A manual of otology for students and practitioners

Front matter

I

Boo] Copyright^?.

f

Preface

Many years' experience in teaching otology to undergraduates and to postgraduate students who are adopting it as a specialty has made the author conversant with its inherent diflficulties. In the preparation of this manual it has been his aim to lighten these difBculties and to supply data which will enable those who thoroughly master them to become capable aurists. The simpler and less serious affections, which fortunately form the larger part of aural practice, have been discussed in full detail as to their diagnosis and treatment, and the orderly sequence in which these topics are taken up should afford the reader a properly correlated conception of the entire subject.

In describing operative procedures applicable to the middle ear and mastoid, the consideration of many minutiae is essential. Careful adherence to the technic advised, with due attention to the measures indicated to avoid dangers and accidents, will insure the sm^geon's becoming a safe and efficient operator.. The chapter on Suppurative Diseases of the Labyrinth sets forth tTie present knowledge of the graver affections of this complex region. It is

IV PREFACE

hoped that this part of the subject has been presented in so clear and definite a manner as to bring it even within the easy grasp of the beginner.

The author desires to express his thanks to Dr. E. B. Dench, with whom he has been many years associated in aural work. He is also under obligations to Mr. K. K. Bosse for his work on the illustrations, and to the publishers, Messrs. Lea & Febiger, for their care in making the book.

C. E. P.

New York, 1916.

Chapter I. Anatoiviy And Physiology Of The Ear

The ear may be divided into three parts: the external, middle, and internal. The external and middle are concerned in transmitting sonorous vibrations and are therefore called the sound-conducting mechanism. The inner ear consists of the acoustic and static labyrinths. The acoustic labyrinth (the cochlea) with its central connections receives and analyzes sonorous vibrations, and is called the sound-perceiving mechanism; while the static labyrinth (consisting mainly of the semicircular canal system) with its central connections, is concerned in the maintenance of equilibrium.

EXTERNAL EAR.

The external ear is that part of the auditory apparatus which is situated external to the membrana tympani, and consists of the auricle and auditory canal or meatus.

Auricle. — The auricle or pinna is an irregular, ovalshaped appendage, composed of a cartilaginous framework covered with integument. It is attached to the side of the head at an angle so that its posterior surface is directed internally, and its anterior surface is directed externally. Its posterior surface is comparatively smooth, while its anterior surface is irregular and more or less concave, thus acting to collect sound waves and conduct 2 them into the external auditory canal. It presents numerous eminences and depressions. These vary in different subjects, both in size and position, but it is usually not difficult to identify t,hose shown in the cut (Fig. 1), a study of which conveys a very good impression of the name and location of these eminences and depressions.

DARWINIAN TUBERCLE rOSSA

TRIANGULAniS

CRURA or

ANTIHELIX cRus or

HELIX

INCISURA ANTERIOR

TUBERCULUM SUPRATRAGICUM

EXTERNAL AUDITORY MEATUS

INCISURA INTERTRACICA

POSTERIOR

AURICULAR

SULCUS

CAVUM CONCHAE

Fig. 1. — Anterior and external view of the auricle. (Gray.)

The muscles of the ear are rudimentary in man and not of surgical importance. The auricle is attached to the temporal bone by three ligaments — the superior, anterior and posterior — w^hich extend from the cartilaginous framework of this structure to the bone forming the entrance to the external auditory canal.

The blood supply of the auricle is received through the

EXTERNAL EAR * 19

posterior auricular from the external carotid, the anterior auricular and some smaller branches from the temporal.

The nerves are the auricularis magnus from the cervical plexus, the auriculotemporal, T\'hich is a branch of the inferior maxillary nerve and some small branches of the small occipital nerve.

Meatus. — The external auditory canal is about one and a half inches long if measured from the tragus. A little more than the outer half is cartilaginous and membranous, the remaining inner part being osseous. If measured from the bottom of the concha, it is about one inch in length, of which the outer third is cartilaginous. The cartilage extends deeper on the anterior than on the posterior wall and is traversed by two vertical fissures (the fissures of Santorini) through which infective processes or purulent accumulations may extend from the canal to the tissues anterior to the ear. The membranous and osseous portions of the meatus form an obtuse angle with each other, the outer part having a direction upward, backward, and inward; while that of the inner part is downward, forward, and inward. The integument lining the outer part contains the openings of the ceruminous glands, and from it project the hairs of the meatus. These structiues are found only in the cartilaginous, never in the osseous portion of the canal — a point of practical importance in the study of certain diseases affecting the external auditory canal. Slightly internal to the junction of the two parts the canal is usually contracted by the prominence of the anterior bony wall. This constricted portion is called the isthmus. Internal to this the canal usually widens anteriorly, forming a depression which extends to the membrana tympani. This renders it difficult at times to obtain a view of the extreme anterior part of the drum membrane, and also aftords a lodgement for foreign bodies from which their extraction may become difficult. The membrana tympani forms the fundus of the canal and lies obliquely, thus making the canal deeper anteriorly and inferiorly than above and

