Book - Buchanan's Manual of Anatomy including Embryology 17

Embryology - 19 Apr 2024        Expand to Translate
Google Translate - select your language from the list shown below (this will open a new external page)
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt    These external translations are automated and may not be accurate. (More? About Translations)

Frazer JE. Buchanan's Manual of Anatomy, including Embryology. (1937) 6th Edition. Bailliere, Tindall And Cox, London.

Buchanan's Manual of Anatomy: I. Terminology and Relative Positions | II. General Embryology | III. Osteology | IV. Bones of Trunk | V. Bones of Head | VI. Bones of Upper Limb | VII. Bones of Lower Limb | VIII. Joints | IX. The Upper Limb | X. Lower Limb | XI. The Abdomen | XII. The Thorax | XIII. Development of Vascular Systems | XIV. The Head and Neck | XV. The Nervous System | XVI. The Eye | XVII. The Ear | Glossary

Historic Disclaimer - information about historic embryology pages

Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Embryology Navigation Teaching Movies Embryonic Systems Abnormal Explore

Talk:Book - Buchanan's Manual of Anatomy including Embryology 17 Jump to:navigation, search CHAPTER XVII

THE EAR

The organ of hearing is divided into three parts—the external, middle, and internal ear.

External Ear.

The external ear consists of the auricle (or pinna) and the external auditory meatus. The former has been already described (see p. 1294)* The external auditory meatus extends from the bottom of the concha to the membrana tympani, and is about 1 inch in length. It consists of two parts—outer, or cartilaginous, and inner, or osseous. The cartilaginous part, which is also fibrous, is about J inch in length, and the osseous part, which lies within the petrous portion of the temporal bone, is about § inch long. The widest part of the meatus is its orifice, which is oval, the long measurement being vertica . The narrowest part is situated in its osseous portion, about T mch from the tympanic membrane, and it is known as the isthmus. There is another constriction of the canal situated near the deep end of t e cartilaginous part, and produced by a projection which is placed antero-inferiorly. The chief direction of the canal is inwards and slightly forwards. At first it is also inclined upwards, then backwards,

and finally downwards. . ,

The cartilaginous part is continuous with the cartilage of the auricle, and is attached to the external auditory process of the temporal bone. Its cartilage is folded so as to form a deep groove which is open at its upper and back part, the cartilaginous deficiency being completed by fibrous tissue. In the anterior wall of the cartilaginous part are two clefts (called the fissures of Santorini ) which are occupied by fibrous Ssuf In important and close inferior relation of the cartilaginous

meatus is the parotid gland (see Fig. 1027). ,

The osseous part has been described in connection with the temporal hone (d IQ4) At its deep end there is a narrow groove, called the sulcus tvmianicus, which forms about five-sixths of a circle, the

deficiency being placed superiorly, at the V^femporaf^The rinp- ic completed by the squamous part of the temporal bone, me

tympanic membrane is set obliquely within the tympanic sulcus being inclined in such a way that its front part is nearer the nudd line of the body than its back, and its lower part nearer the middle line than the upper. The floor and anterior wall of the meatus consequently longer than the roof and posterior wall.

^ 1671

1672

A MANUAL OF ANATOMY

The meatus is lined with skin, which is continuous with that of the auricle. In the osseous part of the canal the skin is very thin, and is provided with vascular papillae, but is destitute of glands and hairs.. It is reflected over the outer surface of the membrana tympani, of which it forms the outer layer. In the cartilaginous part of the canal the skin is thicker, and is provided with hairs, connected with the follicles of which are sebaceous glands. In addition to these there are convoluted tubular glands, similar in structure to sweat-glands, and called the ceruminous glands, which secrete the ear-wax.

Blood-supply.—The arteries are derived from the posterior auricular of the external carotid, the deep auricular of the first part of the maxil

Upper Part ofHelix

Lateral Ligament of the Malleus

1

Incus

Semicircular Canals

Concha

External Auditory Meatus

Malleus Vestibule __ Cochlea x , Tympanum

Tensor Tympani Muscle

Apex of Pet. Portion of Temporal Bone

_Anterior Lig. of

the Malleus

-Internal Carotid

Artery

Lobule

Pharyngotympanic Tube

Parotid Gland

Tip of Styloid Process of Temporal Bone

Tympanic Membrane

Iug. 1027. General View of the Right Organ of Hearing (after

Hirschfeld and Leveill£).

The external ear and middle ear are seen in section.

iary, and the anterior auricular branches of the superficial temporal. The veins follow the course of the arteries.

Lymphatics. These pass to the mastoid glands and to the preauricular lymphatic glands.

Nerves. The auriculo-temporal nerve gives two branches to the meatus, upper and lower, which enter it by passing between the cartilaginous and osseous walls. The upper branch supplies the skin covering the upper part of the membrana tympani, while the auricular branch (Arnold s nerve) of the vagus supplies that of the osseous part of the canal in its lower and back part, and also that covering the lower part of the membrana tympani.

Early Condition of the Meatus.—At birth the osseous part of the

canal is represented by the tympanic annulus and a small portion of

THE EAR

1673

the squamous part of the temporal bone. It is connected by fibrous tissue to the cartilaginous framework of the auricle, and within this fibrous tissue the osseous canal is formed by two outgrowths from the tympanic annulus.

Middle Ear.

I he middle ear, or tympanum, is an irregular space within the petrous part of the temporal bone, which lies between the membrana tympani externally and the outer osseous wall of the internal ear or labyrinth internally. It is lined with mucous membrane, and it communicates with the naso-pharynx by means of the pharyngotympanic tube, through which it receives air. It has three parts: (1) the tympanum proper, or cavum tympani; (2) the attic, or epitympanic recess; and (3) the tympanic or mastoid antrum.

The tympanum proper (or cavum tympani) is situated between the tympanic membrane and the outer wall of the internal ear. Its contents are as follows:

1. A chain of ossicles (malleus,

incus, and stapes), with their ligaments.

2. Muscles.

The vertical and antero-posterior diameters (inclusive of the attic) are fully \ inch. The transverse measurement is from | to £ inch, except opposite the centre of the membrana tympani, where it is only T V inch, and the shape of its cavity may, perhaps, be visualized by likening a cast of it to a biconcave disc about the size of a threepenny piece.

The tympanic cavity has six walls—lateral, medial, roof, floor, anterior, and posterior.

The lateral wall is formed chiefly by the tympanic membrane, which has the handle of the malleus fixed to it, and slightly by the tympanic annulus, within the circumference of which there is a groove, called the tympanic sulcus, in which the membrane is set. The tympanic annulus and sulcus are interrupted superiorly by a notch, called the tympanic notch. In front of the tympanic annulus is the open, inner extremity of the squamo-tympanic fissure, which lodges the processus gracilis of the malleus, and transmits the tympanic branch of the internal maxillary artery. At the inner end of the fissure is the opening of the iter chordce anterius, by which the chorda tympani nerve leaves the tympanum.

The (medial) wall (see Fig. 1028) separates the tympanum from the internal ear or labyrinth. It is very irregular, and is formed by the following parts :

1 The fenestra vestibuli. 3 - The promontory.

2. Projection of the facial canal. 4 - The fenestra cochleae.

5. The sinus tympani.

3. Nerves.

4. Bloodvessels.

5. Air.

A MANUAL OF ANATOMY

^74

The fenestra vestibuli is situated in a depression, called the fossa ovalis, at the upper part of the inner wall, and it leads into the cavity of the vestibule. It is irregularly oval, and is elongated from before backwards. It is occupied by the foot-piece of the stapes, and the annular ligament which connects the circumference of the foot-piece to the margin of the opening, the margin being covered by cartilage.

The projection of the facial canal lies above the fenestra ovalis. The canal, which contains the facial nerve, is here directed backwards, and has walls of a paper-like thinness.

