No. 927.

LONDON, SATURDAY, JUNE 5, 1841.

COURSE OF LECTURESON THE

ANATOMY, PHYSIOLOGY, AND DISEASESOF

THE EAR.

By GEO. PILCHER, Esq., Lecturer onSurgery.

DELIVERED AT THE

WEBB-STREET SCHOOL OF MEDICINE,SOUTHWARK.

Arranged and Prepared from NotesBy THOMAS WILLIAMS, M.B., &c., Demon-

strator of Anatomy at the same School.

LECTURE VI.

jdm/t&M.*—The general outline of theear in the divisions of animals ranging nextin the order of our ascensive method of study,although imperfectly developed, in compari-son with the organ as found in mammalia,presents, notwithstanding, some prominentfeatures distinctive of its superiority whenregarded in juxta-position with the instru-ments of hearing in fishes. If our objectwere, at this department of the course, to in-vestigate, with precision and minuteness, thesteps of development through which theauditory apparatus consecutively passes, incommon with other organs in these cold-blooded vertebrata, it would constitute ourprimary business here to examine into itscondition previously to the occurrence of themetamorphosis in their general structure, bywhich the animals are qualified to sustain thetransition from an aquatic respiration bybranchia to atmospheric breathing by truelu!!g’s. By adhering, however, to the morephy siological division t of the class amphibia,

* Af1rþtf;, utrlnque ; &bgr;Wf;, vita.t [As the real zoological distinction between

the amphibia, properly so called, and rep-tiles is not very generally recognised andreceived, it is desirable to introduce this divi-sion of our subject, by a summary explanationof the grounds upon which it is established.The toad and frog families, constituting the

by which the orders composing it are ar-

ranged, according to a principle founded onthe absence or presence and duration of thebranchiae, the progression of developmentwill be to a great extent followed out. Itwill be immediately anticipated that the earin these inferior genera, as the proteus andsiren, in which, from the persistence of thebranchiae, the respiration, and therefore thewhole mode of living is aquatic, should offerpoints of close affinity in its construction tothe acoustic organ of fishes ; a class fromwhich the lower amphibia are removed by novery obvious grades of organisation. Thisapproximation to the ear of fishes will, like-wise, be found to exist in the superior rep-tiliform groups (of which the genera ofbombinator and rana, or frog-tribes, supplyexamples), during their tadpole stage of

existence, and before the disappearance ofthe means for aquatic life, although, subse-quently, when this temporary period is suc-ceeded by the permanent conditions of theadult animal, the organ acquires elements ofcorrespondent perfection. A considerationof the acoustic properties of the dense medium

genus rana of Linnaeus, were, even at arecent period, regarded as a sub-order ofreptiles ; while the term amphibia was like-wise thought rightly applicable to the highergenera of the true reptiles, from the circum-stance of possessing the power of maintaininga temporary stay in the water. The termamphibia, however, in its physiological sig-nification, respects the change that occurs inthe organic economy of the animal, afterwhich an aquatic residence ceases to be com-patible with its continued existence. In thetransition from the aquatic to an atmosphericmode of living, a general metamorphosis ofstructure takes place, by which the alteredwants of the animal in its new element are

securely supplied. Those animals, then,which are not destined to pass the infancy oftheir existence in a larva state, or that of in-complete formation, subsequently to mount toa standard of higher organisation, are thoseto which the term reptiles is now restricted.These transitions of structure in reptiles, likeall the higher vertebrata, happen while theanimal is yet in ovo.-T. WILLT.AMS.]

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of which they are permanent inhabitants, atonce suggest not the inutility only, but eventhe injuriousness of providing them with atympanum, or even a cochlea, since either, if

Fig. 6.

(a) Membranay tympani covered by the skin.

Fig. 7.

(a) The skin removed.

Fig. 8.

