The Ear | Operating Microscopes

The Ear
THE ANATOMY OF THE EAR, INCLUDIN OLOGY AND HISTOLOGY.
By BURTON ALEXANDER RANDALL, M. A., M. D., Ph. D.,
OF PHILADELPHIA.

Embryology. The human organ of hearing first appears in early embryonic life on each side of the head posteriorly as a pit like involution of the epiblast (Fig. 434), which closes in to form a spherical "otic vesicle" (Fig. 435). Bud like hollow processes grow out from this inward, to form the endolymphatic duct and sac; forward, as the spirally coiled cochleal tube (Fig. 436); outward, upward, and backward, in curving course to meet and coalesce with similar outgrowths and form the three semicircular canals, each with a flask like dilatation where one of its ends springs from the vesicle. This has meanwhile lost its spherical form, dividing into a more spherical anterior saccule," connected with the cochleal tube by a narrowed “canalis reuniens and an ovoid “utricle " communicating with the semicircular canals by five openings one non sacculated termination being common to the vertical and the posterior canal. The tissue which effects this cleavage extends tip into the endolymphatic duct (Fig. 437), so that the two sacs, although in contact, communicate only by this now Y shaped tube.

Within the otic sac, which has now come to deserve the name of membranous labyrinth, there has been marked differentiation of the cells; while externally a fibrous envelope has been formed from the mesoblast, splitting into a periebondrium sheathing the cartilaginous tissues which have been encasing the structure, separated by spaces of growing complexity from the delicate basement membrane which supports all parts of the' labyrinth. Supported by this, the simple rounded neural cells, generally in single layer, flatten into pavement cells throughout most of the extent of the semicircular canals, the utricle, the saccule, and part of the cochleal tube; but at each point where the developing acoustic nerve sends fibers the cells assume a columnar form, surmounted by short, stiff cilia, the " hair cells," above which floats a layer of gelatinous material of doubtful function, but invariable and early presence. Each of the flask shaped ampullve of the semicircular canals presents such an area, the "acoustic crest," upon its concave side; larger areas are present in the saccule and in the utricle, as shown in Fig. 437, the gelatinous " blanket " of each of the latter loaded with small crystals of lime the otoliths. In the cochleal tube the corresponding structure, called after its discoverer “Corti’s organ," is extremely complex and merits more extended description.

Within the open spiral of the cochleal tube there forms a cartilaginous, early ossifying conical axis, "the modiolus," permeated with openings for the fibers of the cochleal branch of the nerve, which fills the end of the internal auditory meatus at the base of the cochlea and sends its separated fibers through a spiral series of openings into the windings of the structure (Fig. 438). From this conical axis a delicate ossifying shelf is pushed out, ensheathing the diverging nerve fibers to their entrance into the cochleal tube, and by a fibrous extension underlying the inferior (mesial) surface of this tube. These structures, winding spirally like the other portions of the cochlea, vary greatly in their relative size from the base to the apex : for the " basilar membranes,'' with its stiff radiate fibers, is narrowest below, where all the other structures are at their largest, and broadens progressively upward at the expense of the bony " spiral lamina." As its tense radiate fibers are probably comparable to the strings of a harp or piano, responding to notes higher and higher in proportion to their shortness, this is a fundamental fact in the anatomy and physiology of the organ, as proven by a growing amount of pathological investigation (see pp. 644 and 773).

The cochleal tube, originally cylindrical, has now been compressed to a triangular section, one side of it, as stated, being flattened by the basilar membrane. Below and above this, the cavities formed between the layers of surrounding mesoblast constitute parallel channels winding spirally upward and known as “tympanic and vestibular scala;" beyond these the ossifying cartilage forms a firm protecting spiral, which gives the ultimate snail shell form from which the cochlea is named. Similar spaces about the saccule, utricle, and semicircular canals bold away the bony walls which ultimately surround them, except at some one point, generally the convexity, as in the cochlea and the semicircular canals. At the attached point, where the vascular supply is best, the neural epithelium is modified to form secreting cells of the endolymph, those in the cochlea] tube being a gland area of cylindrical cells on the outer wall the " stria vascularis " (Fig. 439). The upper wall of the tube becomes exceedingly delicate (" Reissner's membrane "), the neural cells flattening to a delicate pavement, separated by thin fibrous tissue from the endothelial pavement without. The lower wall shows the greatest modification both of the cells and of the supporting mesoblastic tissues. A fibrous crest forms by thickening of the periosteum of the osseous lamina, ending outward in protecting teeth, to which is attached the gelatinous membrane (in. tectoria of Corti) before mentioned as present above all hair cells. Of these four or five rows are present, supported inward and outward by spheroidal cells, large, vacuolated, and piled upward. Within, two more modified rows of pillar cells (Corti's rods) form a tunnel beneath their arch.