The promontory is seen below the fossa ovalis, between it and the fossa rotunda, and slightly in front of both. It is a rounded promin

Mastoid

Antrum

Sinus Tympam Outlet of Facial Canal at Stylo-mastoid Foramen

kiG. 1028. Section through the Petrous and Mastoid Portions of the Temporal Bone, showing the Tympanum and Mastoid Cells.

ence made by the first turn of the cochlea, and is grooved by the nerves of the tympanic plexus.

The fenestra cochlese is situated in a funnel-shaped depression called the fossa rotunda, below and behind the promontory. It leads into the scala tympani of the cochlea, and in the recent state is closed by the secondary membrane of the tympanum.

1 he sinus tympani is a depression behind the promontory, and between the fossa ovalis and fossa rotunda. In close relation to this is the ampulla of the posterior semicircular canal.

The roof of the tympanum is a thin plate of bone, called the tegmen tympani, which forms part of the anterior surface of the petrous part of the temporal bone.

The floor, narrower than the roof, is a thin plate of bone which separates the tympanum from the jugular fossa.

THE EAR

1675

The anterior wall is narrow, owing to the descent of the roof, and the inclination towards each other of the outer and inner walls. In it are the openings of two canals, the upper of which lodges the tensor tympani muscle, whilst the lower is the osseous part of the pharyngotympanic tube. The two orifices are separated by the margin of the processus cochleariformis. The carotid canal lies just in front of the lower part of the anterior wall.

Fig. 1029.—Diagrammatic Outline of Tympanum and Associated

Recesses.

The posterior wall is formed by the anterior or tympanic surface of the petrous part of the temporal bone. From above downwards the following parts are seen: (1) the opening of the mastoid antrum, which communicates with the attic of the tympanum, or epitympanic recess; (2) a depression called the fossa incudis, receiving the short process of the incus* (3) a small conical projection, called the pyramid, at the summit of which is an opening for the tendon of the stapedius muscle (posteriorly the canal within the pyramid, which contains the

Mastoid Antrum

Lateral Semicirc.C. ~ * Pyramid • Sinus Tympanum —

-Epitympanic Recess

-Site of Geniculate Ganglion

Fenestra Vestibuli

Promontory

Fenestra Cochleae

Fir jo^o_ Diagram to show Course and Relations of Facial Canal on

' 'the Medial and Posterior Walls of the Tympanum.

stapedius muscle, passes downwards in the posterior wall of the tympanum, and communicates with the descending part of the canal which contains the facial nerve; this explains how the branch of that nerve to the stapedius reaches the muscle); and (4) the iter chordae

posterius, for the chorda tympani nerve. .

Tympanic Membrane.-This is the membrane which closes the inner extremity of the external auditory meatus. It is situated on the outer wall of the tympanum, of which it forms the chief part, and it is set for the most part in the sulcus tympamcus, which marks the

1676

A MANUAL OF ANATOMY

inner surface of the tympanic annulus. Superiorly, however, where the ring is wanting, the membrane is attached to the tympanic notch (of Rivinus). This part of it is thinner and looser than the rest, and is called the membrana flaccida, or Shrapnell’s membrane. The attachment of the membrana tympani to the sulcus is by a thickened ring of fibres, called the annulus fibrosus. This annulus passes from the extremities of the notch to the short process of the malleus in the form of two bands, the anterior and lateral ligaments of the malleus, which form the lower boundary of the membrana flaccida. The membrane is set obliquely in the tympanic sulcus, so that its lower part forms an acute angle with the floor of the meatus externus, and its upper part an obtuse angle with the roof of the passage.

Superior Ligament of Malleus

Head of Malleus Insertion of Tensor Tympani

Chorda Tympani Nerve

Pharyngo-tympanic Tube

Superior Ligament of Incus Body of Incus

Attic of Tympanum

^ Short Process of Incus

L ’\ST ^Posterior Liga^ ment of Incus

s ^_Long Process of

_Processus Orbicu 1 & • laris of Incus

y ' 0 _Handle of Malleus

—

_Membrana Tympani

Fig. 1031.—The Right Membrana Tympani, Malleus, and Incus (Internal, Posterior, and Superior View) (Spalteholz).

The tympanic membrane is somewhat oval. In the vertical direction it measures about 10 mm., and horizontally from 8 to 9 mm. The handle of the malleus lies between the mucous and fibrous layers of the membrane, and descends to a point a little below its centre, where it ends in a small knob, from which the radiating fibres of the membrane proceed. This knob is firmly attached, and, being directed inwards, the membrane is consequently drawn inwards at that point, and its outer surface presents a slight conical depression, the deepest part of which is called the umbo.

Structure.— The membrane consists of three layers—external, middle, and internal.

The external or cutaneous layer is very thin, and is derived from the skin of the external meatus. It contains no glands, is freely provided with bloodvessels and nerves, and is covered by stratified squamous epithelium.

/

THE EAR

1677

The middle or fibrous layer forms the proper substance of the membrane, and consists of fibrous tissue. The fibres are radial and circular. The radial fibres lie beneath the cutaneous layer, and radiate from the handle of the malleus to the annulus fibrosus. The circular fibres are deep to the radial fibres, and are most numerous towards the circumference of the membrane. Both sets of fibres are absent from the membrana flaccida.

The internal or mucous layer is continuous with the mucous membrane of the tympanum, and is covered by a single layer of squamous epithelium.

The membrana flaccida, pars flaccida, or Shrapnell’s membrane, has cutaneous and mucous layers only. These are united by connective tissue, which is so loosely arranged that the membrane is flaccid. This part is very liable to perforation.

Cone of Light.—Extending from the knob, in which the handle of the malleus terminates, downwards and inwards to the antero-inferior margin of the membrana tympani there is seen a specially bright reflection, triangular in outline, with the apex towards the umbo. This is called the cone of light.

Arterial Supply of the Tympanic Membrane.—(1) Deep auricular branch of the maxillary artery. This vessel, which passes through the anterior wall of the external meatus, supplies the cutaneous layer. It descends from the skin of the roof of the meatus along the course of the handle of the malleus to the umbo, where it divides into branches which radiate towards the circumference of the membrane. (2) The stylo-mastoid branch of the posterior auricular; and (3) the tympanic branch of the maxillary. The former artery enters the tympanum from the facial canal, and the latter through the squamo-tympanic fissure. Branches from them supply the mucous layer, and form an anastomotic ring around the circumference of the membrane. The fibrous layer receives its arterial supply from the vessels of the cutaneous and mucous layers.

The veins join the external and internal jugular.

Nerve-supply.—(1) The auriculo-temporal of the mandibular, (2) the auricular branch of the vagus, and (3) branches from the tympanic plexus.

Secondary Membrane of the Tympanum.—This membrane closes the fenestra cochleae on the inner wall of the tympanum, and separates the tympanic cavity from the scala tympani of the cochlea. It is concave towards the tympanum, and, like the membrana tympani, consists of three layers. The external layer is formed by the tympanic mucous membrane; the middle layer is fibrous; and the internal layer is formed by the lining membrane of the cochlea.

The mastoid antrum and mastoid air-cells are described with the temporal bone on p. 188, while the Eustachian or pharyngo-tympanic tube is dealt with on p. 1378.

1678

A MANUAL OF ANATOMY

Lateral Process Long Process

Ant.

Process

Plandie

Fig. 1032.—The Malleus.

1. anterior view; 2, posterior view.

Ossicles of the Tympanum.

The tympanum contains three small bones, arranged in the form of a chain which extends from the membrana tympani to the fenestra vestibuli. The bones are the malleus, the incus, and the stapes. The malleus is related to the membrana tympani, the stapes to the fenestra, and the incus occupies an intermediate position between these two.

The malleus is so named from its resemblance to a hammer. It is composed of a head, neck, handle, and two processes, long and short.

