Mouth hygiene; a text-book for dental hygienists

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Preface To The Second Edition

Five years have passed since the pubHcation of the first edition of Mouth Hygiene, and during that period a number of schools have been organized for the education and training of the dental hygienist. The experience of these schools has shown that several of the fundamental subjects, such as anatomy, physiology, bacteriology, hygiene etc., require more extensive text than could be adequately covered in any one book. It is, therefore, recommended that standard text-books be utilized for such subjects.

The second edition of Mouth Hygiene endeavors to meet the need of the dental hygienist for those subjects which pertain directly to dentistry and which are essential to her education.

A. C. F.

Bridgeport, Conn., 1921.

LIST OF CONTKIBUTOES.

ALFRED C. FONES, D.D.S.,

Professor of Preventive Dentistry in Columbia University, New York City; Director of the Division of Dental Hygiene in the Bridgeport, Conn., Department of Health.

ARTHUR HOPEWELL-SMITH, ScD., L.R.C.P., M.R.C.S., L.D.S.,

Professor of Dental Histology, Histopathology and Comparative Anatomy in the LTniversity of Pennsylvania, Philadelphia.

EDWARD C. KIRK, D.D.S., Sc.D., LL.D.,

Editor of the Dental Cosmos.

ARTHUR H. MERRITT, D.D.S.,

Specialist in Periodontia, New York City.

LEROY M. S. MINER, M.D., D.M.D.,

Associate Professor of Dentology in the Boston University School of Medicine; Assistant Professor of Oral Surgery in the Dental School of Harvard University, Boston, Mass.; Consulting Oral Surgeon to the Children's Hospital; Oral Surgeon in the Forsyth Infirmary for Children; Chief of the Dental Service in the Massachusetts General Hospital, Boston, Mass.

RODRIGUES OTTOLENGUI, M.D.S., D.D.S., LL.D.,

Editor of Dental Items of Interest.

ROBERT H. W. STRANG, M.D., D.D.S.,

Specialist in Orthodontia; Oral Surgeon to the Bridgeport Hospital, Bridgeport, Conn.

KURT H. THOMA, D.M.D.,

Assistant Professor of Oral Anatomy and Pathology in the Harvard University Dental School, Boston, Mass.

CHARLES R. TURNER, M.D., D.D.S.,

Dean of the School of Dentistry in University of Pennsylvania, Philadelphia.

CHAPTER I. ANATOMY OF THE HEAD.i

By ROBERT H. W. STRANG, M.D., D.D.S.

THE SKULL.

The twenty-two bones that enter into the formation of the osseous framework of the head are united by immovable joints, the lower jaw excepted, called sutures. These form a strong supporting and protecting structure termed the skull (Fig. 1). This may be conveniently studied under four headings: (a) the cranium; (6) the base; (c) the lateral aspect; (d) the anterior aspect or face.

The Cranium. — The cranium comprises that portion of the skull which contains the brain. It is formed by the union of eight bones which are named as follows: frontal, two parietal, occipital, two temporal, sphenoid and ethmoid. In outline it is somewhat egg-shaped and presents for study a superior surface, forming the vertex of the skull, and an inferior surface.

The external surface of the vertex is convex and is covered in the living subject by the tissues that form the scalp. This convexity of surface is ideal for the resisting and warding off of blows. This surface is traversed by three sutures arranged in the form of the letter "H." The anterior cross suture which is situated well toward the top of the skull is called the coronal; the one passing from this to the posterior cross suture is the sagittal; the posterior transverse suture is the lambdoid.

The internal surface of the vertex is concave and is marked with elevations and depressions for the accommodation of the irregular brain surface. Through the center, running anteroposteriorly, is a groove in which lies the superior longitudinal sinus, a blood channel performing the function of a vein and carrying part of the return blood from the brain. To the margins of this groove are attached some of the supporting membranes of the brain.

1 Bibliography: Gray's Anatomy; Piersol's Anatomy; Cryer, Internal Anatomy of the Face; Deaver, Special Anatomy of Head and Neck; Swan, Manual of Anatomy; Chapter on Anatomy of the Teeth, by C. R. Turner, in Johnson's Operative Dentistry. 2

There are also numerous smaller grooves in the bony surface which radiate in various directions and in appearance resemble the branches of a tree. In these run the ramifying branches of the middle meningeal artery which supplies the cranial bones and dura mater with blood.