Each set has broadened feet and heads, the outer set Dearly twice as numerous as the inner, and connected outward with a network of phalanx shaped elements " reticular membrane " through the meshes of which the outer hair cells protrude. These latter are double cells the lower fusiform and firmly attached below to the basilar membrane by a, slightly broadened foot, while the tipper process adheres to the side of the peg shaped hair cell, and probably is attached to the reticular membrane. The acoustic nerve fibers pass out between the plates of the osseous spiral lamina after emerging from the " spiral ganglion," which occupies a canal in the modiolus at the root of the lamina. Losing their sheaths, the axis fibers are sent through a row of openings below the Corti teeth to split into ultimate fibrils, which pass, some up to the inner hair cells, some across the tunnel to the outer hair cells while some wind upward along the spiral tunnel.

Osteology. Embryological study has made clear the greater importance, formatively as well as functionally, of the membranous labyrinth. Yet the bony labyrinth was much earlier known and described, and was deemed the structure giving shape to its softer contents (Fig. 440). Its beauty, when carved out of its setting in the dense petrous bone, as is easy in infancy, or of its cast in metal when liberated by corroding away the bone, has attracted to it study which it does not otherwise deserve. It communicates with the middle ear by two fenestre The lower "round Window," closed by the delicate in drawn membrana tympani secondaria, connects with the lower cochlear scala, hence called tympanic; while the upper "oval window," normally occupied by the foot plate of the stapes, communicates with the vestibule, which contains the utricle and saccule, separated from the oval window by a space of nearly 2 mm. the cisterna perilymphatica." This space is directly continuous with the upper vestibular scala “of the cochlea. Depressed cribriform. areas on the mesial wall of the vestibule admit the nerve fibers to the sacs of the membranous vestibule, the saccule occupying the hemispherical and the utricle the hemielliptical fossa (Fig. 441).

The first inner branchial furrow of the embryo early narrows at its outer portion leaving little lateral extension to represent the future Eustachian tube, which does not begin its development until the labyrinth is almost full formed; then it extends outward as a Darrow cleft, gradually separating the labyrinth from the tissues without, in which the ossicles are developing, largely from Meckel's cartilage (Fig. 442). Near the closed outer extremity of the cleft, seven little cartilaginous nodules grow to form the auricle; while in their midst an invagination of the surface forms the external auditory meatus, which presses inward until only the handle of the malleus and the thin ritembrana propria of the drumhead intervene between the cutaneous lining of the meatus and the mucous membrane of the tympanum. The old theory is baseless that the drumhead grows as a septum across the canal, and might fail to do so, leaving a colobomatous opening. When a fistulous opening persists, it is almost always from imperfect closure of the bronchial cleft, and shows above the tragus, at the root of the helix, as a tiny fistula (see Fig. 483).

Besides the ossifying centers of the labyrinth three principal adjacent foci form one in the petro mastoid cartilage to imbed the inner ear and inwardly bound the tympanic portion. A second, called the “annulus " in the infantile form, is a ring incomplete above (Fig. 443), but develops into an extensive scroll later, forming all but the upper posterior wall of the auditory meatus. A third center in the membranes above the ear forms the squama, the zygomatic process, the outer half of the tympanic roof, and the scutum the plate which roofs in the external meatus and forms the outer wall of the attic and antrum portions of the tympanum. Another (post natal) center forms the styloid process. The temporal bone at birth is readily separated into annulus, squamous, and petro mastoid portions, the last of which is still spongy, and can be cracked away from the enclosed labyrinth as a nutshell from its kernel. Later the sutures unite almost completely, and the bony labyrinth blends imperceptibly with its strong, hard envelope, and we obtain the temporal bone as usually described in the anatomical treatises, with which the reader is supposed to be familiar. The further notes on the adult temporal bone will therefore be topographical and surgical.