T 2 The head is the upper, enlarged,

Facet for incus rounded end. Posteriorly it has

a saddle-shaped facet, directed obliquely downwards and inwards, for articulation with the incus in a synovial joint. The neck is the constricted part below the head. The handle (; manubrium) is directed downwards, inwards, and backwards from the neck; it is compressed from before backwards, slightly curved, and ends in a knob. It lies between the fibrous and mucous layers of the membrana tympani, descending to a point a little below the centre of the membrane. It is firmly attached to the fibrous layer by its periosteum. The tensor tympani muscle is inserted into the inner part close to its root. The long or anterior process (processus gracilis ), which is slender, springs from the front of the neck, and is directed forwards and downwards to the petro-tympanic fissure, where it is embedded in fibres which form part of the anterior ligament of the malleus, and connect it to the margins of the fissure. The long process is in the adult for the most part replaced by fibrous tissue except close to the neck of the malleus. In early life it is continuous with Meckel’s cartilage. The short or lateral process is situated immediately below the long process. It is directed laterally to the upper part of the membrana tympani, with which it is connected by the annulus fibrosus. It is also connected with the extremities of the notch by the anterior and posterior malleolar ligaments.

Ihe incus resembles an anvil. It consists of a body and two processes—short and

long. The body is thick, somewhat four- ^ ~ Head ° f Sta fu

sided, and laterally compressed. Anteriorly y^ 08 '

it presents a saddle-shaped articular surface

for the head of the malleus, with which it forms a synovial joint. The short process is directed backwards, is tipped with cartilage, and articulates with the fossa incudis on the posterior wall of the tympanum. The long process is directed downwards and medially, behind

Facet

for Malleus

/ Short Process

/

Long Process

'*■' Lentiform Nodule for Head of Stapes

THE EAR

1679

and parallel to the handle of the malleus. Its lower extremity is bent inwards, and becomes narrowed into a neck, upon which is placed a disc-like knob of bone, called the os orbiculare, which is covered by cartilage for articulation with the head of the stapes. In early life, and up to the sixth month of intra-uterine life, this process forms a separate ossicle.

The stapes resembles a stirrup. It has a head, neck, two crura, and a foot-piece or base. The head is directed laterally, is concave and covered by cartilage, and articulates with the processus lenticularis of the incus. The neck is the constricted part which lies immediately internal to the head. Posteriorly it gives insertion to the stapedius muscle. The crura are anterior and posterior respectively, and spring from the neck. They diverge as they pass inwards, and are attached to the foot-piece near its extremities. The anterior crus is straighter and shorter than the

posterior. The foot-piece or base is some- Malleus what oval, is directed medially, and occupies incus,, the fenestra ovalis, which it almost completely fills; its circumference is covered by cartilage, being attached to the margins of the fenestra by annular ligamentous fibres. Stapes- The arch formed by the crura and foot-piece _ ~

is occupied by a delicate membrane, which tympanic Os is attached to a slight groove on the inner sicles in Position. aspect of the arch.

Development of the Tympanic Ossicles— The malleus and incus are usually regarded as being developed from the proximal end of Meckel’s cartilage, which forms the cartilaginous bar of the first or mandibular arch. According to some authorities, however, the incus is developed from the hyoid bar. The stapes is developed from the dorsal part of the hyoid bar of the second visceral arch.

Ligaments of the Ossicles.—The synovial joints between the malleus and incus and between the incus and stapes are provided with thin capsular ligaments. The ligaments which connect the ossicles to the walls of the tympanic cavity are five in number, three of them belonging to the malleus, one to the incus, and one to the stapes.

The ligaments of the malleus are anterior, lateral, posterior, and superior. The anterior ligament is arranged as a fibrous band which extends from the root of the long process to the petro-tympanic fissure, through which it passes to be attached to the spine of the sphenoid bone. The lateral ligament (or malleolar fold) extends from the short process to the anterior extremity of the tympanic notch. The posterior ligament extends from the short process to the posterior extremity of the notch. The superior ligament extends from the head of the malleus to the roof of the attic or epitympanic recess.

The ligament of the incus connects the short process, near its posterior extremity, to the fossa incudis.

i68o

A MANUAL OF ANATOMY

The annular ligament of the stapes connects the circumference of the foot-piece, which is covered by cartilage, to the margin of the fenestra vestibuli, which is also covered by cartilage.

Muscles of the tympanum are the tensor tympani and the stapedius.

Tensor Tympani— Origin .—(i) The cartilaginous part of the pharyngo-tympanic tube; (2) the apex of the petrous part of the temporal bone; and (3) the wall of the osseous canal through which the muscle passes.

Insertion, —The medial aspect of the handle of the malleus close to its root.

Nerve-supply.—A branch from the otic ganglion, and through it from the internal pterygoid branch of the mandibular division of the fifth cranial nerve.

The muscle consists of a fleshy belly, about inch long, and a delicate tendon. In passing backwards to the tympanum it lies in a canal, the entrance to which is situated within the petro-squamous angle of the temporal bone. The canal is placed above the osseous part of the tube, from which it is separated by the processus cochlearifor mis. On entering the tympanum the tendon of the muscle bends sharply over the edge of the processus cochleariformis, and then passes laterally to reach its insertion. The tendon forms very nearly a right angle with the fleshly belly.

Action. —To render tense the membrana tympani by drawing inwards the handle of the malleus, and along with it the membrane.

Stapedius Origin. —The wall of the canal within the pyramid, and of the continuation of this canal in front of the descending part of the facial canal.

The tendon emerges from the canal within the pyramid through a small orifice on the apex.

Insertion. —The posterior aspect of the neck of the stapes.

Nerve-supply. —The facial nerve.

Action. To draw the head of the stapes backwards. The result is that the front part of the foot-piece of the stapes is tilted away

from the vestibule, and its back part is pressed inwards towards the vestibule.

Movements of the Ossicles.—The malleus and incus both act as levers of the first kind, the fulcra of which are represented by an axis passing backwards from the slender process of the malleus. When a sound-wave presses the tympanic membrane inward, the handle of the malleus travels inward with it, and the head of the malleus, or short arm of the lever, moves outward. The upper part, or short arm of the incus lever, must move out too, since it is attached to the head of the malleus, and the long process moves inward, thus pressing the

stapes into the fenestra ovalis and compressing the perilymph in the cochlea.

Ihe secondary membrane of the tympanum, stretched across the fenestra rotunda, is bulged outward by the perilymph, thus allowing vibrations to travel through that fluid.

/

THE EAR

1681

Mucous Membrane of the Tympanum.—The tympanic mucous membrane is continuous anteriorly with that of the naso-pharynx through the pharyngo-tympanic (or Eustachian) tube. Posteriorly it is prolonged into the mastoid antrum, and thence into the mastoid cells. It forms the internal layer of the tympanic membrane, and the external layer of the secondary membrane of the tympanum. It also furnishes sheaths for the tendons of the tensor tympani and stepedius muscles, and for the chorda tympani nerve. Two folds extend downwards from the roof of the attic or epitympanic recess, one in front of and the other behind the superior ligament of the malleus. The former is connected with the head of the malleus, and the latter (sometimes described as the superior ligament of the incus) with the incus.

Attic or Epitympanic Recess and its Pouches.—The part of the tympanic cavity which lies above the level of the upper margin of the tympanic membrane is called the attic or epitympanic recess, as distinguished from the atrium or tympanum proper. It contains the head and neck of the malleus, and the body and short process of the incus. These divide it incompletely into two compartments—outer and inner. The outer attic is subdivided into two pouches—superior and inferior. The superior pouch is partially separated from the inner attic by the two mucous folds which have been already referred to as descending from the roof of the attic in front of and behind the superior ligament of the malleus. The inferior pouch of the outer attic is known as the pouch of Prussak. It is bounded laterally by the membrana flaccida, superiorly by the lateral ligament of the malleus, which partially separates it from the superior pouch, and internally by the neck of the malleus. The pouch communicates posteriorly with the tympanic cavity by an opening which is situated a little above the level of the bottom of the pouch. If fluid, therefore, should accumulate in Prussak’s pouch, it may readily lead to perforation of the membrana flaccida.

