The ear of the cat, like that of man, is composed of three parts, the
external, middle, and internal ear.
The =external= ear is limited internally by the tympanic membrane.
The =middle= ear extends from the tympanic membrane to the fenestra
cochleæ and fenestra vestibuli. It contains the bones of the ear, and is
connected with the pharynx by the Eustachian tube.
The =internal= ear is wholly contained within the petrous bone, and on
the surface of the membrane lining its cavities are distributed the
branches of the auditory (eighth) nerve.
=The External Ear.=
The external ear consists of an expanded portion, the =auricle=
(=auricula=) or pinna, and of a canal leading from the auricle to the
tympanic membrane, known as the =external auditory meatus=. The concave
surface of the auricle is directed craniolaterad, and its edges form the
dorsal and lateral boundaries of the auditory opening. From the bottom
of the cavity which it partly encloses, the external auditory meatus
passes medioventrad.
The auricle (Fig. 168) is composed of a thin curved plate of cartilage
(=cartilago auriculæ=), to which numerous muscles are attached and which
is covered on both sides by integument intimately attached to the
cartilage. Distad the edges of the cartilage form a free border (_b_);
proximad the plate is rolled together in such a way that its edges (_g_
and _h_) overlap on the craniomedial side, thus enclosing a tube or
funnel (_e_) (concha) which forms the enlarged outer portion of the
auditory passage.
Within the cavity of the auricle, on its medial surface, somewhat below
the level of the lowest portion of the margin of the auditory opening is
seen a very prominent pedunculated process (_d_). This is really part of
a transverse ridge, extending craniocaudad on the medial wall of the
auricular cavity. This ridge is the =antihelix= (_d_); it divides the
auricula into a distal and a proximal portion. The portion distad of the
antihelix is the =scapha= (_a_); its surface is smooth except for two
slight longitudinal ridges extending distad from near the two ends of
the antihelix. That portion of the auricle that lies proximad of the
antihelix forms a deep irregular cavity known as the =concha= (_e_). A
deep furrow, corresponding to the antihelix, separates scapha from
concha externally. The concha extends caudad as a deep, rounded pocket;
externally this pocket forms a prominent convexity, the =eminentia
conchæ= (_j_), on the caudal surface of the proximal portion of the
external ear. Farther proximad the concha narrows like a funnel and
becomes much compressed laterally, so that a section of its cavity forms
a narrow craniocaudal slit.
The inner surface of the concha is marked by a number of very prominent
ridges and projections. These are usually due to folds in the cartilage,
so that external furrows and depressions correspond to the internal
projections. The edges of these depressions serve for attachment of
muscles (Fig. 169), by contraction of which the form of the concha is
changed,--the folds of the cartilage thus serving as regions of greater
flexibility.
Two or three centimeters proximad of its external opening the cartilage
of the concha ends in a free edge which receives within itself the
distal end of the cartilaginous external auditory meatus (_e′_). The two
are united together by fibrous tissue. The cartilaginous auditory meatus
(_e′_) extends as a nearly cylindrical tube mediad and slightly craniad
to the tympanic membrane in the opening of the bulla tympani. This tube
is lined with integument which continues over the tympanic membrane. The
subcutaneous tissue of the tube contains the ceruminous glands, which
secrete the ear-wax and open on the surface of the skin.
To understand the relations of the muscles of the external ear it is
necessary to consider a little more fully the structure of the auricular
cartilage (Fig. 168), especially as compared with the human ear. The ear
of the cat differs from the human ear in the fact that the edges of the
auricular cartilages are rolled together proximad so as to overlap.
There is thus no gap between the tragus (_g_) and antitragus (_h_), as
in man, but these two structures partly overlap.
[Illustration: FIG. 168.--CARTILAGE OF THE EXTERNAL EAR, CRANIOMEDIAL
VIEW.