The tympanum and vestibule laid open, showingthe columella (a) and otolithe (b) in situ.

present, would contribute little towards thefaculty of audition. With reference to the

general situation of the labyrinth in the lowerorders of amphibious animals, it resemblesthat of the higher fishes, in which it is almostwholly inclosed in the temporal bone, pre-senting only a limited point for communica-tion with the encephalic meninges. This

arrangement is observed to exist in the lowestgenera, as the proteus, siren, menobranchus.Iii these aquatic species, then, neither thetympanum nor the cochlea has yet appeared.Without, therefore, prosec:uting our examina-tion into the mechanism of the organ of hear-ing in the succeeding groups of the amphi-bious division of animals, the propriety willat once appear obvious, of reducing theminto two sections (of which one presents

neither tympanum nor cochlea, the otherpossessing these appendages). The ear inthe former division is distinguished by anelongation of the membranous sacculus intoa pouch, which assumes the functions of anoval fenestra. In the possession of thisfenestra the auditory organ in the proteusand siren, and the larva state of the generapseudobranchus and menobranchus, whichare the lowest in the scale of amphibiousanimals, is distinguished from this organ inthe higher fishes. While, however, we re.mark the complete absence of the membranatympani and the cavity of the tympanum, adelicate columellar bone is observed to haveits expanded inner extremity adjusted incontact with the summit of the fenestral pro.jection of the labyrinth; and to extend in someexamples a little outwards, to rest its exter.nal extremity in the neighbouring integument,indicating thus the situation of the futuretympanic cavity, while itself is the faint re-presentative of the more perfect columella,and more remotely of the elaborated stapes. Insome of the inferior genera this rudimentalstapes is further reduced to the condition ofa simple lamina of bone, attached to thesacculus, as the minute scales of a fish con.ceal and adhere to the spot upon which theyrest.The semicircular canals, and likewise the

vestibular sac, with its contained calcareouslapilli, in their general formation are closelyanalogous to the corresponding parts inosseous fishes, presenting some slight differ.ence only in the dimensions of the sacculusrotundus, which is synonymous with thevestibule of the higher animals. In relationto the calcareous concretions which attain, asformerly shown, their greatest magnitude incartilaginous fishes, it is surprising thatM tiller denies the uniform presence of theseacoustic substances in the class amphibia,stating only that they are present in appre.ciable form in the genera which are nearestallied to fishes, as the menobranchus. It is,however, capable of demonstration, that theseconcretions, of varying magnitudes, are to befound in the whole class, from the proteus atone extreme to the reptiliform genus ofcaeci-liae. at the other.

In an earlier lecture, a detailed descrip-tion was given of the communication be-tween the posterior extremity of the sac-

culus rotundus and the integument on

the side and back of the head; the allegedresemblance of which in office to the fenestraovalis was also considered. It is not, how.ever, improbable that, from the slight rela.tions of the vestibular diverticulum to thesurface, although this arrangement may littleimprove the function of hearing, it may yet,if regarded trunscendentally or structurally,be considered as the analogue of the fenestraovalis in the inferior orders of amphibia. In

both, with trifling exceptions, the whole

organ is covered externally by the integu.

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ment, without any practicable communicationwith the atmosphere. In the lower amphi-bia, therefore, there are no discoverable indi-cations of an external meatus ; the onlymean, consequently, for the conduct of sono-rous impulses to the acoustic instrument,is that afforded by the solid materials of thehead and body.In the second and higher group into which

we have resolved the class, the ear assumes amore complex structure; the tympanum, in-cluding the Eustachian tube, and imperfectossicula being superadded. The tympanum,as an unequivocal element of the ear, is firstseen in the batrachian genera of amphibiousanimals ; a statement, however, to which thebombinator family offers an exception.Although the membrana tympani, by cau-

tious examination, is susceptible of separatedemonstration, it is, notwithstanding, veryintimately blended with the integument con-cealing and overstretching its area. Thetemporal bone presents a circular aperture,the circumference of which is distinguishedby a sulcus, into which the membrane accu-rately fits. The frog affords a good instancein which this construction may be exhibited.The membrane here, as in all other classes ofanimals, deviates somewhat from the cir-cular, by measuring more vertically thanhorizontally. The figure of the membranevaries: in the frog there is a slight centralelevation internally, giving to a section of itthe appearance of nearly a double plane. Inthe batrachian orders the membrana tympaniis rendered visible by a well-marked depres-sion, with an appearance of thinness of theskin.The cavity of the tympanum contains a