Two other pouches are present—namely, the anterior and posterior recesses or pouches of Troltsch. These lie one in front of and the other behind the handle of the malleus, and are produced by the fold of mucous membrane which invests the chorda tympani nerve.

The tympanic mucous membrane is covered for the most part by columnar ciliated epithelium, except over the ossicles and membrana tympani, where the epithelium consists of a single layer of squamous, non-ciliated cells.

The tympanic or mastoid antrum is supplementary to the tympanum proper, or cavum tympani, behind which it is situated. It communicates by a large irregular opening with the attic, and is lined with mucous membrane, which is continuous with that of the attic and cavum tympani. Opening from the antrum there are the mastoid cells, which are lined with mucous membrane, continuous with that of the antrum.

The average measurements of the antrum are as follows: vertical, about Q millimetres; antero-posterior, about n millimetres; and

106

1682

A MANUAL OF ANATOMY

transverse, about 8 millimetres. The roof is formed by the thin tegmen tympani, which enters into the formation of the middle fossa of the base of the skull, and is consequently related to the temporal lobe of the cerebrum and its meninges. The genu and descending limb of the sigmoid sinus lie behind the antrum, and a little farther back is the cerebullum. The facial nerve, as it traverses the descending part of its canal, lies in the posterior wall of the cavum tympani, close to the medial wall and in front of the mastoid antrum. This part of the nerve is on a plane anterior to the mastoid process, and is nearly flush with the opening of the antrum. The ampulla of the lateral semicircular canal of the internal ear gives rise to a slight eminence on the medial wall at its anterior part.

The lateral wall corresponds on the surface with the area of the suprameatal triangle of Macewen, and is formed by the postmeatal plate of the squamous portion of the temporal bone. For a description of this triangle, which is the region selected for mastoidectomy , and also for the mastoid cells, see the description of the temporal bone.

Summary of Important Structures closely related to the Mastoid Antrum.

1. Temporal lobe of cerebrum and its meninges (roof).

2. Genu and descending limb of sigmoid sinus, and farther back the cerebellum (posterior wall).

3 - Facial nerve (posterior wall of cavum tympani, close to medial wall and nearly flush with antral opening).

4 - Lateral semicircular canal (anterior part of medial wall).

The upper part of the antrum communicates, as stated, with the attic of the cavum tympani, but the lower part is shut off by bone from the cavity of the cavum tympani.

The mucous membrane of the mastoid antrum is continuous with that which lines the mastoid cells. Anteriorly it is also continuous with the mucous membrane of the attic and cavum tympani or tympanum proper. The mucous membrane of the cavum tympani is continuous anteriorly with that of the pharyngo-tympanic tube, and the mucous membrane of the tube is continuous with that of the naso-pharynx. This extensive and continuous tract of mucous membrane is covered by columnar ciliated epithelium except in the following regions: (1) the promontory; (2) the tympanic ossicles; (3) the tympanic membrane; (4) the mastoid antrum; and (5) the mastoid cells. In these regions the epithelium consists of a single layer of squamous, non-ciliated cells.

It is of considerable importance to note that micro-organisms may pass from the naso-pharynx through the tube into the cavum tympani and attic, and thence into the mastoid antrum and mastoid cells. Purulent affections of these regions may therefore readily be caused in this manner. Such affections may subsequently involve (1) the temporal lobe of the cerebrum and its meninges, (2) the genu and descending limb of the sigmoid sinus, and (3) the internal ear or labyrinth.

/

THE EAR

1683

Arteries of the Tympanum. —The principal arteries are: (1) the tympanic branch of the maxillary, and (2) the stylo-mastoid branch of the posterior auricular. The tympanic artery enters through the petro-tympanic fissure, and supplies the membrana tympani and front part of the tympanum. The stylomastoid artery enters the facial canal through the stylo-mastoid foramen, and passes from the descending part of the canal into the tympanum. It supplies the back part of the cavity and the mastoid cells, and it forms, with the tympanic artery, a ring round the circumference of the membrana tympani.

In addition to the foregoing two arteries, the following three arteries enter the tympanic cavity: (1) the petrosal branch of the middle meningeal, which enters from the facial canal, into which it passes through the hiatus; (2) the tympanic branch of the ascending pharyngeal, which accompanies the nerve through the tympanic canaliculus; and (3) the tympanic branch of the internal carotid, which enters by a minute foramen on the posterior wall of the ascending part of the carotid canal in company with a sympathetic twig from the carotid plexus.

The veins of the tympanum pass to the pterygoid plexus, the superior petrosal sinus, the internal jugular vein, and the pharyngeal plexus.

Nerves of the tympanum are described on pp. 1325 and 1400.

Internal Ear.

The internal ear is the essential part of the organ of hearing, and is known as the labyrinth from its remarkable complexity. It consists of two parts—namely, the osseous labyrinth and the membranous labyrinth.

Osseous Labyrinth.

The osseous labyrinth is a cavity situated within the petrous part of the temporal bone, and is divided into three parts—namely, the vestibule, the semicircular canals, and the cochlea. These divisions are lined with a delicate periosteum, between which and the contained membranous labyrinth there is a clear fluid, called the perilymph.

Vestibule.—The vestibule is the central division of the osseous labyrinth. The semicircular canals lie behind it, and the cochlea is situated in front of it. In the lateral wall is the fenestra vestibuii, which is occupied by the foot-piece of the stapes and its annular ligament. The medial wall has anteriorly a depression called the fovea spherica or spherical recess, which corresponds to the lamina cribrosa at the deep end of the meatus auditorius internus. It is pierced by apertures for the passage of filaments of the auditory nerve to the saccule. Behind and above the fovea spherica there is a ridge, called the vestibular crest, which lies obliquely. Posteriorly it bifurcates, and between its two divisions there is a small depression, called the cochlear recess, in which are openings for nerve filaments to the canalis cochlese. Anteriorly it becomes somewhat triangular, and forms a pyramid, which is pierced by nerves to the utricle.

The roof of the vestibule, behind and above the crista vestibuii, has an oval depression, called the elliptical recess, which encroaches on the inner wall and lodges the recess of the utricle. It is pierced by nerves to the ampulke of the superior and external semicircular canals. Below the fovea elliptica is the opening of the aqueduct of the vesti

1684

A MANUAL OF ANATOMY

bule, which leads to the posterior surface of the petrous part of the temporal bone about J inch external to the orifice of the meatus auditorius internus. It transmits the ductus endolymphaticus and a minute vein.

Anteriorly the vestibule communicates with the scala vestibuli of the cochlea, and posteriorly are the five openings of the semicircular canals.

Semicircular Canals.—The osseous semicircular canals are situated behind the vestibule. They are three in number—superior, posterior,

Cupola of Cochlea Petrous part of Tern- 5

poral Bone

Facial Canal A

Superior Semicircular Canal

Fenestra Vestibuli

Lateral Semicircular Canal

Posterior Semicircular Canal

Vestibule

Fenestra Cochleae

_ Superior Semicircular Canal

Inner Wall of Canal ' of Cochlea

Helicotrema

Lamina Spiralis

Crus Commune of Sup. and Post. SemicircularCanals

Posterior Semicircular Canal

Lateral Semicircular Canal Elliptical Recess \ Crista Vestibuli Spherical Recess

1 \ .