_a_, scapha or pinna; _b_, helix; _c_, spine of the helix; _d_,
antihelix; _e_, concha; _e′_, cartilaginous auditory meatus; _f_,
crista helicis; _g_, tragus; _h_, antitragus; _i_, processus
uncinatus; _j_, eminentia conchæ.]
The free edge (_b_) of the auricular cartilage corresponds partly to the
=helix= of man. At its mediocranial angle the proximal part of the
auricle is composed of muscle and integument only, so that the cartilage
when isolated presents here a deep notch distad of which is a prominent
projecting angle, the =spine= (_c_) of the helix, which serves for
attachment of the adductor auris superior muscle (Fig. 169, 1).
Proximad of the spine, separated from it by the deep notch, the cranial
edge (_g_) of the auricular cartilage approaches the caudal edge (_h_),
finally overlapping it, so that the cavity of the auricle is now
completely surrounded. This overlapping portion of the cartilage is on
the medial side of the cavity, and in the natural state is covered by
muscles and integument, so as not to be apparent.
That portion of the cranial edge of the cartilage that overlaps the
caudal edge forms a cartilaginous plate which projects proximad as a
blunt point; this plate is the =tragus= (_g_). Along the inner edge of
the tragus is a ridge which forms a continuation of the helix; this is
the =crista helicis= (_f_). The portion of the caudal edge of the
cartilage that is partly overlapped by the tragus is an irregular flat
plate known as the =antitragus= (_h_). At its caudolateral margin the
antitragus extends distad as a thin, pointed, cartilaginous spine, the
=processus uncinatus= (_i_). This supports a sheet of integument which
bears two longitudinal ridges projecting into the cavity of the ear,
with a broad groove between them.
=The Scutiform Cartilage.=--Closely connected with the external ear, but
not forming a part of it, is the scutiform cartilage. This lies in the
temporal fossa, between the integument and the temporal muscle, just
behind the orbit and beneath the craniomedial portion of the auricle
(Fig. 63, 1, page 97). It is a narrow cartilage about two centimeters
long, with its long axis craniocaudal. For the greater part of its
length it forms a slender rod, from which a thin cartilaginous sheet
extends a short distance laterad; at its caudal end the rod forks, the
two arms extending nearly at right angles to the main portion. The
scutiform cartilage serves for the origin or insertion of a number of
the muscles of the ear.
=Muscles of the External Ear= (Fig. 169).--(_a_) The muscles connecting
the cartilages of the external ear with other parts of the head have
been described (page 96), excepting the tragicus lateralis (4). These
muscles are the following: M. intermedius scutulorum (Fig. 63, _a_), M.
frontoauricularis, M. levator auris longus (including the
cervicoauricular) (Fig. 63, _g_, _g′_), M. auricularis superior (Fig.
63, _k_), M. abductor auris longus (Fig. 63, _m_), M. abductor auris
brevis (Fig. 63, _l_), M. zygomaticus (Fig. 64, _d_), M. submentalis
(Fig. 64, _c_), M. depressor conchæ (Fig. 64, _b_), M. frontoscutularis,
M. adductor auris inferior (Fig. 63, _f_), and M. tragicus lateralis
(Fig. 169, 4).
=M. tragicus lateralis= (Fig. 169, 4).--A band about seven millimeters
wide, lying beneath the mass of fat about the ear and running ventrad
lengthwise of the concha and close against it.
_Origin_ (Fig. 96, _a_, page 229) on the caudal end of the mandibula, in
the cavity found between the condyloid process and the angular process.
The muscle passes dorsad and is _inserted_ (Fig. 169, 4) on the caudal
margin of the tragus and in the depression on the concha just caudad of
the tragus.
_Relations._--This muscle lies between the temporal muscle and the
concha.
_Action._--Pulls the ear ventrad and probably rotates it outward.
(_b_) The following three muscles connect the scutiform cartilage with
other parts of the external ear.
=M. rotator auris= or =scutuloauricularis inferior=.--A flat
craniocaudal band of fibres lying between the auricle and the temporal
muscle.