The Adult Temporal Bone (Plate 9). The outer aspect of the bone looks much more downward than is generally realized, although presenting as many minor variations in this as in all other particulars. One important “orientation point " is the upper edge of the zygoma, which is almost invariably horizontal. Less definite for determining its true position in the vertical plane is the auditory canal, the axis of which (so far as the line can be determined for so curved and tortuous a tube) points on an average 10' below and 10' back of the horizontal transverse axis of the head. Its deviation in each direction varies from 0' to 20' in individual cases. Sharing the general proportions of the cranium, the temporal bone varies hugely in size, massiveness, and configuration, its structure rarefying with advanced life to a delicacy like that of childhood, while its mastoid process corresponds to the muscular development to which its growth is a response. The pneumatic cells within hardly appear before adolescence, and probably enlarge and coalesce progressively throughout life; and nature's economy of material gives to the large process the more pneumatic structure. Diploe is present, as in all the cranial bones, but is in inverse proportion to the air spaces. The type of the cranium and the external configuration of the temporal afford no criterion is to the topography of the bone, except that we may generally expect to find a large mastoid, thin walled and large celled. The mastoid may be said to extend up to the curving temporal ridge which extends backward and upward from the root of the zygoma, and has been used as an index of the level of the middle cerebral fossa within, as in Macewen's chosen 11 postmaster triangle; “but this cannot be relied upon as a landmark. UP and back from the canal a small spine with a depression back of it can be almost invariably recognized as marking the back margin of the meatus. This is a most important surgical landmark, as the point of election for opening the mastoid is close behind it. The floor of the middle fossa at the nearest point averages 6 mm. above it and is probably never lower than this spine: I have found it as low but 5 times in 1060.

This fossa, which constitutes most of the superior aspect of the bone, is of uneven surface, marked with the gyri of the cerebrum, and shows the petro squamous suture in childhood and sometimes into adult life. It is covered with thin, strong dura mater, which sends fibrous prolongations into the suture, and is most firmly attached along the back edge of the pyramid, where it passes into the tentorium and is split to form the superior petrosal sinus (see Plate 10, Fig. 1). Another sinus follows the petro squamous suture, after which it is named, and emerges at the root of the zygoma, large in fetal life, but generally tiny after birth, and shrinking to a fibrous thread, although occasionally persisting of Good size through life, and burrowing more or less tinder bridges of the inner surface of the bone. A groove more mesial in the floor accommodates the great petrosal nerve as it passes from the sphenopalatine ganglion backward to the facial canal to blend with the facial nerve at the geniculate ganglion structures which are often devoid of bonv covering (at the hiatus Fallopii) in infancy, and at times in adult life. Below tile tentorium is the posterior or cerebellar fossa, bounded forward by the posterior aspect of the petrous pyramid. Here the internal auditory meatus, of very varying width and depth, is in close relation with the apparent origin of the facial and acoustic nerve in the polls; and, bidden close by in the layers of the dura, the endolymphatic sac communicates through a long cleft the vestibular aqueduct with the interior of the membranous labyrinth. Farther outward and backward the lateral sinus, leaving its course in the edge of the teutorium, sweeps downward and in to find exit into the bulb of the jugular, forming the sigmoid sulcus upon the base of the pyramid, and a still more marked turn as it passes through the foramen lacerum (Plate 10, Fig. 2). No point in human anatomy is subject to greater variation as to size and position than this sinus and the jugular bulb continuous with it. Formed at the torcular by the confluence of the longitudinal sinuses, the lateral sinus, generally quite unequal in size on the two sides, passes out and forward (transverse sinus) to receive the superior petrosal as it curves down at the posterior end of the crest of the pyramid, to give off the mastoid emissary as it turns inward, and to receive the inferior petrosal as it passes forward at the foramen lacerum. As most of the blood current of the superior longitudinal sinus usually flows to the right, this is oftener the larger and grooves more broadly and deeply the temporal bone at the sigmoid sulcus, extending farther for¬ward and outward with its stronger curves. This cannot be counted upon, however, as rendering the left temporal any safer for easily avoiding the sinus in surgically opening the mastoid. The space between the sigmoid sulcus and the antrum is smaller, actually as well as relatively, in adult life, but rarely exceeds 5 mm., while some 10 mm. usually intervene between it and the posterior wall of the external canal. A large curved or bulbous sinus always approaches close to the canal and to the mastoid cortex, and little or no bone may protect it in one or both of these directions. The fossa in which the bulb of the jugular is lodged is also of infinite variation. Its depth occasionally carries it actually into communication with the internal auditory meatus: it generally closely approaches the lower back part of the tympanic cavity (Fig. 446), with the intervening bony septum at times dehiscent, and it so trenches, in rare instances, upon the labyrinth that its walls are marked by the semicircular canals. The mastoid emissary vein curves in variable fashion out and back from the lower curve of the sigmoid and passes by single or multiple channels through the bone, to emerge at or near the occipito temporal suture. It varies from a tiny vessel hardly more than a nutrient of the bone to a large sinus carrying all the blood of the sigmoid sinus to the external jugular. The inferior petrosal sinus, like the superior, passes back from the cavernous and transverse sinu¬ses, gathering the flow of the effer-ents of the pyramid and adjacent parts. It follows the line of the petro occipital suture, and is prob¬ably of great importance in some cases as the channel of infections from the diseased tympanum.