1 '

> Orifice of Aqueductus Vestibuli Recessus Cochlearis

Fenestra Cochleae Scala T.ympani Scala Vestibuli

Fig. 1035. —The Osseous Labyrinth of the Left Side (Lateral View). A, entire; B opened (Hirschfeld and Leveille; B, modified).

and lateral—and they open into the vestibule by five circular apertures, the contiguous ends of the superior and posterior canals having a common orifice. Each canal forms about two-thirds of a circle, and each presents at one end an enlargement, called the ampulla. The superior semicircular canal occupies a vertical position, and lies transversely as regards the long axis of the petrous part of the temporal bone, giving rise to the eminentia arcuata on its superior surface. Its ampullary end (antero-external) opens independently into the upper part of the vestibule above the ampullary orifice of the external canal.

THE EAR

1685

Its non-ampullary end unites with the non-ampullary end of the posterior canal to form the crus commune, and the two open by a common orifice into the vestibule. The posterior semicircular canal arches backwards towards the posterior surface of the pars petrosa, with which it is almost parallel, and, like the superior, it occupies a vertical position. Its ampullary end (inferior) opens independently into the lower and back part of the vestibule, and its non-ampullary end, as just stated, joins that of the superior canal. The lateral semicircular canal arches outwards, and occupies a horizontal position. Its extremities are independent of those of the other two canals, and they open by separate apertures into the upper and back part of the vestibule. Its ampullary end is in front.

Cochlea.—The osseous cochlea is situated in front of the vestibule. It consists of a tube coiled spirally upon itself, like a snail s shell,

Fig. 1036. _Median Section of the Left Osseous Cochlea of Man

from Apex to Base (Arnold).

S.V. Scala Vestibuli S.T. Scala Tympani

L.S.O. Lamina Spiralis Ossea C.C. Central Canal of Modiolus

M.A.I. Internal Auditory Meatus

and is conical. Its base is opposite the lamina cribrosa at the deep end of the meatus auditorius interims; and its apex, known as the cupola is directed outwards and slightly forwards towards the canal which contains the tensor tympani muscle. Its length from base to apex is about i inch. It consists of (1) a winding tube, called the spiral canal of the cochlea ; (2) a central pillar, called the modiolus, round which the spiral canal turns; and (3) a thin plate of bone, called the osseous spiral lamina, which winds spirally round the modiolus and projects into the spiral canal of the cochlea. . .

The spiral canal of the cochlea (cochlear canal or tube) winds round the modiolus which forms its inner wall. It describes two and threenuarter turns, and its basal turn or coil gives rise to the promontory on the inner wall of the tympanum. At the cupola it ends in a blind extremity. It gradually diminishes in size from base to cupola; its

i686

A MANUAL OF ANATOMY

length is about 32 millimetres; and its diameter is about 2 millimetres at the base, where it is greatest.

The modiolus is the central pillar round which the spiral canal of the cochlea turns, and it forms the inner wall of that canal. It commences at the cochlear area of the lamina cribosa at the deep end of the internal auditory meatus, and extends almost to the cupola, gradually tapering. It is traversed by minute canals for branches of the cochlear division of the auditory nerve. One of these canals occupies the centre of the modiolus, and is called the central canal of the modiolus. This canal begins at the foramen centrale of the cochlear area of the lamina cribrosa, and it transmits the nerve-filaments for the apical coil. The other canals, which have no special name, commence at the tractus spiralis foraminosus of the cochlear area of the lamina cribrosa, and they transmit the nerve-filaments for the other coils— middle and basal. At successive levels these canals change their direction, and pass outwards to the attached margin of the lamina spiralis, to be presently described. Here they coalesce and form a winding canal, called the spiral canal of the modiolus, which lodges the spiral ganglion or ganglion of Corti. From this canal secondary canals for nerve-filaments pass into the lamina spiralis.

The osseous spiral lamina is a thin plate of bone, which winds spirally round the modiolus, to which it is attached. It projects from the modiolus into the spiral canal of the cochlea throughout the windings of the latter, and it extends for about half-way towards the outer wall of the cochlear canal. It divides that canal incompletely into two passages or scalar —an upper or scala vestibuli, and a lower or scala tympani, the commencement of which is at the fenestra cochleae. Close to the cupola the lamina spiralis terminates in a hook-like process, called the hamulus. The spiral lamina consists of two plates of bone, between which there are canals for nerve-filaments, these canals being offsets of the spiral canal of the modiolus, which, as has been said, contains the spiral ganglion or ganglion of Corti. They extend to the free margin of the spiral lamina. The free margin of the lamina spiralis is, in the recent state, attached to the outer wall of the spiral canal of the cochlea by means of the basilar membrane or basilar lamina, and the scala vestibuli and scala tympani are now completely separated, except in the region of the hamulus, where they communicate through an opening, called the helicotrema.

The lamina cribrosa, at the deep end of the internal auditory meatus, will be found described in connection with the temporal bone (p. 190).

At the lower end of the scala tympani is the upper opening of the aqueductus cochleae, which passes downwards and medially to the posterior border of the petrous part of the temporal bone. It transmits a small vein to the inferior petrosal sinus, and establishes a communication between the scala tympani and the subarachnoid space.

THE EAR

1687

Membranous Labyrinth.

The membranous labyrinth is situated within the osseous labyrinth, and its constituent parts receive the terminal branches of the auditory nerve. It is separated from the periosteal lining of the osseous labyrinth by the perilymph, and it contains the fluid known as the endolymph. In the case of the vestibule and the osseous semicircular canals the. membranous labyrinth corresponds more or less with them; but in the case of the osseous cochlea it forms part of the septum between the scala tympani and scala vestibuli, and contains a passage called the membranous canal of the cochlea (ductus cochleae).

Fig. 1037. —Diagram of Membranous Labyrinth.

Vestibular Part of the Membranous Labyrinth.—The vestibule contains two membranous sacs—namely, the utricle and the saccule

_which are in close contact, but do not communicate with each other

directly. These sacs contain endolymph.

The utricle is the larger of the two sacs, and into it the membranous semicircular ducts open. It occupies the upper and back part of the vestibule, a portion of it, known as the recessus utriculi, lying in the fovea elliptica. Near the crista vestibuli the wall of this recess receives fibres of the auditory nerve and is thickened, this part of it being called the macula utriculi. From the anterior and medial part of the utricle a minute canal, called the ductus utriculi («ductus utriculo-saccularis ), passes to join the ductus sacculi, and so form the ductus endolymphaticus (see Fig. 1037).

1688

A MANUAL OF ANATOMY

The saccule, which is somewhat oval, lies in front of the utricle, and occupies the fovea spherica, where it is near the opening leading into the scala vestibuli of the cochlea. Through the openings of the fovea spherica it receives filaments of the auditory nerve, and this portion of the saccule, being thickened, is known as the macula sacculi. Interiorly the saccule is connected with a small canal, called the ductus reuniens, which opens into the canal of the cochlea, or ductus cochlearis, not far from its closed vestibular end. From the posterior part of the saccule a minute canal, called the ductus sacculi, passes off, which is soon joined by the ductus utriculi, and so the ductus endolymphaticus is formed. This latter duct traverses the aqueductus vestibuli, and, having reached the posterior surface of the petrous part of the temporal bone, it ends in a small blind dilatation, called the saccus endolymphaticus , which lies beneath the dura mater. The saccule and utricle are thus indirectly connected by means of the ductus sacculi and ductus utriculi; and the saccule communicates with the ductus cochlearis by means of the ductus reuniens.

Semicircular Ducts.—The membranous semicircular ducts correspond in outline to the osseous semicircular canals, within which they lie; they form about two-thirds of a circle, and each has an ampulla at one end, which is situated within the ampulla of the osseous canal. They are elliptical in transverse section, and open into the utricle by five orifices, the non-ampullated ends of the superior and posterior canals being united, so that these two open by a common orifice forming the crus commune. The convex wall of each canal is attached to the periosteal lining of the osseous canal, whilst the concave wall is practically free from the osseous wall, and is bathed by the perilymph. These canals contain endolymph.