The inferior surface of the cranium corresponds to the cerebral surface of the base of the skull (Fig. 2) . It is divided by two transverse ridges into three planes, arranged like terraces with the anterior one on the highest level. These planes bear the name of fossce and are called according to their position, anterior, middle and posterior. Their surfaces are more or less irregularly concave, grooved to accommodate bloodvessels and perforated in many places to allow these vessels and also nerves to pass in and out of the cranium.

ANTERIOR NA SPINE

GNATHIONi

Fig. 1. — Side view of typical skull. According to the present nomenclature the bone marked malar should be zygoma and that zygoma should be zygomatic arch. (Cryer.)

Description of the Fossae. — Anterior Fossa. — The points of interest in this fossa are (a) the prominent bony spine in the median line called the crista galli (cock's crest); (6) near the front end of this on either side, slit-like openings for the passage of the nasal nerves into the nasal cavity; (c) the cribriform plate of the ethmoid bone placed on a somewhat lower level than the rest of the floor of the anterior fossa forming what is known as the olfactory groove. This groove is divided anteroposteriorly by the crista galli, accommodates the olfactory bulb of the brain and has its floor pierced with many

THE .SKULL 19

openings for the passage of the olfactory nerves to the nasal cavities; (d) the anterior and posterior ethmoidal foramina, situated at the outer edge of the cribriform plate, the former at about the middle and the latter at the posterior end of the plate. The bone forming the floor of the anterior fossa roofs over the orbital cavities.

Middle Fossa. — In the middle fossa are seen (a) two openings that communicate with the orbits. The smaller of these is the ojJtic foramen, transmitting the optic nerve and ophthalmic artery to the eye; the larger one is the siihenoidal fissure or anterior lacerated foramen, for the passage of the third, fourth, ophthalmic division of the fifth and the sixth cranial nerves, a s^inpathetic nerve, and also arteries and veins to and from the orbits; (/>) in the center of the fossa a bony formation that resembles a saddle and for this reason is called the sella turcica (Turkish saddle) . In this bony structure is situated the pituitary body, one of the so-called ductless glands, which secretes important hormones that exert a marked influence on growth and development; (c) on either side of this, four openings. The two anterior ones are of particular interest because through them pass the divisions of the fifth cranial nerve that go to the upper and lower teeth. The anterior opening is the foramen rotundum and it transmits the superior maxillary division of the fifth nerve. Behind this is the foramen ovale through which passes the sensory and motor portions of the inferior maxillary division of the same nerve. The smallest of the openings is the foramen spinosum through which the middle meningeal artery enters the skull. The largest of these four foramina is called the middle lacerated foramen. This is closed in the living subject with cartilage. On its posterior wall, however, is seen (rf) the inner opening of the carotid canal through which the internal carotid artery gains entrance to the cranium.

The bone at the posterior aspect of the middle fossa acts as the roof for the middle and internal divisions of the ear and is somewhat irregular in conformation with their make-up.

Posterior Fossa. — The surface of this fossa is deeply concave and accommodates the cerebellum. It is marked with (a) grooves for the lateral sinuses carrying return blood from the brain. To the edges of these grooves is attached the membrane supporting the cerebellum. (6) The foramen magnum, centrally located, through which passes the spinal cord; (c) the anterior condyloid foramina for the passage of the h}T)oglossal nerves to the tongue; {d) the jugular or posterior lacerated foramina which affords a means of exit to the ninth, tenth and eleventh cranial nerves as well as the lateral sinuses; ie) the internal auditory meati for the passage of the auditory nerves and arteries and the facial nerves.

The Base of the Skull. — The cerebral surface of the base has just been described under the heading of the Inferior Surface of the Cranium (Fig. 2).

Groove for super, sagittal sinus

Grooves for anter. meningeal vessels

Foramen ccecum

Crista galli

Slit for nasociliary nerve

Groove for nasociliary nerve

Anterior ethmoidal foramen

Orifices for olfactory nerves Posterior ethmoidal foramen

Ethmoidal spine

Olfactory grooves

Optic foramen

Chiasmatic groove

Tuberculum sellae

Anterior clinoid process

Middle clinoid process

Posterior clinoid process

Groove for abducent nerve

Foramen lacerum

Orifice of carotid canal

Depression for semilunar ganglion

Internal acoustic meatus

Slit for dura mater

Groove for superior petrosal sinus

Jugular foramen

Hypoglossal canal

Aquceduetus ve^tibuli

Condyloid foramen

Mastoid foramen Posterior meningeal grooves

Fig. 2. — Base of the skull. Inner or cerebral surface. (Gray.)