Cartilage oj /^

Promont.

Int. carat, a

Membrana / •

tympani /

Cartilage of the cxt. ouditory meatus

Fig. 2. — Horizontal section of the external auditory meatus and tympanum. (Gray.)

FENESTRA OVALIS LOSED BY STAPES

Fig. 3.— Vertical section of the external auditory meatus and tympanum.

(Gerrish.)

EXTERNAL EAR posteriorly (Figs. 2 and 3). The osseous canal is formed principally by the outgrowth of the tympanic ring. At birth this ring forms an incomplete circle (Fig. 4). The part between the ends of the ring is called the segment of Rivinus, and is filled in by the squamous portion of the temporal bone. In the development of the bone this grows pari passu with the outgrowth from the tympanic ring and completes the canal above. This takes place

1 for squamous porfion, including zygoma : Sd month.

1 for ti/mpanic plate.

6 for petrous

and mastoid

portions.

2 for styloid process.

Fig. 4. — Tympanic ring. (Gray.)

in extra-uterine life. At birth there is no osseous canal, the tympanic ring, and therefore the drum membrane attached to it, lie practically on the surface. The inclination of the drum head is as great or slightly greater than in adult life, its external surface being directed downward, forward, and outward. The membranous canal is attached directly to the tympanic ring and its direction is upward and outward, the auricle being placed higher on the squama than in the adult.

The nerve supply of the external auditory canal is derived from the auriculotemporal, the auricularis magnus, and the pneumogastric through its auricular branch (Arnold's nerve). It is claimed that the facial supplies sensory and trophic filaments to the external auditory canal and auriqle. This claim is based largely upon pathological investigations which are discussed under Herpes Zoster Oticus.

Tympanic Membrane. — The tympanic membrane forms the fundus of the external canal and part of the external wall of the tympanum and is therefore common to the external and middle ear. It is divided into two parts which are very unlike in structure and appearance. These are the membrana tensa and the membrana flaccida or Shrapnell's membrane. The membrana tensa is that part which is attached to the tymi)unic ring; while Shrapnell's membrane fills in the Hivinian segment, having its peripheral attachment at the internal end of the squamous portion of the canal. The meml)raiui tensa consists of three layers — an outer one of epithelium which is continuous with that lining the meatus; a middle one of fibrous tissue, the fibers of which are arranged in either a radiating or circular manner and enclose the manubrium; while the inner layer is composed of mucous membrane which is continuous with that lining the tympanic cavity. Shrapnell's membrane is much smaller, and is really an extension of the skin of the su])erior canal wall, lined to a greater or less extent on its iimer surface with the tympanic mucous membrane.

MIDDLE EAR.

Tympanum. — The middle ear lies internal to the tympanic membrane. It is an irregularly cuboidal cavity and presents six walls for examination: namely, internal, external, anterior, posterior, inferior or floor, and superior or roof. The cavity lies obliquely so that the internal wall is also partly superior. This must be remembered, but for pjurposes of description is ignored as leading to confusion. The internal wall (Fig. 5) is principally formed of the bony labyrinth and presents tive points for examination, as follows: (1) Superiorly, the prominence of the anterior end of the horizontal semicircular canal. (2) Immediately below this and extending horizontally is a ridge which is the external wall of the tympanic part of the Fallopian canal containing the

JUNCTION BETWEEN MAS TOlO ANTRUM AND EPITYMPANIC RECESS TEGMEN TYMPANI

EPITYMPANIC RECESS

: OF EXTERNAL ) CANAL PROMINENCE OF AQUEDUCT OF FALLOPIUS

TENDON OF STAPEDIUS MUSCLE PLICA STAPEDIUS

PROCESSUS

COCHLEARIFORMIS

^^ TENSOR TYMPANI

MUSCLE (cut through)