_Origin_ on the scutiform cartilage, just caudad of the insertion of the
frontoscutularis, of which this muscle seems to be a continuation. The
muscle passes caudad as a band seven or eight millimeters wide, curving
about the medial surface of the auricle, and is inserted on the
caudomedial surface of the eminentia conchæ.
_Relations._--Lateral surface with the auricular cartilage; medial
surface with the temporal muscle.
_Action._--Rotates the external ear mediad and caudad.
[Illustration: FIG. 169.--CARTILAGE OF THE EXTERNAL EAR, WITH MUSCLES.
_a_-_i_, as in Fig. 168. 1, M. adductor auris superior (cut); 2, M.
adductor auris medius (cut); 3, M. helicis; 4, M. tragicus lateralis
(cut); 5, 5′, M. tragicus medialis; 6, M. antitragicus; 7, M. adductor
auris inferior (cut); 8, M. depressor conchæ (cut); 9, M. conchæus
externus; 10, part of M. auricularis externus; 11, M. abductor auris
longus (cut).]
=M. adductor auris superior= (Fig. 169, 1; Fig. 63, _f_). (Part of the
auricularis anterior of man.)
_Origin_ on the dorsal surface of the scutiform cartilage for its entire
length. The fibres form a continuation of those of the frontoscutularis;
they pass dorsocaudad and are _inserted_ into the spina helicis or
craniomedial margin of the auricular cartilage. This muscle lies in the
fold of skin that connects the craniomedial angle of the auricular
cartilage with the head, so that it is covered on both sides by
integument.
_Action._--Draws the auricle craniad.
=M. adductor auris medius= (Fig. 169, 2). (Part of the auricularis
anterior of man.)--A very thin band of fibres eight to ten millimeters
broad.
_Origin_ on the middle two-thirds of the caudoventral edge of the
scutiform cartilage. The fibres pass nearly ventrad and are _inserted_
along the medial or caudal surface of the tragus.
_Relations._--Outer surface with the auricular cartilage. Inner surface
with the temporal muscle or with the pad of fat separating this from the
auricle.
_Action._--Pulls the concha dorsocraniad.
(_c_) The remainder of the muscles of the ear interconnect parts of the
auricular cartilage.
=M. transversus auriculæ= (Fig. 63, _i_).--A band about six to eight
millimeters wide, on the medial convex surface of the auricle, between
the distal ends of M. levator auris longus (_g′_) and M. auricularis
superior (_k_).
_Origin_ on the medial surface of the concha, just proximad of the
furrow which corresponds to the antihelix. The muscle bridges this
furrow, passes distad about two centimeters, and is inserted on the
auricular cartilage on a line which forms a caudal continuation of the
line of insertion of the levator auris longus (_g′_).
_Relations._--Outer surface with the levator auris longus (_g_, _g′_),
the auricularis superior (_k_), and the integument; inner surface with
the auricular cartilage.
_Action._--Flexes the scapha mediad on the concha, thus enlarging the
external opening of the concha.
=M. auricularis externus= (Fig. 169, 10; Fig. 64, _r_).--This consists
of small scattered bands of interrupted fibres on the convex surface of
the auricle, caudad of the transversus auriculæ. The largest of these
bands runs parallel with the transversus auriculæ about five or six
millimeters caudad of the latter. It has origin on the eminentia conchæ
distad of the insertion of the abductor auris longus. Thence the fibres
run distad five or six millimeters and are interrupted by an attachment
to the cartilage. Distad the fibres begin again, and the muscle runs
thence distad for about one centimeter, the distal end being attached to
the cartilage.
Caudad of this, especially of its proximal portion, are one or two small
bands of fibres usually running parallel with it, but unconnected with
it and with each other. The extent and distribution of these bands
varies, and the direction of fibres is also variable.
_Relations._--Outer surface with the integument; inner surface with the
auricular cartilage.
_Action._--Flexes the auricular cartilage.
=M. helicis= (Fig. 169, 3; Fig. 64, _m_).--A muscle about two
centimeters in length and five or six millimeters in width, on the inner
surface of the ear, along its cranial margin.