THE SKULL Incisors

Canine

Incisive, canal

Transmits left nasopalatine nerve Traiunaits desceiuiiiuj palatine vesseh Transmits right nasopalatine nerve

Lesser palatine foramina

Posterior nasal spine Musculus uvvlae Pterygoid hamulus

Sphenoidal process of palatine -Pharyngeal carml

Pharyngeal tubercle Bituaiion of auditory tube and semicanal for Tensor tympani Tensor veli palatini Inferior tympanic canaliculus Aqtiaednctus rorfJeae Jugular foraimn Mastoid canaliculus Tympanomastoid fissure

Fig. 3.— Base of tlio skull. External surface. (Oray.)

22 AX ATOMY OF THE HEAD

The external or inferior surface of the base (Fig. 3) (the mandible removedj presents the following points for study: (a) In front is the hard palate bordered by the teeth. Behind the incisor teeth is a depression in the palate known as the anterior imlatine fossa. In the floor of this fossa are four foramina for the passage of the naso-palatine nerves and bloodvessels from the nose. On the hard palate opposite the last molar teeth are the posterior -palatine foramina transmitting the descending palatine arteries and the anterior palatine ners'es to the hard palate. (6) Behind the hard palate are seen the posterior openings of the nasal cavities on the outer sides of which are the two pterygoid processes of the sphenoid bone, (c) External to these processes are the zygomatic fossce which contain three of the large muscles of mastication, the inferior maxillary division of the fifth nerve and the internal maxillary artery. These fossae communicate with the orbits by means of the large sphenomaxillary fissures, (d) Numerous foramina the most important of which are: ovale, external opening of the carotid canal, stylo-mastoid, posterior lacerated, condyloid and magnum, (e) Two pairs of articulating surfaces, the one to receive the condyles of the mandible and named the glenoid fossce, the other to articulate with the first vertebra, (f) The styloid and mastoid processes which form pronounced landmarks and serve for the attachment of muscles.

The Lateral Aspect of the Skull fFig. 1). — The following landmarks present themselves for study, (a) The malar bone that forms the prominence of the cheek, (b) The zygoma which lies very superficially and affords attachment to the masseter muscle, (c) The external auditory meatus and (d) the styloid and mastoid processes. It is of interest to note that practically all of the bone that enters into the formation of the side of the skull above the zygoma is covered by the largest of the muscles of mastication, i. e., the temporal.

The Anterior Aspect of the Skull or Face. — The anterior portion of the skull is termed the face. The follo'^dng fourteen bones enter into its make-up: two nasal, two lachrymal, the vomer, two superior maxillary, two malar, two inferior turbinates, two palate and the mandible. Passing from above downward the following points of interest are noted : (a) The supra-orbital foramina or notches through which pass arteries and nerves bearing the same names, (b) The orbits, in which well-protected cavities lie the eyes, (c) The nasal fossae. (d) The infra-orbital foramina which transmit the infra-orbital arteries and the end-branches of the superior maxillary nerves, (e) The prominent malar bones, (f) The teeth of the upper and lower jaws supported by their alveolar processes, (g) The mental foramina through each of which an artery and nerve of the same name emerge. (h) The mandible or lower jaw.

The Orbits. — These are irregular, conical cavities, with the base toward the exterior and the apex inward. The outer edge of the base is in the form of a strong bony ridge which projects a little beyond the e^'e and thus protects it from injury. Seven bones enter into the

THE SKULL 23

formation of* the walls of the orl)its. On the superior aspect of the outer wall near the base is a depression for the lachrymal gland. Each orbit is in communication with various other cavities and fossse by means of the following openings: (a) The optic foramen and (6) sphenoidal fissure open into the middle fossa of the cranial cavity; (c) the sphenomaxillary fissure gives entrance into the sphenomaxillary and zygomatic fosstt; (d) on the inner wall, the anterior and posterior ethmoidal foramina, which transmit vessels of the same names and the former also the nasal nerve, lead into the anterior fossa; and (e) the nasal duct opens into the nose. The posterior opening of the infra-orbital canal is seen on the floor of the orbital cavity.

i

J],-Ostium rnaxillare

First molar First molar

Fig. 4. — Vertical transverse section of typical skull. (Cryer.)

The Nasal Fossae (Fig. 4). — These are large, irregular shaped cavities extending from the floor of the cranium to the roof of the mouth. They are separated from each other by a thin partition made up of bones .and cartilage and called the nasal septum.