WALL OF LABYRINTH

/ PROMINENCE OF / SEMICIRCULAR < / PROMIl / OF FALl

Fig. 5. — Internal tj^mpanic wall. (Gray.)

facial nerve. (3) Immediately below this is the oval window^ at the bottom of a depression or niche. In the recent state the niche is occupied by the stapes, the foot-plate of which, together with its annular ligament, closes in the oval window. The shell of bone between the facial nerve and the oval window is very thin and sometimes absent — a point of surgical importance. (4) Below and behind the oval window, separated from it by a process of bone which is part of the promontory, is the niche of the round window at the bottom of which is the fenestra rotunda, or round window. This is closed in by a thin membrane called the membrana tympani secondaria. (5) The promontory which lies between and anterior to the two fenestrse is of ivory-like bone, and a part of the labyrinthine capsule which forms the first cochlear whorl or turn. Anterior to this promontory the internal wall is receding, and the transverse dikmeter of the tympanum greater. The second cochlear whorl in this location does not form a prominence as the first.

The outer wall is formed by the tympanic membrane, the tympanic ring, and above by the internal end of the superior wall of the external auditory meatus. The anterior wall is formed internally by the bone covering the carotid canal, while externally is the opening of the Eustachian tube, and above this and separated from it by a bony process, the processus cochleariformis, is the canal for the tensor tympani muscle. The posterior wall of the tympanum is absent above on account of the opening into the mastoid antrum (additus ad antrum) » on the floor of which is the fossa incudis in which lies the short process of the incus. Below is the pyramid, a hollow bony process, which contains the stapedius muscle. The tendon of this muscle emerges at the apex of the pyramid and is attached to the posterior crus of the stapes. At times there is a recess leading back into the bone from the posterior wall, called the sinus tympani.

The roof, or tegmen tympani, separates the tympanic cavity from the middle cranial fossa. It is a thin shell of bone and part of the inner table. A few small fragile cells may be found in this situation or the bone may present defects or dehiscences, thus allowing the middle ear to come into still closer relation with the cranial cavity. The tegmen is continuous posteriorly with the roof of the antrum, and anteriorly with the bone covering in the canal for the tensor tympani muscle. The floor or inferior wall of the tympanum is in relation with the jugular bulb. The thickness of the bone in this situation varies. It may be very thin or even absent. If absent, the dome of the jugular bulb may extend into the tympanic cavity. When this condition is present there is danger of wounding the vessel in performing the operation of myringotomy. This has occurred, but fortunately this anomaly, which renders this accident possible, is rare. The floor is perforated by a small opening near the internal tympanic wall for the passage of the tympanic branch of the glossopharyngeal (Jacobson's nerve).

Eustachian Tube. — The Eustachian tube connects the tympanum with the nasopharynx. It is about one and a half inches in length. Its external one-half inch is osseous and lies adjacent to the tympanic cavity. The remaining part of the tube, about one inch in length, is membranous and cartilaginous, and is adjacent to the nasopharynx. At the junction of these two parts the lumen of the tube is contracted and the isthmus is thus formed. The inner end is trumpet-shaped, the pharyngeal orifice being much larger than the lumen at the isthmus. Its direction is downward, forward and inward, the amount of obliquity varying more or less with the shape of the skull. The tube is lined with mucous membrane, covered with ciliated epithelium, continuous with the mucous membrane of the t^Tnpanum and nasopharynx. Immediately above the tube and separated from it by a thin shell of bone — the processus cochleariformis — is the tensor tympani muscle which arises from the base of the temporal bone and adjacent part of the Eustachian tube, and is inserted into the manubrium.

Ossicles. — The tympanic cavity contains the ossicular chain (Fig. 6). This consists of the malleus, incus, and stapes. The malleus or hammer (Fig. 7) presents a head, neck, long and short process and handle for examination. The handle or manubrium lies in the tympanic membrane and partakes of the movements of this membrane and affords attachment to the tensor tympani muscle. The short process is a small conical process

Firji. 6. — Chain of ossicles and their ligaments, seen from the front in a vortical, transverse section of the tympanum. (Gerrish.)