The lower aspect of the temporal bone is marked by the canals of the carotid artery and the fossa for the head of the jugular, beginning in close proximity, but curving strongly in opposite directions. The first bend of the carotid presses its convexity into close relation to the forward part of the tympanum, and the wall, sometimes dehiscent, is always penetrated by vascular twigs. The outer wall of the canal is in like proximity to the Eustachian tube, and dehiscence is here more frequent a fact to be borne in mind in bougieing this passage. External to this is the glenoid cavity, its posterior boundary formed by the tympanic scroll its juvenile foramen often persistent (Fig. 444) constituting the ,anterior wall of the auditory canal. The open Glaserian fissure at its inner extremity marks the sutural line and gives place to the gracilis process of the malleus, vessels, and the chorda tympani. The fossa of the digastric grooves deeply the under surface of the mastoid, paralleled mesially by that for the occipital nerve and vessels. While the tip of the mastoid process is wholly external to these, it must not be forgotten that thin walled pneumatic cells commonly occupy the mesial boundary and may be the source of pus burrowing in the digastric fossa.

The styloid process, partially ensheathed by a lamina of the tympanic scroll, extends down, in, and forward toward the lesser horn of the hyoid. It represents the cartilaginous and, fibrous axis of the second branchial arch, undergoing no ossification during the early years of life, but sometimes developing almost to the form of the stylo hyoid of the dog. It may thus become 10 cm. in length, with articulations, real or suggested, in its length. It may occasionally be recognized clinically in the lateral wall of the pharynx. Between it and the mastoid lies the foramen of exit of the facial nerve. The deep notch between the apex of the pyramid and the lower anterior margin of the squama is occupied by the tip of the great wing of the sphenoid, with its spinous foramen, from which the middle meningeal artery courses over the inner surface of the squama.

The External Ear. The auricle pinna, or helix is formed by the outward growth of the cartilage, carrying the skin covering in close contact with it. Its form, subject to many minor variations, is fairly constant, and, except at the lobule, closely moulded upon the cartilaginous framework. Its apparent attachment to the head posteriorly is some 20 mm. back of the supra meatal spine, and fairly corresponds to the region of the mastoid where the lateral sinus is most near the surface (see Plate 10). The rest of the mastoid surface is hairless, and seems to cling closely to the surface which it reveals beneath ; but its marked swelling in diseased conditions proves the presence of much areolar tissue and a chain of lymphatic glands connected with those below and in front of the ear. These are frequently involved in ear disease, acute or chronic, and may need evacuation or extirpation steps complicated by the passage of the facial nerve through the superficial group forward and by the adhesions of the deeper set to the sheath of the jugular and carotid. The layers of the cervical fascia concern the aural surgeon greatly in his work, and explain why rupture of mastoid empyema into the digastric fossa lifts the sterno mastoid and all the neck tissues, and how it may burrow to the posterior pharynx wall, down to the clavicle or even into the mediastinuin or pericardium. Parotid abscess is also said to endanger the ear by bursting through the fissures of Santorini of the cartilaginous canal a path about as easily forced were the cartilage devoid of such gaps.

The external muscles of the ear are rudimentary and unimportant, although many have ability to call the attollens into play. The others are occasionally seen in spasmodic action, twitching generally without the consciousness of the person.