_Origin_ on the medial surface of the concha, just caudad of the
proximal end of the tragus. The fibres pass distad to be inserted on the
auricular cartilage at the caudal angle of the notch which lies beneath
the cranial edge of the auricle, along with the caudal fibres of the
adductor auris superior (1).
_Relations._--Lateral surface with the integument; medial surface with
the tragus.
_Action._--Draws proximad the cranial margin of the auricle.
=M. antitragicus= (Fig. 169, 6; Fig. 64, _n_).--A small muscle about one
centimeter in length and three millimeters in thickness, attached at one
end to the caudal border of the antitragus. The fibres pass mediodistad
and are inserted on the tragus, in common with the fibres of the
tragicus medialis (Fig. 169, 5, 5′).
_Relations._--Outer surface with the adductor auris inferior and the
integument; inner surface with the cartilage.
_Action._--Constricts the external auditory opening.
=M. tragicus medialis= (Fig. 169, 5, 5′).--A small, flat muscle, in two
parts, on the medial surface of the concha.
_Origin_ on the ventral end of the tragus. The cranial fibres form a
thick nearly cylindrical bundle which extends proximad about one
centimeter, and is inserted on the cranial surface of the concha just
proximad of a deep fold in the cartilage. The caudal fibres form a thin
sheet which spreads on the medial surface of the concha and is inserted
on an obliquely proximodistal line on the medial surface of the concha,
less than one centimeter from the free distal edge.
These two bundles are distinct except at their origin, and are sometimes
considered as separate muscles.
_Relations._--Outer surface with the tragicus lateralis (4) and the fat
about the concha. Inner surface with the concha.
_Action._--Flexes the concha.
Strauss-Durckheim describes under the name =cornetoconchæus= a small
muscle bridging the deep groove caused by the folding of the medial
surface of the concha, its proximal end being close to the insertion of
the abductor auris brevis (page 100). This muscle was absent in all of
the cases examined for its presence.
=M. conchæus externus= (Fig. 169, 9).--A quadrilateral muscle on the
lateral surface of the concha. The cranial end of this muscle is
attached to the concha a short distance distad of the antitragus along a
dorsoventral line six to eight millimeters in length; the fibres pass
thence caudodorsad a distance of about one centimeter, where they are
again attached to the concha. The muscle thus bridges a shallow
depression on the lateral surface of the concha.
_Action._--Constricts the concha.
=The Middle Ear.=
[Illustration: FIG. 170.--TYMPANIC MEMBRANE, VIEWED FROM THE INNER
SIDE.
The outer wall of the bulla has been removed, carrying with it the
tympanic membrane with the malleus and incus; these are now viewed
from the side that was turned toward the cavity. _a_, bony ring
forming inner end of external auditory meatus; _b_, _b′_, malleus;
_c_, tympanic membrane; _d_, incus; _e_, lenticular process; _f_,
chorda tympani nerve; _g_, cartilaginous support for chorda tympani
nerve.]
The middle ear is enclosed in a cavity within the tympanic bulla. Its
outer boundary, as well as the inner boundary of the external ear, is
formed by the =tympanic membrane= (Fig. 170, _c._) This is a thin,
semitransparent membrane attached to the bony ring (_a_) surrounding the
inner end of the external auditory meatus. It is oval in outline. Across
the dorsal third of its inner surface runs the handle of the malleus
(_b_), to which the membrane is firmly attached and which by pulling on
the membrane gives it the form of a cone instead of a flat surface. The
apex of the cone is directed mediad.
The middle ear, or =cavum tympani=, is an ellipsoidal cavity situated in
the tympanic bulla. On removing the medial wall of the bulla, a bony
plate is seen separating its cavity into two chambers. This plate of
bone extends in from the lateral wall of the bulla, forming a complete
partition except at the dorsocaudal part, where a notch in the bone
forms a free communication between the two chambers. The lateral
chamber, or tympanum proper, contains the bones of the ear and is
limited externally by the tympanic membrane. At its cranial end is seen
the opening of the =tuba auditiva=, or =Eustachian tube=.