well-developed ossiculum. These ossicles,in some examples, appear to be fusedinto one, and assume the characters of acolumella, stretching as a pillar of commu-nication between the oval fenestra of thelabyrinth and tympanic membrane. Theouter termination of this acoustic column, isdivisible into two or three cartilaginouspieces, attached to the plane of the membraneof the tympanum, and thought by some ana-tomists to present remote but typical resem-blances to the ossicles, which in the amphibia,reptiles, and birds are apparently absent,—the malleus, namely, with the incus and orbi-cular.This form of ear is found in the families of

frogs and toads. There are some orders,however, of the amphibious class in whichthe membranai tympani acquires a cartilagi-nous structure ; with this state two ossiculaare generally associated. In all the generain which the tympanum is developed the Eu-stachiau tube is necessarily present, since thefurmer is nothing more than a prolongationfrom the mucous membrane of the fauces-its foimation, consequently, involves that ofthe communicating tube. In the frogs andtoads, and other batrachians, the Eustachian

tubes terminate in the fauces by a cartilagi.nous and prominent aperture : *. in othergenera the extremities of the tubes from op-posite sides unite, and open only by oneaperture in the median line of the pharynx.In the amphibia, then, it has been seen thatthe cochlea and fenestra rotunda are unde-veloped : in the succeeding class, reptiles,they appear.

Fig. 9.

The figure to the left is the columella of the toad,and that on the right the columella of the frogwith its cartilage.The organ of hearing in reptiles, in compa

rison with that of the preceding classes ofanimals which we have examined, acquiresconsiderable complexity, from the superaddi-tion of a cochlea and fenestra rotunda, witha tympanic cavity traversed by a well-formedcolumella. There are genera, however,comprehended in the class of reptiles, as

the ophidia, or serpent tribe, in which the

tympanum, as a cavity, appears to be almostentirely wanting. The twofold division,therefore, of the organ of hearing in theseanimals is suggested according to the obviouspresence or incomplete formation of the tym-panic cavity. It will be remembered that aparallel classification was adopted in the exa-mination of the auditory apparatus in am-phibious animals ; in which case its proprietywas further supported by a correspondencebetween the habits of the animal with refer-ence to aquatic and aerial living, and the or-ganisation of the ear. But this conformityof structure with office and mode of existencein the inferior genera of reptiles, does not sosatisfactorily appear. It is right to statethat the grouping of this class into the two

leading divisions of those, firstly, in whichthe tympanum is very imperfectly developed,or entirely absent; and those, secondly, inwhich its presence can be readily demon-strated, communicating with the pharyngealcavity through the inter-channel of theEustachian tube, is given upon the authorityof Miiller. There are not wanting otherphysiologists, however, whose names are

weighty as authorities in the science of com-parative anatomy, who, in part, deny thecorrectness of the data upon which this clas-sification is founded. The statement of theformer physiologist is that, in the ophidianreptiles, the chamber of the tympanum is

wholly unformed; while the accounts and

dissections-of Grant and Owen, in common

356

with other naturalists, tend to prove thatthese inferior reptiles possess this accessorypart of the acoustic instrument, although ina condition of rudimentary development. Theadmission of accuracy in the description ofthe latter authors, therefore, involves the ne-cessity of qualifying the proposed classifica-tion.

hiternal Ear of the TU1’tle.Fig. 10.

(a) Cartilage corresponding to the membranatympani.

(b) Tympanum enlarged at both extremities, inan external and internal chamber, connectedby the isthmus tympani.

(c) The columella.(d) The vestibule with its membrane.(e) One of the semicircular canals.