The auditory canal, as has been stated, varies greatly in its direction and size, with the result that the structures clinically visible at its bottom are inconstant. The lower anterior segment of the drumhead cannot always be brought to view; while up and back the incus shank, stapes, and stapedius tendon, and down and back the niche of the round window with its prominent anterior lip may often be looked for in vain. The length of the upper back wall usually concerns the surgeon, since it marks the depth of the structures which he may have to attack, while it also gives, as above noted, a clue to the depth of' the facial and horizontal semicircular canals which he must avoid (Fig. 445, Pl. 10). This length to the spina suprameatum is from 12 to 17 mm., and the soft tissues of the canal lengthen it to about 25 mm. The front wall is longer, since the oblique plane of the annulus places the lower anterior parts of the drumhead some 5 mm. farther in ; while the tragus protrudes a little outward, making it some 35 mm. long. The impression is common based upon the position of the drumhead as part of the under surface of the prepared bead of the infant that the infantile canal is much shorter than the adult, and the drumhead more superficial. This has hardly any truth, although accepted and taught by many authorities; for the length of the undeveloped bony meatus is fully represented by its fibrous precursor (Plate 10). Only in so far as the cartilaginous canal is smaller may we look for a few mm. less depth of file drumhead. The caliber is smaller in infancy, and the rather collapsed tube follows the upward curve of the squared, requiring a downward and outward traction to straighten it. The adult meatus is very tortuous, as a rule, although occasionally broad and straight. Its lumen is usually oval; vertically at the exit, but inclining obliquely forward as it passes in, until the axis may be less than 45' from the horizontal. With this there is curvature in the vertical plane and sometimes in the horizontal, so that the inner portion has often a downward trend, although the axis of the outer part pointed 10' to 20' upward. There is commonly some widening of the inner, anterior part, constituting a sulcus close to the drumhead a difficult place from which to remove wax and foreign bodies, as many have found. The diameter of the canal is very various, as is the type of its oval, the narrowest isthmus being generally the most compressed portion, with a minor diameter sometimes as small as 5mm. In the unmacerated canal the strong curves of the soft parts (Plate 10) still farther complicate the tortuosity, although these can be largely eliminated by good traction, usually up and back and out. Yet the gain is great, in getting access surgically to the depths of the canal, if we lay forward the soft parts and work in the shorter, broader tube of the naked bony meatus. In clinical cases, where free access is needed or the widest exit for a foreign body, the jaw must be dropped so as not to press upon the front wall of the canal. That it compresses the outer parts is demonstrable at a glance or by moving the jaw with the finger thrust into the meatus; but it is generally forgotten that the bony wall often presents in adult life the foramen of lacking ossification near the drumhead, which is a constant feature in the early years of life, and that pressure of the jaw condyle may be exercised here also.

The middle Ear. The Tubo tympanic Cavity. The drum cavity, like the labyrinth, although varying much in its dimensions in individual cases, is practically of full size at birth, and undergoes little or no change in the development of the temporal bone as a whole. Its full significance anatomically and pathologically is partly lost by those who do not follow Leidy in recognizing its threefold division in considering the antrum, as much a part of it as is the attic or atrium. Much good surgical comprehension and practice has crystallized about the name attic for the epitympanic space, and further clarification will be general when the antrum is no longer considered as one of the mastoid cells. The loose vagueness of ideas and terminology which calls the tympanic membrane “the drum " is only less manifest when we ignore this and the relation of the scutum (pars ossea of the drumhead, Walb) to the upper and posterior tympanic cavities.

Embryology shows it’s that the Eustachian tube, tvmpanum, and mastoid cells are one complex and slowly developing structure; and, while the last are possibly merely adventitious adnexa, we may yet learn to better appreciate their unity. Absent, however, at birth, when the organ is otherwise so complete, we may now regard them as unimportant. The tubo tympanic cavity is a portion of the tipper air passages as much as are the accessory cavities of the nose lined with an extension of the same nasal mucous membrane with all its pathological attributes. Much of this mucous membrane is at the same time virtually a periosteum, which magnifies its importance physiologically and increases greatly the importance of its lesions.

The tympanum or drum cavity is in man situated in the midst of the temporal bone, some 20 35 mm. from the opening of the external canal. Its lower portion, the drum cavity proper, or atrium, is bounded outward and down by the annulus and tympanic membrane, and has somewhat the shape of the body of a vertebra a short cylinder with concave ends: the inward traction of the center of the drumhead and the protrusion of the promontory opposite bring these walls within about 2.5 mm. of each other. This dimension might well be called the height, as it is more vertical than the longer line from floor to roof, which is conventionally so called, the extreme obliquity of the annulus being forgotten. From front to back it measures some 12 mm. of which 9 mm. is bounded by the drumhead. Roof the atrium has none, for the attic is continuous with it above, defined externally by the tympanic margin of the squama, but less definitely elsewhere. Including the 5 mm. of the attic, there is a distance of about 15 mm. from floor to roof, but only two thirds of this belong to the atrium. The attic space above is broadest at the top, and overhangs markedly the inner end of the canal, with the Scutum, which separates them, but a thin wedge of bone. It merges almost imperceptibly into the antrum out and back, a hour glass contraction (aditus) being made by the protrusion of the facial and horizontal semicircular canals.