Structure.—The walls of the utricle, saccule, and membranous semicircular canals consist of three layers: an outer or fibrous layer, which is vascular; a middle layer, or membrana propria, which is translucent; and an inner or epithelial layer. In each ampulla the middle layer, or membrana propria, projects into the cavity of the canal from the peripheral wall, this projecting part being known as the septum transversum. It partially divides the interior of the ampulla into two compartments, and its free margin, which is covered by the auditory epithelium, is called the crista acustica or ampullaris. The epithelial layer consists of a single stratum of squamous cells, except in those regions to which the filaments of the auditory nerve are distributed. These regions are as follows: (i) the macula (acustica) utriculi; (2) the macula (acustica) sacculi; and (3) the crista of each ampulla.

The macula utriculi is the thickened part of the antero-inferior wall of the recessus utriculi, and is lined with auditory epithelium. The macula is covered by calcareous particles, called otoconia, which consist of crystals of calcium carbonate. The macula sacculi is the thickened part of the anterior wall of the saccule, and is also lined with auditory epithelium covered by otoconia. The crista ampullaris, as

THE EAR

1689

we have seen, is the free margin of the septum trailsversum in each ampulla, and is covered by auditory epithelium.

The auditory epithelium is of the columnar variety, and consists of two kinds of cells, auditory and sustentacular. The auditory cells are nucleated, and each is provided at its free extremity with a slender, tapering, hair-like filament, which projects into the cavity. These filaments are sometimes spoken of collectively as auditory hairs, and the cells are hence called hair-cells. Their deep extremities fall short of the membrana propria. The sustentacular cells lie between the haircells, and are elongated and nucleated. Their deep extremities are attached to the membrana propria, and their free extremities give rise to a kind of limiting membrane. The auditory nerve-fibres pierce the membrana propria, and, having lost their medullary sheaths, the axons end in arborizations round the deep ends of the auditory or hair cells.

Superior Semicircular Duct

Lateral Semicircular Duct i

Posterior Semicircular Duct

Facial Nerve

Crus Commune of Superior and ' l Posterior Semicircular Ducts

j r Ampulla

_ l Nerve to Ampulla Nerve to Utricle ~ . Nerve to Saccule _ Cochlear Nerve

Fig. 1038. —The Membranous Semicircular Ducts, showing the Distribution of the Branches of the Auditory Nerve to their Ampullae

(Breschet).

Membranous Cochlea.—The membranous cochlea is situated within the osseous cochlea, and fills the gap which is left by the lamina spiralis. It consists of two membranes, the basilar membrane and the vestibular membrane (membrane of Reissner, Fig. 1040), which enclose between them the ductus cochlearis, or scala media. 1 he osseous cochlea in the recent state is therefore divided into three spiral passages—the scala tympani, the scala vestibuli, and the ductus cochlearis. The scala vestibuli is continuous with the scala tympani at the cupola through an aperture, called the helicotrema ; and at the base of the cochlea it opens upon the anterior wall of the vestibule. The scala tympani begins at the fenestra cochleae, and in the recent state is separated from the tympanic cavity by the secondary membrane of the tympanum. The scala media, or ductus cochlearis, communicates near its lower end with the saccule by means of the ductus reuniens. The scala vestibuli and scala tympani contain perilymph, which is continuous with the perilymph of the vestibule and osseous semicircular canals. The scala media contains endolymph, which is continuous with that of the saccule,

1690

A MANUAL OF ANATOMY

Basilar Membrane.—The basilar membrane extends from the free margin of the lamina spiralis to the crista basilaris, or lower part of the spiral ligament, a thickening of the periosteum of that part of the outer wall of the cochlea which forms the outer wall of the scala media, or ductus cochlearis. It separates the ductus cochlearis from the scala tympani, and is divisible into two zones, inner and outer. The inner is called the zona arcuata, and supports the spiral organ. The outer is known as the zona pectinata, and extends from the foot-plates of the outer rods of this organ to the crista basilaris. The basilar membrane consists of a homogeneous membrana propria, with fibres embedded in it, the fibres being most numerous in the zona pectinata.

Vestibular Membrane, or Membrane of Reissner.—This is a delicate membrane which extends from the upper surface of the lamina spiralis a short distance from its free margin to the outer wall of the cochlea, where it is attached to the periosteum a little above the outer attachment of the basilar membrane. It separates the cochlear duct from the scala vestibuli, and consists of very delicate connective tissue lined on each side with a single layer of squamous epithelium.

Osseous Spiral Lamina

Vestibular Membrane Basilar Membiane

Scala Vestibuli

Ductus Cochlearis

Sc' NYll ESy/: ,'-.;

| Scala Tympan*

Fig. 1039.

-Vertical Section of the Cochlea of a Fcetal Calf, showing THE SCALAE AND MODIOLUS (KoLLIKER).

The scala media, or ductus cochlearis, is situated between the basilar membrane and the vestibular membrane. It is triangular in transverse section, and has a roof, an outer wall, and a floor. The roof is formed by the vestibular membrane (see Fig. 1040). The outer wall is the wall of the cochlea and its periosteum, between the external attachments of the basilar membrane and the vestibular membrane. The periosteum in this region is much thickened, and forms the spiral ligament of the cochlea, the lower part of which gives rise to the crista basilaris. The floor is formed by the basilar membrane, and a part of the upper surface of the lamina spiralis. It has been seen that the basilar membrane separates the ductus cochlearis from the scala tympani. The ductus ends above in a closed extremity at the cupola, and it has a similar ending at the base of the cochlea. Near its lower blind extremity it receives the ductus reuniens, by which it communicates with the saccule.

It has just been shown that part of the floor of the cochlear duct is formed by some of the lamina spiralis. In the recent state it is of some thickness, which is due to a thickening of its periosteal

/

THE EAR

1691

covering. This fibrous thickening forms the limbus laminae spiralis (see Fig. 1040).

Its outer margin is crescentic, the deep notch being called the sulcus lamince spiralis. The sulcus has two lips, upper and lower. The upper is called the labium vestibulare, the upper surface of which is marked by several interlacing prominences and grooves. At the free margin of this labium the prominences assume the form of tooth-like projections, which are known as the auditory teeth. The lower lip of the sulcus is called the labium tympanicum. It is continued into the basilar membrane, and is perforated by a great number of apertures for the branches of the cochlear division of the auditory nerve.

Spiral Organ (of Corti).—Over the upper surface of the inner part (zona arcuata) of the basilar membrane the epithelium undergoes

Fig. 1040.—The Organ of Corti (Wiedersheim, after Lavdowsky).

1. Spiral Ligament

2. Limbus

3. Sulcus Spiralis

4. Inner Rod of Corti

5. Outer Rod of Corti

6. Tunnel of Corti

7. Phalangeal Process of Outer Rod

8. Inner Hair-cells

9. Outer Hair-cells

10. Cells of Deiters

11. Lamina Reticularis

12. Cells of Hensen

13. Cells o Claudius

14. Spiral Ganglion

15. Cochlear Nerve

16. Nerve-fibres to Hair-cells

remarkable modification, and gives rise to the spiral organ or organ of Corti. The constituent parts of this very complicated organ are as

follows:

1 The rods of Corti. 4. The cells of Hensen and of Claudius.

2 The auditory or hair cells. 5 - The lamina reticularis.

3! The cells of Deiters. 6. The membrana tectona.