[Illustration: FIG. 171.--MALLEUS AND INCUS, UNITED IN THE NATURAL
POSITION.
1, malleus; 2, incus. _a_, handle of malleus; _b_, neck; _c_, head;
_d_, process for attachment of tendon of M. tensor tympani; _e_,
wing-like plate and process of neck; _f_, _g_, the two processes of
the incus.]
The tuba auditiva, or Eustachian tube, is a cartilaginous tube about one
and a half to two centimeters long, passing from the nasopharynx to the
middle ear and placing these two cavities in communication. The
pharyngeal opening is at about the middle of the length of the
nasopharynx, near the dorsal border of the lateral wall; it is a narrow
slit two or three millimeters long. Thence the tube passes caudolaterad
on the ventral surface of the sphenoid bone, and enters the tympanic
cavity through the opening just laterad of the styliform process of the
tympanic bulla. The tube has a thick, cartilaginous medial wall, while
the lateral wall is thin and formed of connective tissue. The lumen of
the tube is a curved slit in cross-section.
=Bones of the Ear= (ossicula auditus).--There are three small bones in
the middle ear, the malleus (hammer) (Fig. 171, 1), incus (anvil) (Fig.
171, 2), and stapes (stirrup) (Fig. 172).
The malleus (Fig. 171, 1) consists of a slender handle (_a_) with a
small neck (_b_) attached at an angle to the caudal end of the handle.
At the end of the neck is a rounded head (_c_) which articulates with
the incus (2), and together with one part of the incus lies in a small
fossa dorsad of the fenestra vestibuli. From the medial aspect of the
neck extends a short process of bone (_d_) to which is attached the
tendon of the tensor tympani muscle. One side of the neck is produced
into a thin wing-like plate, bearing on its edge a pointed process
(_e_).
The =incus= (Fig. 171, 2) bears much resemblance to a two-fanged molar
tooth. It lies in the fossa with the head of the malleus. Its head or
crown is directed craniad and has a concave surface which articulates
with the head of the malleus. One of the fangs (_f_) extends caudad in
the fossa above mentioned and is held in position by a ligament. The
other fang (_g_) extends ventromediad and articulates with the head of
the stapes. This fang terminates in a minute rounded tubercle known as
the =lenticular process=; this in early stages of development is a
separate bone, the =os lenticulare=.
[Illustration: FIG. 172. STAPES.
_a_, head; _b_, base; _c_, process for attachment of the stapedius
muscle.]
The =stapes= (Fig. 172) has the form of a stirrup, and is the last in
the chain of bones. The small end is termed the head (_a_) and the
broader part the base (_b_). The head of the stapes articulates with one
of the processes of the incus. The base of the stapes fits into the
fenestra vestibuli of the petrous bone and completely closes it. The
stapes is held in position by the stapedius muscle, which is attached to
a minute process (_c_) near the head.
Thus the three bones of the middle ear form a chain which connects the
medial surface of the tympanic membrane with the fenestra vestibuli. As
the fenestra vestibuli forms a portion of the wall of the vestibule, the
vibrations of the tympanic membrane are transmitted through the chain of
bones to the fluid within the vestibule.
=Muscles of the Middle Ear.=
=M. tensor tympani.=--The tensor tympani muscle is a short conical
muscle having origin in a small fossa (Fig. 25, _d_, p. 34) in the
petrous bone dorsocraniad of the fenestra vestibuli. Its very short
tendon passes laterad and is inserted into a projection (Fig. 171, _d_)
on the neck of the malleus.
=M. stapedius.=--The stapedius muscle is a short muscle taking origin
from a fossa (Fig. 25, _f_) in the lateral surface of the petrous bone
caudad of that for the incus. It is inserted into the head of the
stapes.
=The Internal Ear= (Fig. 173).