As viewed by the clinician, the drum cavity seems bounded by the annulus, and through the transparent drumhead or any perforation various inner structures are seen. Anteriorly, the opening of the Eustachian tube makes a deep depression, as its lumen is nearly in line with the axis of the external meatus (they meet at 150' on an average). Down and back the dark niche of the round window (see Fig. 445) is bounded in front by the prominent lip of the promontory (1), marking the large beginning of the first turn of the cochlea. Up and back more or less of the shank or descending process of the incus can be seen (f, Fig. 447), commonly parallel to the malleus handle, with a horizontal line stretching back from it the stapedius tendon (h). Of the stapes little or nothing can be normally seen in the depths of the pelvis ovalis. The curved line of Troeltsch's posterior pocket seems to broaden the handle of the malleus as it spreads upward, until at the incus shank it meets the whitish line of the chorda tympani, which edges the backward sweep of the rest of it. This helps to shadow the stapes, the bead and anterior crus of which are hidden by the incus, and come to light only when its shank is displaced or lost. In the floor posteriorly small depressions between trabeculve of bone give irregularity to what has been called the recessus hypotympanicus, important as being often almost or quite trenched upon by the bead of the jugular. Above the short process (a) and the anterior (e) and posterior folds, which can generally be discerned even in the normal drumhead, is the flaccid membrane sometimes defined into anterior(c), middle (d), and posterior (g) portions by visible "suspensory folds” occupying the gap between the extremities of the annulus, where the tympanic mar gin of the squama completes the ring. This “Rivinian segment “is usually strongly notched, but varies, and gives varying size to the 11 Shrapnell's membrane," as it is called after the Englishman who first pointed out its flaccid character.” Rivinian “it is also termed, as the site of the tiny pinhole which Rivinus, following Riolalus, pointed out as a frequent 11 foramen “here, and which Bochdalek and others have claimed to be congenital. Embryology offers no explanation of its occurrence, and anatomists and clinicians generally unite in denying its usual presence. Unknown in fetal or infantile specimens, it grows more common through childhood, averaging 10 per cent.; and in adult life it may be clinieally recognized as a sear or patent opening in 25 per cent., of cases frequently symmetrically. It is almost certainly the remains of a pathological perforation in spite of any negative history.

This upper region, like many other parts of the tympanum, varies much in its visibility; as the conformation and direction of the canal may render illumination and observation difficult or easy. Especially is this the case in the region of the stapes, which is bidden more or less completely when the canal is horizontal, but shows better the more upward is its inclination Non transparency or variability of the drumhead structures has influence in concealing these important parts; but the surgeon who desires access to them should note carefully the axis of the canal in relation to the horizontal plane of the head, as given by the eyebrows, eyes etc., since it is a criterion for what he may expect as to their apparent location.

The drumhead or tympanic membrane is the thinned remains of the tissues which separated the in growing external meatus from the outgrowing tubotympanic space. Long before birth it has become a delicate membrana propria of strong, slightly elastic fibers the denser, outer laver radiating from the malleus handle, while the inner laver is circular. Both sets merge peripherally into the fibrous tissue of the tendinous annulus which form ,, the thick margin inserted into the sulcus of the bony tympanic annulus. To the circular fibers, which are most numerous near the annulus and least numerous in the intermediate portion, is largely due the characteristic funnel shape of the drumhead, which increases when the structure is cut loose from its attachments. They also explain the maintenance of depressed conditions of the drumhead after the cause has been removed. This fibrous basis is clothed externally by the thin skin of the external canal and internally by the mucous membrane of the tympanum. This basis is present only throughout the extent of the membrana tensa filling the tympanic annulus. Above, in the Rivinian notch, the flaccid membrane of Shrapnell is almost devoid of fibrous tissue interposed between skin and mucosa. The normal membrane is capable of sustaining a pressure upon either of its surfaces of 15 pounds to the square inch, but will generally rupture under greater stress.

The handle of the malleus, partly sheathed in cartilage, is inserted between the layers of the membrana propria, although protruding much upon the mesial surface. Folds of the mucous membrane, with, more or less of fibrous basis, run backward and forward from the manubrium, in relation in part with the chords tympani, but forming with the drumhead inverted pockets, anterior and posterior. Much variability marks these, like many other tympanic structures, but generally they are well defined and can be seen through the transparent membrane. The annulus is very obliquely setits plane (not a true plane, as it is slightly spiral) facing downward, outward (37'), and forward (370). The planes of the two drumheads extended downward and forward would meet at 128'. As the external canal is nearly transverse, averaging 100 upward and 10' forward as it passes inward, but with its innermost portion sometimes curving downward below the horizontal, it meets the drum membrane at a very acute angle. This is best measured upon casts of the canal, which show the upper back wall to pass without demarcation into the drumhead (Figs. 448, 449), while downward and forward the angle may be as small as 30' or less. This depends in part Upon the indrawn funnel shape of the drumhead, which does Dot lie in the plane of the annulus, but bulges slightly outside of this plane peripherally. It is centrally drawn 1 mm. or more within it by the traction of the tensor tympani.