In front these fossae communicate with the exterior by means of two large openings called the anterior nares. In back they open into the pharynx through the posterior nares or choancB.

The lateral walls are very irregular and are divided by shelf-like bones named turbinates (scroll-like) into three or more sections called meati. The turbinate bones are normally three in number and according to their position receive the names of inferior, .niddle and superior.

The floor of the nose is formed by the same bones that make up the hard palate, i. e., the palatal processes of the superior maxillary and the horizontal processes of the palate bones. The superior surface of these processes receives the name, "floor of the nose," while the inferior surface is called the "roof of the mouth."

The nasal fossse are in communication by means of openings and canals with the following cavities: (a) The cranium, (6) the orbits, (c) the pharynx, (d) the mouth, (e) three sinuses, i. e., maxillary, frontal and sphenoidal, and (/) three sets of air cells, i. e., anterior, middle and posterior ethmoidal.

According to function the nasal fossge are divided into two parts, the olfactory and respiratory. The olfactory area is in the upper portion and extends down to include the middle turbinate bones on the one side and two-thirds of the septum on the other. The respiratory portion takes in the remainder of the cavity.

The nose is lined with mucous membrane which in the olfactory portion is non-ciliated but contains cells that are speciaHzed to receive the sensations productive of smell. That in the respiratory portion is much thicker, contains large plexuses of veins and its cells are of the ciliated variety. Many glands are. found in the mucous membrane of both portions of the nasal cavities. Their secretion is poured upon the free surface of the membrane keeping this moist. The inspired air is warmed as it passes over this membrane due to the heat imparted from the great amount of blood found in the large venous plexuses.

The hlood supply to the nasal cavity comes through the internal maxillary, the ophthalmic and the facial arteries.

The nerve supply is of two kinds: (a) that of special sense through the first cranial or olfactory nerve and (6) that of common sensation through the fifth cranial or trifacial nerve.

The Bony Sinuses and Air Cells.— In all of the bones of the skull that have any great bulk we find cavities. The largest of these cavities are called sinuses while the smaller ones are called air cells. Their function is to reduce the weight of the bone and in the region of the mouth and nose to render the bone more resonant for the purpose of speech. The most important of these sinuses and air cells are the following :

Maxillary or Antra of Highmore.

Sphenoidal.

Frontal.

Anterior, Middle and Posterior Ethmoidal.

Mastoid.

The Maxillary Sinuses or Antra of Highmore (Fig. 4). — ^These are two in number, situated within the bodies of the superior maxillary bones, external to the nose and below the orbits. In shape they are somewhat pyramidal, with their bases directed toward the nose and the apices at the prominence of the cheek. They open into the middle meati of the nose at points known as the infundibula. The antra are often divided and partitioned by bony septa. They are lined with mucous membrane which is directly continuous with that of the nose

THE SKULL and is covered with ciliated epithelium. The mucous membrane also contains glands. Often the roots of the molar and biscuspid teeth form elevations on the floors of the sinuses and when diseased frequently infect the mucous membrane with most serious results.

The Sphenoidal Sinus (Fig. 5). — This air cavity may be a single one but is usually partitioned into two distinct cells. It is situated within

Section turned •

Sella turcica

Auditory orifice

Posterior ethmoidal cells

Right frontal sinus

Left frontal sinus

Infundibulum

Anterior

ethmoidal cells Hiatus

semilunaris Unciform

process

Middle meatus

Inferior concha Probe passing

into lacrimal

duct Inferior meatus

Hard palate

Alveolar process

Fig. 5.

-An anteroposterior section within the nasal cavity, with the middle concha and portion of the cell walls turned up. (Cryer.)

the body of the sphenoid bone at the posterior aspect of the roof of the nose. It has an opening into the superior meatus and is also lined with mucous membrane continuous with that of the nasal cavity. Often the posterior ethmoidal air cells communicate with this sinus.

The Frontal Sinuses (Fig. 5). — These are two fairly large cavities within the frontal bone. They are located immediately above the orbits and their position is marked approximately by the eyebrows.

They are really a continuation of the anterior ethmoidal air cells of their respective sides. They open into the middle meati of the nose. Congestion of these sinuses is a usual sequence in a so-called " cold in the head," and gives rise to the accompanying headache so frequently noted in this condition.

