short; it soon abuts upon a cribri-form or perforated plate of bone closing thepassage, through which it passes, and thendividing into various portions, spreads it.self over 11,P. internal s,hamhcre of the ear
while there is not throughout the whole ofits course one anastomosis. Its root cannot,like the first and second nerves, be tracedto distant parts, for it at first appears onthat track of medullary matter to which theothers are ultimatelv referred. It does notseem so much a prolongation of a part ofthe brain, and yet it springs, as it were,bodily, and at once, from the floor of the ven-tricle. Human anatomists have spoken offasciculi of fibres proceeding from above andbelow to form the root—something likestriae I have seen; but the fibres, if theydo exist, are indistinct.
There is a circumstance respecting one
of the divisions of this nerve, that we havenot observed in any portion of the first orsecond nerves, namely, that those fibreswhich go to the trumpet-like extremitiesof two of the semi-circular canals, passthrough a little knot or ganglion. I mustnot at present draw any conclusion fromthis isolated fact, but I shall have hereafterto remind you of the ganglion on a part ofthe 8th nerve of peculiar sensation. Theminute ramifications of this nerve are af-fected only by the vibrations of the air.
Comparative Bulk of the Auditory Nerve.-When I come to treat of hearing, I shallhave to point out many peculiarities, ac-
counting for the greater acuteness of tiiissense in the quadruped, than in the humanbeing, arising from the external ear—thetympanum, the ossiculi, &0. ; but at present1 must onlv point out to you in each of thebrains before tis, the greater bulk of the
auditory nerve compared with that in man;on the other hand, the adaptation of thehuman ear to the various modulations oftone, will be an interesting subject of in.quiry.
Cnmparisoo of these Nerves of pecnliccr Sen-sation.—Now, then, Gentlemen, let us forone moment recapitulate. We have ex-
amined the three cerebral nerves of pecalarsensation,-those of smell, sight, and liear-ing. We have traced each of them to that
portion of the medullary matter at the cen-tre of the brain, which answers to, or mavbe considered as a prolongation of, thesurface of the spinal chord;—we have fur-ther traced them to the centre of that sur-face ; we have observed, that they seemto be prolongations of the substance of thebrain, or to spring bodily from that organ,and that through the whole of their coursethey have no anastomosis. To the two
first of these peculiarities 1 shall Lereatterrecall vour recollection, when 1 shall haveto speak of the relative situation of nervesof certain functions-the prolongation ofthe substance of the brain—the long root
that can be traced ueep into the brain,seems natural enough, wheu we consider,that from these three nerves the animal de-
f rives almost all his knowledge of-’urround-, ing objects-all the materials for thoughti and improvement. It seems reasonablei that these impressions should be conveyed
securely and deeply, there to be presened,t arranged, and compared. They are as muchf identified with the mind and with animal, life, as the well-protected organs of the- thorax are with the inferior organic prinei-f ple,-and the wisdom and benevolence ofd the arrangement by which these impres-e sions of smell, and hearing, and sight, are
y preserved distinct and pujf, and by winchno anastomosis with other nerves is per-
B mittted, needs no illustration of mine. But
I proceed to other nerves, differently situ-ated and of different function, viz. thenerves of pure motion.The Motores Oeutor2cm, or Third Pair of sNerves.—About the middle of the crura cere-bri in the horse, (and to this animal, if an xespecial exception is not made, all these aanatomical details refer,) and rather nearer .
to the corpus albicans than to the pons va- ’ rolii, the third pair of nerves take their origin. They seem to rise superficially B. from the crura cerebri; they cannot be tracedat all into the substance of the brain, ex-cept in a way that I shall presently notice,and are clearly referable to another surfaceof the spinal chord, or its supposed pro-longation--the inferior one. Let me hereremark, once for’all, that from the difer-ence of the attitude of the human beingand the quadruped, the anterior surface of
the spinal chord in man, is the inferior onein the brute ; and the posterior one in thebiped, is the superior one in the quadruped.These nerves spring from the central columnof the inferior surface, that track which Ihave alreadv traced under the pons varolii,and along the crura cerebri. These nerves,do not, however, rise bodily from the crura,but by several minute fibriculi 1’anged in aline, following the course of this medullarytrack. These fibriculi approximate to eachother, and unite either before they pene-trate the dura mater, or as they are leavingit, and form a nerve. This is very differentfrom what we have observed in either ofthe nerves of neculiar sensation.