INSERTION

OF TENSOR

TYMPANI

MUSCLE

AURICULAR SURFACE FOR BODY OF INCUS

INSERTION OF EXTERNAL LIGAMENT F MALLEUS

ORT PROCESS

ANUBRIUM

Fig. 7. — Posterior view of the right malleus. (Gray.)

surmounting the upper end of the handle and is directed outward. The long process lies in the Glaserian fissure. The neck is the constricted portion of bone connecting the handle with the head of the ossicle. The latter articulates with the incus. This articulation is of a rachet nature and allows the handle to make excursions outward without acting upon the incus, but movement inward causes the joint to bind and the incus to be moved. This is brought about by the teeth or cogs, which separate when the manubrium makes an extensive movement outward, but come in contact upon inward movement of the handle of the malleus. The incus (Fig. 8) presents

Fig. 8. — Mesial and front view of the right incus. (Graj-.)

a body which articulates with the head of the malleus, a short process which lies in the fossa incudis, and a long process which is connected with the capitellum of the stapes, forming the incudostapedial articulation. The stapes (Fig. 9) or stirrup has a head or capitellum articulating with the long process of the incus, two crura, an anterior and posterior, which connect the head with the foot-plate. To the posterior crus is attached the tendon of the stapedius muscle. The foot-plate with its annular ligament fills in the oval window, forming what is termed the stapediovestibular articulation.

Ossicular Ligaments. — The ossicles are held in position by various ligaments which connect them with the tympanic walls and with each other. The ligaments which connect the malleus with the tympanic wall are the anterior, external, and superior. The anterior consists of two parts, the band of Meckel and the anterior ligament of Helmholtz. The band of Meckel was formerly considered as a muscle and described as the laxator tympani. It is of fibrous tissue and is attached at the base of the long process and extends through the Glaserian fissure to the spine of the sphenoid. The anterior ligament of Helmholtz extends from the neck of the malleus to the anterior margin of the Rivinian segment. The external ligament extends from the junction of the head and neck of the malleus to the posterior part of the Rivinian segment. The posterior fibers

GROOVE FOR OBTURATOR MEMBRANE

ANTERIOR if M O'^^^^^^

CRUS~ff A \g POSTERIOR CRUS

BASE OF STAPES

Fig. 9. — The right stapes., viewed from alcove. (Gray.)

of this ligament constitute the posterior ligament of Helmholtz and together with the anterior ligament of Helmholtz form what is termed the axis band. The superior or suspensory ligament is a small ligament extending between the head of the malleus and the tegmen tympani. The short process of the incus is held in position in the fossa incudis by a band of fibrous tissue. The ossicles are connected with each other at their articulations by fibrous tissue arranged more or less in the form of annular ligaments.

Mechanism of Ossicular Movement. — The axis band above described constitutes the axi3 upon which the movement of the ossicular chain takes place. The mechanism is a lever the long arm of which is the manubrium and the short arm the long process of the incus, the ratio of the arms being as three to two. The effect of this mechanism is therefore to diminish the amplitude but to increase the intensity of sonorous vibrations in transmitting them from the membrana tympani and consequently the manubrium to the stapes, and through it to the labyrinthine fluid. The tensor tympani, by its contraction, depresses and renders the drum membrane tense, causing it to assume a proper condition to be affected by sonorous vibrations. The action of the stapedius muscle is antagonistic to the tensor tympani, as its contraction acts to draw the foot-plate of the stapes away from the labyrinthine fluid, thereby diminishing intralabyrinthine pressure. Acting thtis it protects the delicate structures within the labyrinth against loud noises or force exerted inward upon the drum membrane. Mucous Membrane. — The mucous membrane of the middle ear is continuous with that in the tube and through it with the nasopharyngeal mucous membrane. It lines the tympanic cavity, the internal surface of the drum membrane, and is continuous with the membrane in the mastoid antrum and cells. It covers the ossicles and their ligaments and in so doing several vascular folds are formed. The anterior fold of the malleus extends from the anterior end of the Rivinian segment to the upper end of the handle of the malleus, and envelops the chorda tympani nerve, the long process of the malleus and the anterior ligament. The posterior fold extends from the upper end of the manubrium to the posterior margin of the Rivinian segment, and envelops the posterior tympanic part of the chorda tympani, and the external ligament of the malleus. Both of these folds have free concave margins and the depressions lying between the folds and the tympanic membrane are called the pouches or recesses of von Troltsch, anterior and posterior respectively. Prussak's space lies between Shrapnell's membrane externally, the neck of the malleus internally, and the external ligament posteriorly. Besides those described above there are various other folds, some more or less constant, others peculiar to the individual subject, which are of importance from their role in pathological middle-ear processes. Throughout the greater part of the tympanic cavity the mucous membrane is covered with columnar ciliated epithelium. That covering the internal surface of the drum membrane and the ossicles is of the squamous non-ciliated variety.