The Ethmoidal Air Cells (Fig. 5). — There are three sets of these found within the lateral masses of the ethmoid bone and named according to their position, anterior, middle and posterior. They are lined with mucous membrane which is a continuation of that lining the nasal passages into which each set of cells opens. These cells are often interconnected and frequently the posterior set communicates with the sphenoidal sinus. ^

The orifices of the canals that lead from the nose to the anterior ethmoidal cells are intimately associated with the openings into the antra and into the frontal sinuses. Thus it is that the antrum, the anterior ethmoidal cells and the frontal sinus of each side are made intercommunicating and their mucous membrane linings practically continuous with each other. These anatomical facts make it very possible for an infection arising within one cavity to travel to one or both of the others. Cases are not uncommon in which an abscess on the root of an upper molar or bicuspid tooth infects the mucous membrane of the antrum and the discharge from this tissue passing into the nose infects the lining membrane of the canal that leads to the anterior ethmoidal air cells. The infection traveling up this canal will eventually involve these cells. The mucous membrane of the frontal sinus may also become involved from the same source, either by an extension from the nasal mucous membrane or directly from that of the anterior ethmoidal cells. Such a pathological condition may be continued into the middle and posterior ethmoidal cells and the sphenoidal sinus.

The mastoid air cells or antra are situated within the mastoid portion of the temporal bones and will be mentioned under the description of the ear.

THE EYES.

These are the organs of vision and consist of two globular bodies situated within the orbits. They are freely movable by means of a ball-and-socket joint formed between the eyeball and a tough, fibrous membrane arranged in the form of a socket. This membrane receives the name of the ca'psule of Tenon. Movement of the eyeball is performed through the agency of six muscles that arise from the bony wall of the orbit and are attached to the ball at various points.

The anterior portion of the eye is covered with a modified mucous membrane which is reflected on to the lids and lines the inner side of these. This membrane is called the conjunctiva and covers that portion of the eye that is commonly called the "white."

The eyeball (Fig. 6) is made up of three coats within which are three refracting media. The coats are named:

THE EYES

1. Outer or fibrous.

2. Middle or vascular.

3. Inner or nervous.

The refracting media are:

1. Aqueous humor.

2. Crystalline lens.

3. Vitreous humor.

Conjunctiva

Rectus lateralis

Zonular spaces

Hyaloid canal

Rectus medialis

Sclera

Choroid

Retina

A. centralis retinae

Fovea centralis /u.wwwu-as

Nerve sheath Fig. 6. — The right eye in horizontal section. (Toldt.)

The Coats of the Eye. — Fibrous Coat. — This is divided into two parts: (a) Cornea, forming the anterior sixth of the sphere and (&) the sclera, forming the remainder, (a) The Cornea. This tissue is made up of cells that will transmit the rays of light and so may be likened to a window. It is more highly convex than the rest of the eyeball, giving this portion of the ball a bulging appearance. Its posterior edge is continued into the sclera. (6) The Sclera. This is a firm, inelastic, fibrous membrane forming the posterior five-sixths of the eyeball.

Vascular Coat. — This is divided into three parts:

The iris.

The ciliary body.

The choroid.

(a) The iris may be likened unto a circular curtain attached at the periphery and perforated in the center. This "hole" in the center is called the pupil. The iris contains pigment of varying tint and is the tissue that gives the color to the eye. In structure it consists for the most part of muscular fibers of two kinds, circular and radiating. When the circular contract the pupil becomes smaller and when the radiating are active the pupil enlarges.

The iris is suspended in a cavity formed by the cornea in front and the lens behind. It is nearly in contact with the latter structure. This cavity is filled with a modified lymph called the aqueous humor.

(6) The ciliary body is made up of the ciliary processes and the ciliary muscle. The ciliary processes are composed of foldings, as it were, of the tissues making up the middle coat and are continuous at their posterior ends with the choroid and at their anterior ends with the suspensory ligament of the lens and the iris. They vary from sixty to eighty in number. The ciliary muscle is arranged in the form of a circular band of involuntary muscle fibers lying on the outer surface of the middle coat of the eye between the iris and the choroid. Its function is to control the convexity of the lens so that the rays of light may be properly focused. The ciliary body is an exceedingly vascular area and consequently is very liable to be the seat of an infection, hence it has been designated as the "danger area" of the eye.

(c) The choroid is that portion of the middle coat that lies posterior to the ciliary body. It is made up of areolar tissue, bloodvessels, and considerable pigment.