Course of the Nerve.—The motor oculitakes its course obliquely across the cruscerebri, and winds downwards, and entersthe cavernous sinuses, and escapes throughthe foramen longum orhitale into the orbit ;- there it divides, and the smaller branchgoes to the levator oculi, while the largersubdivides, and goes to all the other musclesof the eye, except the superior obliqueand the abductor, for the purpose of movingthe eye in its socket. Then it is a nerve of
its Anastomosis.—As we follow its course lwe observe another circumstance in whichit differs from either of the sensitive nerves,- it connects itself with other nerves. Itcommunicates with minute branches of the
ophtha’.mic division of the fifth pair, andmore particularly unites with them thatcurious ganglion the ophthalmic, whencethe iris derives its supply of nervous in-fluence. I enter not into the cause of thisvet. We shall know more about it by-and-by, for a thousand anastomoses will comebefore us, the great utility of which will beevident on the slightest inspection ; but Icontent myself with pointing out to youhow far we are departing from the characterof a nerve of peculiar sensation.
T7te Pathetiei, or Fourth Pairof Nerves.—I feel considerable difficulty in classing’this nerve. It goes to the trachealis or
superior oblique muscle of the eye, and
governs its motion, but whether it is a
nerve of pure voluntarv motion, or merelvassociating the motion of the eye withsome organic function, and probably thatof respiration, t am not assured. It arises
from a singular part of the brain,—from thecrus cerebri at the base of the testes, and,consequently, from the superior surface ofthe prolongation of the spinal chord; itarises by filaments. I can plainly tracetwo, but it does not arise either from thecentre, or the side, of the crus-more in-
clining’, however, to the latter than to the
former;—and when I make a longitudinal· incision through the cerebellum, and layf open the Wlaole of the fourth ventricle, Iplainly see that it does spring from thatportion of the crus which may be con
sidered as a possible prolongation of thea lateral column of the medulla oblongata.I In this respect it differs from the voluntary, motor nerve which I have just considered,and from everv other voluntary motor
nerve of the brain or the spinal cltord.On the other hand, it curiously winds its
way through the other nerves, and proceedsto this single muscle without an anasto-
mosis ; nor can I trace to the tracbealismuscle any fibre from any voluntary motornerve. This is very singular, for wnhregard to every other muscle (the visceralones excepted), if there be communica-tions from an organic nerve to associate theaction of that muscle in the discharge ofsome vital function, there are also commu-nications from some spinal cerebral nervefor the purpose of voluntary motion.
Difficulty about the Fourth Pair of Nerves.- -Sir Charles Bell, to whom, more than toany other physiologist, we are indebted for.our ’knowledge of the nervous system, con-siders this as a respiratory nerve, partlyfrom its situation and there I confess I ama good deal disposed to acquiesce with hisopinion ; but I do not understand the func-tion which he assigns to it, of relaxing, notcontracting, the trachealis muscle, and tims
. causing’ the eye to roll upwards, under the
3 superior lid, in every forcible effort ofl respiration, as sneezing or coughing, and
so tending to the security and preservationof the eye. I can form no clear conceptionof this relaxatima of a muscle by the direct
agency of the nerve that supplies it-nor
can I see the great danger to which the eyeis exposed in more than usually forcible
respiratory efforts, nor a purpose suffici-
ently important to lead to the introductionof a separate nerve, and from so singular an.origin. I must, therefore, at present class it
i with the nerves of voluntary motion. I
know it to be a voluntary motor nerve as itregards the usual action of the superior ob-lique muscle, and yet there is somethingabout this nerve which I do not under-stand, and with which I am not quite satis-fied. I shall enter a little more into this,and you will be better prepared for the in-quiry, when the muscles of the eyeball comein order before us.