Arteries. — The arterial supply of the tympanum is quite free and derived from various sources. The tympanic branch of the internal maxillary artery enters through the Glaserian fissure and anastamoses with the stylomastoid branch of the posterior auricuhir which gains access to the tympanum through the Fallopian canal. Besides these there are the petrosal branch of the middle meningeal which enters through the hiatus Fallopii, some small branches of the middle meningeal entering through the petrosquamosal fissure, branches of the carotid reaching the tympanic cavity through the anterior tympanic wall, and the tympanic branch of the ascending pharyngeal which enters the tympanum through the foramen for Jacobson's nerve.

Nerves. — The nerve supply is derived from the tympanic branch of the glossopharyngeal which receives branches from the carotid plexus of the sympathetic. These are the caroticotympanic and small deep petrosal nerves which enter through the anterior tympanic wall. The tympanic plexus also receives branches from the superficial petrosal nerves. The plexus thus formed lies in grooves upon the internal tympanic wall and sends branches to the various structures in the middle ear and, according to some authors, a branch to the chorda tympani {Ravms commiinicans) , The stapedius is supplied by a branch from the facial. The tensor tympani receives its nerve supply from the otic ganglion.

Facial Nerve. — The facial nerve as it passes through the temporal bone is of great interest to the otologist, both from the symptoms arising from its involvement and the danger of its being injured in surgical procedures. The deep origin of the facial is from a nucleus in the floor of the fourth ventricle, although the fibers supplying the occipitofrontalis, the orbicularis palpebrarum and the corrugator supercilii, have a different origin and join the nerve before it leaves the brain. They are, therefore, not involved in nuclear facial paralysis. In the internal auditory meatus the nerve lies above and anteriorly, being separated from the eighth nerve by the pars intermedia of Wrisberg. It enters the superior and anterior compartment at the fundus of the internal auditory meatus. This is the beginning of the aqueductus Fallopii, or Fallopian canal, which passes through the temporal bone, containing the nerve which emerges at the stylomastoid foramen. Its course is at first outward, lying in the angle formed by the superior semicircular canal and the cochlea. As it comes into relation wdth the internal tympanic wall, it turns backward. This part of the Fallopian canal (tympanic portion) forms a ridge on the internal tympanic wall, which is situated between the oval window and the anterior end of the horizontal semicircular canal. In this part of its course the tympanic wall of the Fallopian • canal not infrequently presents defects (dehiscences) which render its involvement in middle-ear processes or its injury during an operation more apt to take place than when these dehiscences are absent. At the posterior tympanic wall the nerve again turns, passing directly downward to the stylomastoid foramen. In this part of its course it lies immediately behind the posterior tympanic wall, at the base of the pyramid containing the stapedius muscle, to which it gives a small branch. In passing through the temporal bone the nerve has thus three directions: the first outward, lying in the coronal plane; the second backward; and the third downward, the second and third being in the sagittal plane. At the junction of the first and second portions is the geniculate ganglion, which is the origin of the large and small superficial petrosal nerves. These reach the middle fossa through an opening in the Fallopian canal (hiatus Fallopii). The former joins with the large deep petrosal to form the Vidian nerve, which is one of the roots of Meckel's ganglion; while the small superficial petrosal connects with the otic ganglion. These nerves have been considered by some as taste paths, carrying the centripetal impulses received through the chorda tympani. This has been denied by others who believe that the pars intermedia of Wrisberg is the taste path for stimuli reaching the geniculate ganglion through the chorda tympani.

Chorda Tympani. — The chorda tympani is the nerve of taste for the anterior two-thirds of the tongue, being capable of responding to stimuli for the four primary tastes: acid, bitter, sweet, and salty. Its fibers accompany the lingual from which they separate beneath the ramus of the mandible, and passing backward enter a small canal (iter chorda anterious), the canal of Hugier, which runs parallel with the Glaserian fissure, and emerges on the anterior tympanic wall near the anterior margin of the Rivinian segment. The nerve then passes through the tympanum, being enveloped by the anterior and posterior folds of mucous membrane, and lying in the angle formed by the articulation of the malleus and incus. It enters the iter chorda posterious, an osseous canal, which extends from the posterior wall of the tympanum near the posterior margin of the Rivinian segment to the Fallopian canal at the stylomastoid foramen. The chorda joins the facial slightly internal to this foramen and its fibers accompany those of the facial to the geniculate ganglion. In infants that part of the nerve between the tympanum and the facial lies on the surface of the mastoid process beneath the periosteum, merely dipping beneath the upper end of the tympanic ring to enter the tympanic cavity.