The rods of Corti are arranged in two rows, inner and outer (see Fig 1041). Each rod consists of a foot-plate or base, an intermediate portion, and a head. The foot-plate, which is expanded, rests upon the zona arcuata of the basilar membrane, and the foot-plates of the inner rods are separated from those of the outer rods by a slight interval. As the rods rise the intermediate portions of the inner and

1692

A MANUAL OF ANATOMY

outer rods incline towards each other, and the heads of the two sets of rods come into contact. In this manner a triangular tunnel is enclosed between the two sets of rods and the basilar membrane, which is called the tunnel of Corti. This extends along the entire length of the ductus cochlearis.

phalangeal

PROCESS

IfSNER ROD

BASILAR. MEMBRANE

The inner rods are more numerous than the outer, there being from 5,000 to 6,000 of the former and about 4,000 of the latter. They incline upwards and outwards. The head of each has a concavity on its outer side, above and below which there is a projecting portion, so

that it resembles the upper extremity of the ulna, with its great sigmoid cavity and olecranon and coronoid processes. The concavity on the head of the inner rod receives the round head of the outer rod. Finally, the tic. 1041. A Pair of Rods of inner rods are shorter than the outer. Corti from the Rabbit’s Coch- The ou + pr rn d<? lpcc nnmprrmQ

lea (Side View, highly mag- An ? ouler * oas j ^ re . Aess nui ? e 5 , 0US

nified) (Schafer, in Quain’s ? nd lon § er than the inner, and they

‘Anatomy’). ~ incline upwards and inwards. The

head of each is divisible into two parts—inner and outer. Ihe inner part is round, and is received mto the concavity on the outer aspect of the head of the inner rod. The outer part is prolonged into a beak-like projection, called the phalangeal process, which forms part of the lamina reticularis, to be presently described.

The auditory or hair cells are arranged in two sets—inner and outer. Ihe inner hair-cells lie internal to the row of inner rods, and form a single row. They are from 3,000 to in number. Their

free extremities, which lie close to the heads of the inner rods, are each provided with a tuft of short, hair-like filaments. The deep, nucleated ends of the cells are related to the terminal arborizations of nervefibres. Internal to the row of inner hair-cells there are two or more rows of columnar cells, which are continuous with the columnar epithelium of the sulcus spiralis laminae. The outer hair-cells are disposed in three or four rows external to the outer row of rods. They are much more numerous than the inner hair-cells. Their free extremities, like those of the inner cells, are each provided with a tuft of short, hair-like filaments, and their deep, nucleated ends are related to the terminal arborizations of nerve-fibres.

1 he cells of Deiters, which are sustentacular, are situated between the rows of outer hair-cells. Each cell is nucleated and contains a slender filament, known as the sustentacular filament or phalangeal process. This filament is attached by its base to the basilar membrane, and is prolonged into the tapering upper end of the cell. It terminates in an expansion, which forms a phalanx of the lamina reticularis, to be presently described.

The cells of Hensen are disposed as a continuous layer external to the lamina reticularis.

THE EAR 1693

External to the cells of Hensen there are the cubical or columnar cells of Claudius: these are merely an epithelial lining layer.

The reticular lamina extends between the heads of the rods of Corti and the cells of Hensen. It consists of phalanges, which are arranged in two (or more) rows—inner and outer. The phalanges of the inner row are formed by the phalangeal processes of the heads of the outer rods of Corti. The phalanges of the outer row (or rows) are formed by the phalangeal processes of the cells of Deiters. Between the phalanges there are openings through which the outer ends of the outer hair-cells, with their crescentic tufts of hair-like filaments, project.

The membrana tectoria, or membrane of Corti, which is elastic, is the most superficial structure in connection with the spiral organ. It extends from the limbus spiralis near, and external to, the attachment of the vestibular membrane to the region of the outer hair-cells. It covers (1) the limbus laminae spiralis; (2) the labium vestibulare,

- Hehcotrema

Scala Vestibuli

Membranous Spiral Lamina

Scala Tympani

1

1

Expansion of Cochlear Nerve

Fig. 1042.—Section of the Cochlea, showing the Distribution of the Cochlea Branch of the Auditory Nerve (magnified) (Hirschfeld and Reveille).

and the auditory teeth; (3) the sulcus laminae spiralis; (4) the inner hair-cells; (5) the inner and outer rods of Corti; (6) the cells of Deiters; and (7) the lamina reticularis.

Auditory Nerve.—The auditory nerve, within the meatus auditorius internus, breaks up into two divisions—vestibular and cochlear.

The vestibular nerve, as it traverses the meatus auditorius internus, has a gangliform enlargement, the vestibular ganglion, and divides into three branches. These enter the vestibule through the foramina in the superior vestibular area of the lamina cribrosa at the deep end of the internal meatus. They are distributed to the macula utriculi and to the cristae acusticas of the ampullae of the superior and external semicircular canals.

The cochlear nerve in the meatus auditorius internus divides into two branches—one to the macula sacculi, and the other to the crista of the ampulla of the posterior semicircular canal. The filaments of the former, which has a gangliform enlargement, pass through the foramina in the inferior vestibular area of the lamina cribrosa, and the

A MANUAL OF ANATOMY

1694

latter passes through the foramen singulare in the lamina cribrosa. The cochlear nerve, having parted with these two branches, breaks up into filaments which pass through the foramina of the cochlear area of the lamina cribrosa, and so reach the base of the modiolus of the cochlea. They traverse the canals of the modiolus, from which they pass into the canals between the two layers of the lamina spiralis. In doing so they have to cross the spiral canal of the modiolus, which is situated close to the attached margin of the lamina spiralis. This canal contains a ganglion, called the spiral ganglion, which follows the windings of the canal, and contains bipolar nerve-cells. As the auditory fibres pass from the canals of the modiolus into those of the lamina spiralis the course of each fibre is probably interrupted by a bipolar cell of the spiral ganglion. The nerve-fibres, leaving these bipolar cells, traverse the canals between the two layers of the lamina spiralis. Having lost their medullary sheaths, they pass through the foramina of the labium tympanicum on the outer margin of the limbus spiralis, and enter the basilar membrane, where they end in arborizations which are connected with the inner and outer hair-cells.

Blood-supply of the Labyrinth.—The labyrinth derives its blood from the internal auditory, which is a branch of the basilar artery or of the anterior inferior cerebellar. The vessel traverses the meatus auditorius internus, and divides at its deep end into two branches— vestibular and cochlear. The vestibular artery supplies the utricle, saccule, and semicircular canals, and the cochlear artery supplies the cochlea.

The veins of the labyrinth ultimately join to form one vessel, called the internal auditory vein, which opens into the inferior petrosal sinus. The aqueductus cochleae and the aqueductus vestibuli each transmit a vein; that which passes through the former joins the inferior petrosal sinus or the bulb of the internal jugular vein, and that which passes through the latter opens into the superior petrosal sinus.

Development of the Ear.

Internal Ear—Membranous Labyrinth. —The membranous labyrinth is developed from the surface ectoderm in a manner similar to the development of the crystalline lens. Over a circumscribed area, corresponding to the upper end of the first visceral cleft, and upon the side of the hind-brain, the ectoderm becomes thickened and invaginated. A depression is thus formed, which is called the auditory pit. This pit becomes deepened, its mouth becomes constricted, and its lips, coming together, unite. The auditory pit then becomes transformed into a closed sac, called the auditory or otic vesicle, or otocyst. The auditory vesicle now becomes isolated from the surface ectoderm, and sinks into the adjacent mesoderm, taking up a position close to the side of the hindbrain.

The auditory vesicle, which is at first almost spherical, soon becomes pyriform, this being due to the formation of a process, called the recess of the labyrinth or vestibule, which is prolonged from its dorsal wall. As this process lengthens it gives rise to the ductus endolymphaticus, which occupies the aqueductus vestibuli of the petrous portion of the temporal bone. The expanded terminal part of the ductus endolymphaticus is called the saccus endolymphaticus, and it lies underneath the dura mater. The ventral aspect of the vesicle gives off a tapering diverticulum, which gives rise to the ductus cochlearis, or scala media, of the cochlea. This duct describes a bend, within which lies the cochlear ganglion of the cochlear division of the auditory nerve. As the duct elongates it continues to bend in a spiral manner, and so the ductus cochlearis is formed. The cochlear ganglion elongates, and follows the spiral turns of the duct, from which circumstance the ganglion is known as the ganglion spirale.