The internal ear or =labyrinth= is completely enclosed in the petrous
bone. In the dry bone the internal ear communicates with the middle ear
by the fenestra cochleæ and the fenestra vestibuli, which have been
described in the account of the petrous bone (page 33). In the recent
state the fenestra vestibuli is closed by the base of the stapes, while
the fenestra cochleæ is covered by a delicate membrane,--so that the
internal ear forms a closed cavity. It is possible to distinguish a
=bony= labyrinth and a =membranous= labyrinth (Fig. 173), the former
being the cavity in the petrous bone, while the latter is the real
auditory organ, situated within the cavity. The membranous labyrinth
repeats in general the form of the bony labyrinth, so that it will not
be necessary to distinguish the two in our account of the main features
of the structure. The auditory nerve is distributed to the membranous
labyrinth. For an account of the minute structure of this organ
reference should be made to a text-book of general histology or
physiology.
[Illustration: FIG. 173.--MEMBRANOUS LABYRINTH.
_a_, vestibule; _b_, cochlea; _c_, external semicircular canal; _d_,
superior semicircular canal; _e_, posterior semicircular canal; _f_,
scala tympani; _g_, scala vestibuli; _h_, fenestra cochleæ; _i_,
fenestra vestibuli.]
The labyrinth (Fig. 173) consists of a cavity, the =vestibule= (_a_),
and connected with this a coiled tube, the =cochlea= (_b_), and the
three =semicircular canals= (_c_, _d_, and _e_).
The =cochlea= (_b_) is situated in the promontory,--the rounded
elevation of the petrous bone extending craniad from the fenestra
cochleæ. It consists of a bony tube coiled spirally about a central
column known as the =modiolus=. The coil has nearly the form of a
snail’s shell, and is somewhat more lengthened in the cat than in man. A
plate of bone, the =lamina spiralis=, incompletely divides the cavity of
the canal lengthwise into two parts, known as =scalæ= (_f_ and _g_). One
of these opens at the fenestra cochleæ (_h_) into the tympanic cavity
(or would so open but for the thin membrane covering the fenestra); this
is known as the =scala tympani= (_f_). The other scala is connected with
the vestibule (_a_) and is known as the =scala vestibuli= (_g_).
The =vestibule= (_a_) is a pyramidal cavity situated mediad of the
caudal end of the cochlea. It is connected with the scala vestibuli of
the cochlea, and with the three semicircular canals. The fenestra
vestibuli (_i_) is an opening in its bony wall, filled in the recent
state by the stapes. On the medial surface of the vestibule are a number
of minute orifices through which the auditory nerve enters.
The =semicircular canals= (_c_, _d_, and _e_) are three curved tubes,
each forming about two-thirds of a circle, and opening at both ends into
the vestibule. They are imbedded in the petrous bone. The planes of the
three tubes intersect nearly at right angles; from their position in man
they have been named =external= (_c_), =superior= (_d_), and =posterior=
(_e_). The external one (_c_) is nearly horizontal in position, and
surrounds the curved fossa (Fig. 25, _f_) for the stapedius muscle,
caudad of the fenestra vestibuli. The superior (Fig. 173, _d_) and
posterior (_e_) are in nearly dorsoventral planes, but at right angles
to each other. The superior (_d_) lies in the ridge forming the caudal
boundary of the appendicular fossa. The posterior (_e_) is situated in a
continuation caudad of the ridge which forms the dorsal boundary of the
internal auditory meatus. One end of each canal is slightly enlarged to
form an =ampulla=. The superior (_d_) and posterior (_e_) canals join at
one end and enter the vestibule by a common opening. One end of the
external canal (_c_) opens into the vestibule so close to one end of the
posterior canal (_e_) that they appear to have a common opening, and
this is commonly stated to be the case; according to Jayne, however, the
openings are really separate. The two remaining ends (of the superior
(_d_) and external (_c_) canals) also enter the vestibule separately, so
that the three canals enter the vestibule by five openings.