INNER EAR.

The best way for the student to acquire a knowledge of the osseous labyrinth is by dissection. The dry bone may be used, but if it is soaked in water for a day or so it will dissect much easier. The instruments used are engravers' tools which may be purchased at the hardware stores at a small expense. The forms which will be found most useful are diamond- and V-shaped. One may by these dissections not only gain a practical knowledge of the canals, cochlea and other parts of the temporal bone, but make some valuable specimens for future study.

The labyrinth occupies a central position in the temporal bone which corresponds to the external part of the petrous portion. It consists of the cochlea anteriorly, the semicircular canals above and posteriorly, and the vestibule uniting these two parts. The soft structures lying within are enclosed by dense ivory-like bone — the labyrinthine capsule. From a physiological standpoint, the labyrinth is divided into two parts, the acoustic and static. The cochlea, being concerned with receiving sonorous vibrations, is called the acoustic labyrinth. The semicircular canals, perhaps also the vestibule, are parts of the apparatlis for maintaining equilibrium and are therefore spoken of as the static labyrinth.

Semicircular Canals. — The semicircular canals (Figs. 10-13), three in number, are half-round and consist of a thick ivory-like cortex enclosing a small canal; the diameter of which is about one-twentieth of an inch, except at one extremity where there is an expansion forming the ampulla. This enlargement is for the accommodation of the crista ampullae, which is a little mound of nerve tissue and an end-organ of one of the branches of the vestibular division of the eighth nerve. Each canal has its ampullated end and its smooth or non-ampullated extremity. An accurate conception of their location and the relative position of their ampullated ends is essential to an understanding of the functional tests of the static labyrinth. The horizontal or external canal lies approximately in the horizontal plane. Its ampulla

J

^9

Ki m0 n§k at^

^1^ k

Fig. 10. — Ri<^ht scmic ircular canals. From above. For rolation of canals to the skull compare with Fig. 14.

HORIZONTAL CANAL

SUPERIOR CANAL

POSTERIOR CANAL

Fig. 11.— Key-plate to Fig. 10.

is anterior and its smooth end enters the vestibule posteriorly. Of all the canals it is the one which is most exposed, as its ivory-like cortex lies bare in the floor of the antrum. The superior or supero vertical canal is, as its name implies, vertical. Its plane is nearly transverse to the axis of the petrous portion of the temporal bone. Its ampulla is in front, below and external, adjacent to that of the horizontal canal. Its smooth end joins with the upper end of the posterior canal to form

Fig. 12. — Left semicircular canals. External view.

a common branch, the crus commune, which opens into the vestibule. Its convexity produces an eminence on the floor of the middle fossa, the eminentia arcuata. In its concavity on the cranial aspect is the subarcuate fossa. In the infant this is a well-marked depression outlining the canal, but in the fully developed bone it is filled to a greater or less extent with osseous tissue.

It nevertheless conveys a vein and a process of dura mater, and through it infection may reach the cranial cavity from the antrum. The posterior or posteriorvertical canal is also vertical. Its plane is nearly parallel with the axis of the bone. Its ampulla is at its lower extremity and its upper end joins with the superior canal to form the crus commune. Its convexity is directed toward the mastoid portion, and in cellular bones may be exposed while performing the mastoid operation.

HORIZONTAL CANAL

SUPERIOR CANAL

PROMONTORY.

OVAL WINDOW

ROUND WINDOW

Fig. 13.— Key-plate to Fig. 12.

The relative position of the canals is that the plane in which each lies is practically perpendicular to the plane of the other two (Fig. 14). The axis of the temporal bone forms with the median line (approximately) an angle of 45 degrees, and the axes of the two bones make with each other an angle of about 90 degrees, or a right angle. As the superior canal is transverse to the axis of the bone and vertical, it follows that it must be in a plane which is parallel to that of the posterior canal on the opposite side, since this canal is vertical and its plane is in the axis of the opposite bone. The horizontal canals on the two sides are in the same plane.