Ossicles. The two larger ossicles (see Fig. 457) develop from the axis of the first branchial arch (Meckel's cartilage), and are gradually insulated by the development of the tympanic cavity around them. The gracilis process of the malleus, "long" in Infancy, but often absorbed or fused to the Glaserian fissure in adult life, is in the seventh week the largest part of the chain. The malleus handle, or manubrium, has been seen to develop in closest relation with the drumhead, which partly ensheathes it; and with the major blood supply along its posterior border there must be very serious destruction to impair its nutrition. The bead is suspended and vascularized from the roof, so that it is less prone to caries than the neck, in contact with which septic collections are apt to be held by Prussak's pouch; so the ossicles may be cut in two. The incus articulate with the saddle shaped surface by a sort of clutch joint, engaging snugly when the handle moves inward, but loosely at other times. Neither this nor the incudo stapedial joint has a definite structure: equally careful students find it a synchondrosis, a true synovial joint with interposed meniscus or an intermediate form. There is a firm capsular ligament with a strong reinforcing band mesially, which constitutes the major support of the incus. When this is destroyed, the delicate attachment to the stapes, if not already severed by the erosion of the incus shank, the most vulnerable part of the chain, and the stronger semi articulation of the tip of the horizontal process to the mouth of the antrum, rarely retain the incus in place. The stapes is the virtual key to the value of the chain, and in the absence of the rest can fulfill most of the needs of hearing. Its annular ligament may be considered a synchondrosis with the edge of the oval window, and its footplate can be torn away without necessarily tearing the membranous closure of this fenestra. Membranous hands unite the crura with each other and the sides of the niche in most variable manner; but these may be the most important factors in the great majority of impairments of bearing, and their minute and extended study is greatly Deeded.

The Eustachian tube, some 35 mm. in length in the adult, is a trumpet shaped canal extending from the pharynx to the tympanum. It is usually collapsed, and presents on transverse section a narrow vertical fissure capable of no great lateral distention. The distal third of its length is supplied with bony walls by the temporal bone; but the longer mesial portion has only *cartilaginous support furnished by the " book cartilage " above (Fig. 450), which in the median half extends down on its posterior wall and constitutes the basis of the prominent posterior lip of the pharyngeal mouth (Fig. 451). The tympanic end also broadens from the narrow isthmus at the junction of the. bony and cartilaginous portion, to merge imperceptibly into the anterior part of the tympanum. The tube is lined with columnar ciliated epithelium and its submucosa is full of racemose glands. The basement membrane is in close relation with the cartilage throughout its extent; but where cartilage is lacking, is in like relation to the membranous wall. Two important muscles act upon the tube, the so called tensor, and the levator veli palati. The latter lies beneath the tube, and acts upon it only by reason of the upward pressure of its shortening belly, permitting rather than causing the 'Walls to separate. The tensor is more complex in its structure and relation. It arises in part from the hook of the cartilage, which it tends to open by its contraction; other fibers arise from the membranous anterior wall of the tube and tend to draw it away from contact with the opposite wall. Still other fibers arise from the basal cartilage above; but any compressive action upon the hook is probably slight, and I have found a bursa interposed in some cases to facilitate play. Passing downward and forward, the tensor fibers converge to the hamulus of the sphenoid, where the tendon turns inward and spreads in the velum. Weber Liel has shown that some of the fibers of the tensor are inserted upon the hamulus, and could act, therefore, only as a dilator of the tube. Some of the fibers which arise from the book of the cartilage seem to merge with the fibers of the tensor tympani which extend in the opposite direction; and certainly they have common enervation and consonant action. The latter muscle occupies with its belly the canal, partial or complete, which lies along the roof of the bony Eustachian tube, and extends backward and out to the inner wall of the tympanum, where the little tube like trochlea (Fig. 445) permits its tendon to turn directly outward and, crossing the drum cavity, to insert itself upon a tiny tubercle on the handle of the malleus nearly directly in from the short process. Disadvantageous as is its leverage, it has abundant power to keep tension on the drumhead through the manubrium. The stapedius, the other intratympanic muscle, is still more snugly lodged in a bony case the pyramid (see Fig. 445) at the back part of the drum cavity, between the facial canal and the round window niche. Below, this is open to receive nerve supply from the facial above, its cavity curves forward to give exit to its thread of a tendon close to the head of the stapes, to which it is attached. Tts action is supposed to be rather antagonistic to the tensor tympani and to limit the pressing of the stapes into the oval window.