Nervous Coat. — This is commonly called the retina. It is continuous with the optic nerve behind and extends forward about as far as the ciliary body where it ends in a jagged margin. It may be called the end-organ of the optic nerve with the special sense of vision as its function.

The fibers of the optic nerve are distributed to all parts of this membrane and end in physiological relationship with special cells, the rods and cones, of the neuro-epithelium lining the retina. The rods and cones receive the visual impressions and transfer them to the nerve fibers.

In examining the retina a white circular area is noted at the point of entrance of the optic nerve. This is called the optic disc and is the one point on the membrane where the rays of light will make no impression. In other words, it is a blind spot. For this reason it is located eccentrically so that the same rays of light will not be received on a blind spot in each eye, damaging the field of vision. Its position is somewhat to the inner side of the center. Directly in the center of the retina is the area where the most acute vision is to be had. This is called the macula lutea (yellow spot) because of its color. In the center of this is a depression known as \h.e fovea centralis.

The Interior of the Eyeball. — ^The interior of the eyeball is unequally divided by the lens. That portion in front of the lens is further sub-

THE EARS 29

divided by the iris into two compartments known as the anterior and posterior chambers. The anterior chamber communicates with the posterior chamber through the pupih

The Refracting Media. — The Aqueous Humor.— This is a watery fluid fining the anterior and posterior chambers of the eye. It is derived from the vessels within the ciHary body and any excess is carried off through spaces and canals that empty into the ciliary veins.

The Crystalhne Lens. — This is a biconvex, circular body made up of transparent fibrous tissue the component parts of which are cemented together with a transparent cement substance and the entire mass of tissue is surrounded by a capsule. It lies in a depression on the anterior surface of the vitreous body and is held in position by the susyensory ligament of the lens. The function of the lens is to bring the rays of light to a proper focus upon the retina.

The Vitreous Humor or Body. — In contact with the retina and filling the interior of the eyeball behind the lens is the vitreous humor. It is composed of a soft, jelly-like substance, perfectly transparent, and made up of semisolid connective tissue. This is surrounded by a membrane that is thickened anteriorly to form the suspensory ligament of the lens which holds the lens in position and affords attachment to the ciliary processes. The anterior surface of the vitreous body presents a cup-like depression into which the lens fits.

The Lachrymal Apparatus. — This consists of (a) the lachrymal or tear gland which is situated in a depression at the outer angle of the orbit at its upper aspect and from which several ducts lead and open through the conjunctiva of the. upper lid just before this is reflected on to the eyeball; (b) the lachrymal sac, placed at the inner angle of the orbit and gathering in the tears by means of two small canals leading from the inner corner of each lid; and (c) the nasal duct, a passage that leads from the sac to the inferior meatus of the nose and discharges its contents into this cavity.

THE EARS.

The organ of hearing (Fig. 7) is divided into three portions:

1. The external ear.

2. The middle ear.

3. The internal ear or labyrinth.

The External Ear. — This consists of the cartilaginous structure that is commonly called the "ear" and the external portion of the auditory canal. The latter, known as the external auditory canal, is about one inch in length and runs inward and somewhat forward. It is separated from the middle ear by the tympanic membrane or drum. Its external opening is termed the external auditory meatus.

The Middle Ear. — This extends from the drum to the internal ear. It approximates one-sixth of an inch in length and is nearly one-half an inch in its vertical diameter. In this cavity are the three ear bones or ossicles, as they are called. The middle ear is in communication with the mastoid antrum through a small opening and with the nasopharynx via the Eustachian tube. Through the middle ear, embedded in its mucous membrane lining, passes the chorda tympani nerve, a branch of the facial, on its way to join the lingual branch of the mandibular nerve. As will be noted later, this nerve transfers the special sensations of taste from the anterior portion of the tongue to the brain. Hence disease of the middle ear may be attended with loss of the sense of taste on the corresponding side of the tongue owing to involvement of this nerve.

Cartilage of auricula Attic Incus

Malleus

Tympanic cavity

Tensor tympani

Tympanic membrane

Cartilaginous

part of ext.

acoustic meatus

Bony part of

ext. acoustic

meatus

Fig. 7. — External and middle ear, opened from the front. Right side. (Gray.)

The Tympanic Membrane or Drum. — This consists of an oval membrane, obliquely attached to the sides of the auditory canal, the upper portion being nearer the external opening. It presents a concave surface to the exterior. Between its layers is bound the "handle" of the malleus, one of the ossicles.