As a general accouut of the ear in the classnow under review, it may be observed, thatan intimate analogy is presented between theorgan in reptiles and birds. But while thespace which it occupies in the cranial parietesin the former division of animals is morelimited than in the latter, the essential element of the labyrinth, the vestibule, acquiresdisproportionate dimensions. The semicir-cular canals, on the contrary, experienceobvious diminution of size, the number con-tinuing unchanged. It is in this order of

animals, whose respiration, organisation, andhabits, make them terrestriul, that the initialgerm of the cochlear appendage is seen. Theprogress of our examination, therefore, provesthe cochlea to be subsequent to the tympa-nic cavity in the progress of development.By a careful dissection of this element in

reptiles, the question, once disputed, may bedetermined, that the cochlear chamber is notderived from a spiral elongation of the propermembranous labyrinth, but consists rather ofa diverticulum from the vasculo-fibrinous

lining of the osseous labyrinth, itself being,originally, a derivation from the cerebralmeninges. This fact has been already ex-plained ; yet from the interest which attaches,

to the origin of this singular addition to theauditory apparatus, there is probably nocharge of tedium incurred by its repetition.Since we possess a very incomplete amountof knowledge, in reference to the true phy-siological signification of the convoluted formof the cochlea, that course of study becomesat once desirable and important, which seeksto determine its office in the process of audi-tion, by taking advantage of the analysiswhich nature herself presents, and thus pro-ceed from the simple to the complex, fromthe more intelligible to the less easily deter-mined. In reptiles, then, the cochlea ap.pears under the character of a plain and un.convoluted pouch, the interior of which isdivided into two imperfect scalce by a verti.cal partition or septum, which constitutes theuncomplicated prototype of the future laminaspiralis. The whole appendage is containedin a portion of cartilage, itself lodged in anexcavation provided in the petrous bone, towhich the external surface of the former isintimately adherent. This structure, whichmay be termed the cochlear cartilage, loses itsindividual form in birds and mammifera, inconsequence of an ossific transformationwhich identifies it with the surrounding bone.The cochlea in external figure in this groupof animals resembles a short pyramidal andcascal canal, in continuity with the parietesof the osseous labyrinth. The septum, bywhich its cavity is multiplied into two minorchambers, is delicate, and membranous instructure, proceeding longitudinally from thebase to the apex of this conoidal cochlea.At the summit or apex the partition per.

forms a semicurve, and by growing deficientleaves an aperture of communication be.tween the two cavities, which latter consti-tutes the obscure beginning of the elaboratelyorganised vestibular and tympanic scalae ofthe higher mammiferous animals.The soft structures which invest the inte.

rior of the osseous cochlea, are theoretically,and, indeed, demonstrably, twofold; that,firstly, which subserves the purposes of a

periosteum to the bone, being a productionof the dura mater; and that, secondly, whichlies in immediate contact with the containedfluid, or perilymph of Breschet, being a va.ginal process obtained from the arachnoidtunic, and, therefore, serous in its character.It appears probable, from the simplicity ofthe plan upon which the cochlea of reptilesand birds is constructed, that the membranousseptum, of which a description has just beengiven, owes its formation to the folding ofthe lining membranes upon themselves, andtheir attachments to the opposite walls of thecavity.

If this explanation be so far correct, it reoquires no ingenuity to conceive that such anarrangement would leave a space at the sum-mit of the cone unoccupied, which wouldnecessarily correspond with that portion ofthe cochlea which, in the mammalia, M

357

known as the cupola, or helicotrema; allowingthus the undulations in one scala to propa-gate themselves uninterruptedly to the other.The position of this rudimental cochlea, rela-tively to that of the vestibule and semicircularcanals, is inferior and anterior, not unlike thesituation that it holds in quadrupeds, inwhich it occupies an anterior relation to thevestibule, being situate between the meatusauditorius internus behind and internal, andthe canalis caroticns in front. In reptiles,the inferior and external wall of the cochlea

presents an aperture, the analogue of thefenestra rotunda or cochleas, in the higheranimals. The partition which removes the ca-vities of the cochlear sac from that of the tym-panum, which in all reptiles, as already ex-plained, is present, in a more or less perfectform, is cartilaginous and slender, havingthe fenestra which conducts to the scalatympani as a depression, differing only fromthe remainder of the walls in the more deli-cate structure of the membrane by which itis closed. This fenestra is likewise knownunder the term of membrana tympani secon.daria, in consequence of its resembling in

structure, and, perhaps, somewhat in func-tion, the proper membrane of the tympa-lll1m.