The three semicircular ducts are developed from the upper or cephalic part of the auditory vesicle, this part representing the future utricle. This portion of the vesicle presents two folds—vertical and horizontal. From the vertical fold the superior and posterior semicircular canals are formed, whilst the horizontal fold gives rise to the lateral semicircular canal.

The auditory vesicle now becomes transversely constricted, and is divided into two sacs—large and small. The large sac is called the utricle, from which the semicircular ducts have just been developed. The small sac is called the saccule, from which the ductus cochlearis has been developed. The portion of this duct which communicates with the saccule becomes constricted, and forms the canalis reuniens of Hensen.

The constriction which completely divides the auditory vesicle into utricle and saccule also involves the vesicular end of the ductus endolymphaticus, and divides it into two ducts, called the ductus utriculi and ductus sacculi. This arrangement constitutes the only communication which now exists between the utricle and the saccule.

As stated, the cochlear ganglion, or ganglion spirale, of the cochlear division of the auditory nerve lies within the spiral turns of the ductus cochlearis. As the osseous cochlea becomes formed, the ganglion spirale comes to occupy the spiral canal of the modiolus. The vestibular ganglion of the vestibular division of the auditory nerve lies in the internal auditory meatus after the completion of ossification.

The membranous labyrinth, hitherto considered, is entirely epithelial. Certain of its epithelial cells undergo important specializations to fit them for sensorial purposes. These cells form six groups in definite regions. These groups are as follows: (1) The crista acustica ampullaris, of which there are three, one in the ampulla of each of the three epithelial semicircular canals; (4) the macula (1 acustica) utriculi’, (5) the macula ( acustica) sacculi', and (6) the spiral organ {of Corti), specialized from the epithelial ductus cochlearis. The groups connected with the ampullae of the semicircular canals, the utricle, and the saccule, receive their nerve-fibres from the vestibular ganglion of the vestibular division of the auditory nerve. The organ of Corti receives its nerve-fibres from the cochlear ganglion, or ganglion spirale, of the cochlear division of the auditory nerve.

Osseous Labyrinth. —The membranous labyrinth is surrounded by mesodermic tissue, which becomes disposed in four layers. These layers, from within outwards, may be spoken of as (1) the connective-tissue layer, (2) the gelatinous layer, (3) the perichondrial layer, and (4) the cartilaginous layer. The connectivetissue layer forms the connective tissue of the various parts of the epithelial labyrinth. The fibrous tissue of the gelatinous layer acquires vacuoles, and gives place to the various perilymphatic spaces, which contain a fluid called the perilymph. In the case of the cochlea, the scala vestibuli and scala tympani alone contain perilymph, the cochlear duct containing endolymph. The perichondrial layer forms the lining perichondrium of the periotic cartilaginous capsule, and subsequently becomes the lining periosteum of the osseous labyrinth. The cartilaginous layer forms the periotic cartilaginous capsule, which undergoes ossification, and gives rise, amongst other parts, to the osseous labyrinth, which jg q£ i ar ger size than the membranous labyrinth. The osseous semicircular canals conform in shape to the membranous semicircular ducts, but are of larger size. The osseous vestibule differs from the membranous vestibule in being a single osseous case, whereas its contents are the utricle and the saccule.

Whilst the perilymph lies external to the membranous labyrinth, the interior of the semicircular ducts, utricle, saccule, and scala media contain endolymph, the latter being in communication with the saccule by means of the canal is reuniens.

The middle ear or tympanum and pharyngo-tympanic tube (see p. 73 et seq.)

are developed from the tubo-tympartic recess of the pharynx. This is the widest part of the early pharynx, opposite the second and third arches, and by the time the embryo has reached the second month of development the recess is definitely indicated; it projects laterally with its contained cavity, compressed dorso-ventrally, widely open into the general pharyngeal cavity. Its roof supports the otic capsule, while the outer parts of the first two visceral grooves are seen in its floor, with the second arch between them. It is bounded in front by the first arch, and caudally by the third arch. The first lateral pouch, on its lateral edge, is in contact with the persistent upper end of the first outer groove; the second pouch, which has lost contact, is at its postero-lateral angle. The recess deepens, and the third arch grows forward, cutting off the outer part of the cavity from the pharynx and narrowing the anterior part, which still remains continuous with the pharynx; the outer and larger part is the early tympanum, and the constricted front portion becomes the tube.

The otic capsule enlarges and chondrifies, and as it grows depresses the inner part of the tympanum, slightly rotating it, so that its former roof now becomes its inner wall, applied to the surface of the capsule. Its floor, also rotated, now slopes downwards and medially, and is in close relation with the meatal plate, a cellular ingrowth from the upper end of the first outer groove; this has grown in below the tympanum, and by a later process of hollowing forms the ectodermal lining of the outer meatus and membrane.

Meckel’s cartilage develops in the mesoderm of the first arch, therefore antero-lateral to the recess, in front of the position of the first pouch. It sends an extension over the pouch to invade the second arch area behind this, passing under the floor between the endoderm and the meatal plate; this extension forms the basis of the tympanic membrane and the manubrium, and its upper part probably also forms the incus. The hyoid bar (Reichert’s cartilage), the bar of the second arch, is behind this, immediately in front of the second pouch; an exten sion from its upper end over the roof gives rise to the stapes. The stapes becomes associated with the cartilaginous capsule, which has enlarged considerably; the enlargement extends postero-laterally, and leads to the fusion of Reichert’s bar with the capsule (tympano-hyal) and the position of the remnant of the second pouch in the fossula rotunda (fenestra cochleae).

The chorda tympani crosses the first pouch; the level of the early tympanum might roughly be taken as extending up to this nerve. The higher level of the adult cavity is attained by a later extension. In the early condition of the tympanum, the malleus, incus, stapes, and chorda tympani nerve lie embedded in the mesodermic tissue which intervenes between the epithelial or mucous roof of the membranous tympanum and its osseous roof. This mesodermic tissue disappears, and the mucous (originally epithelial) lining of the tympanum now comes into direct contact with the malleus, incus, stapes, and chorda tympani nerve, all of which it encloses within folds. Though these structures apparently lie within the tympanic cavity, this is not their actual position. They are really outside the cavity, inasmuch as they lie external to the mucous lining of the cavity. This may be illustrated by stating that the handle or manubrium of the malleus and the chorda tympani nerve do not lie in the tympanic cavity, but are placed between the middle, or fibrous, layer and the internal, or mucous, layer of the membrana tympani.

External Ear.— AThe external ear consists of (1) the external auditory meatus, including the membrana tympani, and (2) the pinna.

The external auditory meatus is developed from the upper part of the first external or ectodermic furrow, which corresponds in position to the first internal visceral cleft.

The membrana tympani is developed from the closing membrane which separates the first internal visceral cleft from the first external ectodermic furrow.

This closing membrane is a trilaminar structure. Its inner layer consists of entoderm; its middle layer of mesoderm; and its outer layer of ectoderm. The membrana tympani, which is developed from it, is therefore trilaminar. The outer layer is ectodermic, and is continuous with the cuticular lining of the external auditory meatus; the middle layer is mesodermic, or fibrous; and the inner layer is entodermic, or mucous, and is continuous with the mucous lining of the tympanic cavity.

The component parts of the pinna are developed from six projections, consisting of mesoderm, covered by ectoderm. These appear on the upper ends of the first and second visceral arches, where these bound the upper part of the first external ectodermic furrow, which gives rise to the external auditory meatus. The helix is developed from two of these tubercles, and each of the other four gives rise to the antihelix, tragus, antitragus, and lobule. The mesodermic tissue of the projections becomes differentiated into connective tissue and cartilage, and the ectoderm covers these.