The Abducentes, or Sixth Pair of Nerves.-These nerves arise from a transverse sulcuson the medulla oblongata, between the pons
varolii and the proper motor origin of the7th pair, and they are clearly. referable tothe central columns of this prolongation ofthe spinal chord. Therefore we find themon the inferior surface of the spinal chord,and springing from its central column; andwhen we look more attentively at it, wecan trace thefibriculi springing in a linefrom which it derives its origin, and unitingto form it. We trace it to the cavernoussinus, and through the foramen laceruminto the orbit, where it divides, and somebranches of it are distributed over the re-tractor muscle, a muscle peculiar to quadru-peds, answering the purpose of hands inwithdrawing the eye from danger, if thatdanger cannot be warded off, and the mainbulk of the nerve going on to the rectus ex-ternus or the abductor of the eye, by meansof which the eye rotates outwardly, andwhich is also a kind of antagonist to theTectus internus, and the superior oblique.Then I can clearly understand that this isa nerve of voluntary motion.
Its Anastomoses with the Great OrganicNerve.—There is a peculiarity about it,however, which I do not understand. Thereare some minute anastomoses with the other ‘motor nerves of the eue, and with the ophthal. Imic branch; that I can comprehend ; butthere is a branch, a very considerablebranch, given to each of the abducentesfrom the great organic nerve. They evi-dently go not from the sixth to the organic,but they come up in bulk through the baseof the skull, and unite with the sixth. Ishall, by-and-by, be assured, and I shallunderstand the reason of it, that this or-ganic nerve freely anastomoses with everyother nerve and every part, but the reasonof these considerable branches being sentto the sixth, is not yet apparent to us. Forthe present I leave this part of our subject,again reminding you, however, that thisabducens is a kind of antagonist of thesuperior oblique, the character of whosenerve a little puzzled us. If it should
prove to be an organic nerve, why here isplain and palpable union with the greatorganic to correspond with it.The Linguales, or Twelfth Pair of Nerves.-
Still proceeding posteriorly, and in thesame track, we find the lingualis, the last
of the simple nerves of the cranium, arisingalso from the inferior surface of the medullaoblongata-arising also, but not so decided.ly, from the central column, and formedfrom fibriculi arising in a line. And here I
begin to have a clearer conception of anothernervous character, of which I had not atrace in the nerves of pure sensation. Thisnerve is scarcely formed before it anasto-moses, and that by many a filament withthe cerebro-visceral-its union with the
sub-occipital and the first cervical cannotescape my observation ; and when I dis.sect it carefully, and especially its descend.ing branches, I find it allied with thesecond cervical, with the spinal accessory,and with the great organic nerve; and thenits superior branch unites with the gusta-tory branch of the fifth, in endless anasto-moses, making a complete plexus, or net-work, of nervous filaments. I have manyan important office to be performed by thislittle agile member the tongue, and 1 mustcall in the aid of many a neighbouring mus.cle, or almost all of them, in order to effectthem. After all these anastomoses with motorand organic nerves, the main branch, ac-
companied by the fifth, penetrates into thesubstance of the tongue, pervades everypart of it, and is lost in innumerable minuteramifications. This then is plainly a nerveof voluntary motion.
Characters of the Cerebral urerves of Vntun-tary Motion.—And now, Gentlemen, after asomewhat long and intricate, yet I trustnot quite unpleasant journey, we havearrived at a resting place. This is the lastof the pure voluntary motor nerves ; and,comparing them together—leaving out at
present the fourth, with which I have saidI scarcely know what to do-we obtainthese characters of the cerebral nerves ofvoluntary motion, which may guide us ma-terially in our future inquiries. Theyarise from the inferior surface of the prolon-gation of the spinal chorcl, and from its centralcolumn, by numerous fibriculi attached in aline to the surface of the brain, and theyanastamose freely with neighbouring nerves.
TEMPERATURE OF WHITES AND NEGROES.
IN a manuscript memoir on his voyage to CentralAfrica, presented to the Academy of Sciences byM. Douville, he has mentioned some experimentsmade in Africa on the difference which exists be.tween the temperature of these two races, accord-ing to age, sex, &c. it results from the researchesof M. Louville, that the temperature of the ncgro iscœt. par. much superior to that of the white:—thatthe heat of negresses is greater than that m iiegioesup to the fifteenth year of their age, but after that
period less, but still greater than that of whites;-that the negroes diminish in temperature as theygrow old; and, finally, that the old negroes havet still a higher temperature than the whites.