In relation to the semicircular canals inreptiles, the observation has been made thatthey present a reduced size, while numeri-cally they amounted, as usual, to three. Theposition of the cochlea being inferior and an-terior, that of canals is consequently superior

. and posterior. Although the mean propor-tions of the canals is diminished in reptileswhen estimated in comparison with the sameparts of the labyrinth in fishes, birds, andmammalia, individual variations of diameterexist. The size of the anterior verticalcanal exceeds considerably that of the pos-terior ; this again being greater than the mid-dle. The anterior extremities of the supe-rior and horizontal canals communicate withthe sinus placed on the front of the vestibule,undergoing, before their entrance into it, anampullary dilatation; and the posterior ves-tibular sinus, according to the description ofDe Blainville, receives the posterior crura ofthe same canals," ex trémité d u canal verti-cal posterieur, dont l’ouverture, ainsi que laposterieure du canal horizontal, ont lieudans le sinus vestibulaire posterieur. Lesdeux autres extremites des canaux verti-caux se reunissent toujours en un canalcommune qui n’a qu’un seul orifice dansle vestibule." The labyrinth in reptiles isfilled by a fluid, partaking of a gelatinouscharacter, and corresponding to the peri-lyinph of birds and mammifera. Blainvillestates, that a communication, even by meansof an aperture, exists between the labyrinthand cerebral cavity in reptiles, an accountthe accuracy of which is questionable.Other anatomists more correctly describe theentire of the auditory apparatus alike in am-

phibia, reptiles, birds, and mammalia, aswholly incased in the cranial bones.

[The physiological error has been com-mitted of supposing a necessary relation be-tween the tympanic cavity and cochlear ap-pendage, and of imagining that wher-ever the former element of the ear exists, thepresence of the latter with its membrana

tympani secondaria is consequently implied.Comparative anatomy, however, and thematerials furnished by these lectures, afforddata for the refutation of such a notion. Thefact is now universally known, that the highergenera of batrachian amphibia, of whichfrogs and toads present us with prominentexamples, possess a tympanum very advancedin development, traversed at its upper andposterior extremity by an extended columella ;and it is a circumstance equally well ascer-tained, that the cochlea, and, therefore, itstympanic fenestra, are wholly unformed. Itthen obviously follows, that the cavity in theseamphibious animals has, for its exclusiveoffice, to insulate the columellar stapes in itsextension from the oval fenestra to the mem-brane of the tympanum, rather than to containair for actively assisting the process of audi-tion. Undulations excited in the air occupy-ing its cavity, evidently cannot affect thelabyrinth during the absence of the fenestrarotunda, since vibrations are propagated withvery neulralised intensity from one substanceto another, when the diflerence of their densi-ties is so great as that which exists betweenair and solids. The inference, therefore, isnatural, that the existence of an unequivocalcochlea and fenestra rotunda, presuppose thedevelopment of a tympanic cavity ; butthat the formation of the latter is no neces-

sary ground for predicating the presenceof the former appendage. It should, how-ever, be known, that the fenestra rotunda,closed by the secondary membrane tympanimay exist without a cochlea. The proof thatthere is no unvarying constancy in the rela-tionship between the round fenestra and

cochlea, is furnished us in the chelonian sub-

genera of reptiles, in which the labyrinth

presents a fenestra rotunda, although destitute

of proper cochlea.—T. W ILLIAlBIS,]

ILLUSTRATIONS OF THE

PATHOLOGY AND TREATMENT OFAMAUROSIS.

By EDWARD HOCKEN, M.D.(Continued from page 225.)(Continued from page 225.)

PART V.Amaurosis from Atony of the Retina.-

Treatment.

Tiiis functional variety of amaurosis is byno means uncommon; according to my ex-perience always increasing in severity aslong as its predisposing and exciting causesremain in operation, although it may conti-