Plications of the mucous membrane serve with their fibrous basis to suspend the malleus and incus from the roof of the tympanum, to form an external ligament fastening each to the upper tympanic margin and to accompany each of the tensor tendons and the gracilis process respectively. Themselves variable, these bands are reinforced by others less constant, which serve to subdivide the attic space more or less completely into several or, at times, many cavities. One fairly constant “pouch “lies between the neck of the malleus and the flaccid membrane, having the short process for its floor, as described by Prussak. Politzer has found this often subdivided, while Kretschmann thinks the usual condition is for the pouch to extend backward along the body of the incus. Much in the pathology of inflammation in the attic depends upon the individual variations of these parts and the retention of secretions within the spaces thus isolated (Fig. 452).

In the antrum comparable septal bands are often present; and its lining mucous membrane, which extends into the communicating pneumatic cells of the mastoid, often narrows greatly the entrances of these. Although sufficient for good drainage in health, these openings are apt to become stenosed by inflammatory swelling, and by retaining the secretions give rise to mastoid empyema. There is usually 4 radiate or racemose arrangements of the pneumatic cells about the antrum, due to the fact that they are outgrowths of the tubo tympanic space. At birth there is merely diploetic structure of the tiny mastoid and adjacent pyramid; and in the developed bone this may never be wholly displaced (Fig. 453), although probably steadily decreasing as the pneumatic cavities enlarge. Little of it remains in the great majority of adult bones which I have examined (Fig. 454); and study of' a thousand indicates that hardly 2 per cent., could be classed as diploetic and only some 10 per cent., as combining a notable amount of diploe with the pneumatic spaces. No mastoid is absolutely pneumatic, although some senile bones show a single thin walled cell occupying the greater part of the process; but no demarcation can be drawn as to bow far the air cavities may be expected to extend. In some specimens they invade the occipital bone backward ; they may occupy the zygomatic process and hollow out the pyramid to its very tip anteriorly usually they pass close to the sigmoid sinus. Some generally connect with the beginning of the Eustachian tube, lying in its floor in close relation to the carotid. Bordering on the digastric fossa, they are thin walled and large on the mesial as well as on the distal side. The paramastoid or paracondyloid outgrowth sometimes seen upon the occipital, as well as even the coudyle itself, may be occupied by these pneumatic extensions of the tympanum. Their function, if any, is doubtful; and they probably show merely Nature's economy of material and only rarefy these structures within as they grow externally larger. Too utterlv variable for the condition on the one side to form much of a criterion for the other, they have no, cry great surgical importance. Hyperostotic inflammation can solidify the bone with equal promptness whether it be pneumatic or diploetic caries is apt to be equally destructive and extensive in each; possibly pyemia extension is less ready from the pneumatic mastoid than when considerable diploe is present. When aural surgery regarded the antrum as merely one of the mastoid cells, the others seemed of little inferior importance; but more precise views of the anatomy and pathology are now dissipating this view. The surgeon, in undertaking to open a mastoid empyema, acts upon the indications as to the location of the pus, whether in the antrum alone, in the mastoid tip, or throughout the process, and considers the cell spaces met only as holding out of his way the more important dural structures which lie desires to avoid in opening his track usually to the antrum. His concern is principally that no anomalous forwardness brings the sinus into his field nor undue lowness of the middle fossa exposes the dura to unintended attack. Be must know that while 10 mm. is the average width of his field, the lateral sinus may wholly occupy it; that above the spin a there may be none of the 6 mm. of space usually to be expected; and that every bone, whether left or right, in brachyeephalic or dolichocephalic, must be treated as though presenting the most dangerous relation possible, until exploration has proved the contrary. Boring instruments must give place, therefore, to gouge and spoon; and the mastoid surface must be bared and well scrutinized, and not blindly attacked even at the well chosen point. For the antrum this is usually about 5 10 mm. horizontally back of the suprameatal spine, and the cavity should be reached at a depth of 10 15 mm. by a channel parallel to the meatus, but directed slightly more upward. Probably the facial and semicircular canals on the farther side of the antrum are never less than 15 mm. from the mastoid Surface; but they are rarely more than 25 mm. a depth which may be taken as the maximum permissible penetration.

Much important anatomical detail has been here omitted, such as the origin and course of the fibers of the acoustic nerve and their distribution. Those wishing more than is shown in Fig. 441 are referred to the exhaustive works.

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