THE ORGANS of the senses may be divided into (a) those of the special senses of taste, smell, sight, and hearing,
and (b) those associated with the general sensations of heat, cold, pain,
pressure, etc.
The Organ of Smell
(Organon Olfactorius; The Nose)
The peripheral olfactory organ or organ
of smell consists of two parts: an outer, the external nose, which projects from the center of the face; and an
internal, the nasal cavity, which is
divided by a septum into right and left nasal chambers.
The External Nose (Nasus Externus; Outer Nose)—The external nose is
pyramidal in form, and its upper angle or root
is connected directly with the forehead; its free angle is termed the apex. Its base is perforated by two elliptical orifices, the nares, separated from each other by an
antero-posterior septum, the columna.
The margins of the nares are provided with a number of stiff hairs, or vibrissae, which arrest the passage of
foreign substances carried with the current of air intended for respiration.
The lateral surfaces of the nose form, by their union in the middle line, the dorsum nasi, the direction of which
varies considerably in different individuals; the upper part of the dorsum is
supported by the nasal bones, and is named the bridge. The lateral surface ends below in a rounded eminence, the ala nasi.
Structure.—The frame-work of the
external nose is composed of bones and cartilages; it is covered by the
integument, and lined by mucous membrane. The bony frame-work occupies the upper part of the organ; it consists
of the nasal bones, and the frontal processes of the maxillae.
The cartilaginous frame-work (cartilagines
nasi) consists of five large pieces, viz., the cartilage of the septum, the two
lateral and the two greater alar
cartilages, and several smaller pieces, the lesser alar cartilages. The various cartilages are connected to
each other and to the bones by a tough fibrous membrane.
The cartilage of the septum (cartilago
septi nasi) is somewhat quadrilateral in form, thicker at its margins than at
its center, and completes the separation between the nasal cavities in front. Its
anterior margin, thickest above, is connected with the nasal bones, and is continuous
with the anterior margins of the lateral cartilages; below, it is connected to the
medial crura of the greater alar cartilages by fibrous tissue. Its posterior margin
is connected with the perpendicular plate of the ethmoid; its inferior margin with
the vomer and the palatine processes of the maxillae.
It may be prolonged backward (especially
in children) as a narrow process, the sphenoidal
process, for some distance between the vomer and perpendicular plate of the
ethmoid. The septal cartilage does not reach as far as the lowest part of the nasal
septum. This is formed by the medial crura of the greater alar cartilages and by
the skin; it is freely movable, and hence is termed the septum mobile nasi.
The lateral cartilage (cartilago nasi
lateralis; upper lateral cartilage) is situated below the inferior margin of
the nasal bone, and is flattened, and triangular in shape. Its anterior margin is
thicker than the posterior, and is continuous above with the cartilage of the septum,
but separated from it below by a narrow fissure; its superior margin is attached
to the nasal bone and the frontal process of the maxilla; its inferior margin is
connected by fibrous tissue with the greater alar cartilage.
The greater alar cartilage (cartilago
alaris major; lower lateral cartilage) is a thin, flexible plate, situated immediately
below the preceding, and bent upon itself in such a manner as to form the medial
and lateral walls of the naris of its own side.
The portion which forms the medial wall (crus mediale) is loosely connected with the corresponding portion of
the opposite cartilage, the two forming, together with the thickened integument
and subjacent tissue, the septum mobile nasi.
The part which forms the lateral wall
(crus laterale) is curved to correspond
with the ala of the nose; it is oval and flattened, narrow behind, where it is connected
with the frontal process of the maxilla by a tough fibrous membrane, in which are
found three or four small cartilaginous plates, the lesser alar cartilages (cartilagines
alares minores; sesamoid cartilages). Above, it is connected by fibrous tissue
to the lateral cartilage and front part of the cartilage of the septum; below, it
falls short of the margin of the naris, the ala being completed by fatty and fibrous
tissue covered by skin. In front, the greater alar cartilages are separated by a
notch which corresponds with the apex of the nose. The muscles acting on the external nose have been described in the section
on Myology.
The integument of the dorsum and sides of the nose is thin, and loosely
connected with the subjacent parts; but over the tip and alae it is thicker and
more firmly adherent, and is furnished with a large number of sebaceous follicles,
the orifices of which are usually very distinct.
The arteries of the external nose are the alar and septal branches of the
external maxillary, which supply the alae and septum; the dorsum and sides being
supplied from the dorsal nasal branch of the ophthalmic and the infraorbital branch
of the internal maxillary. The veins
end in the anterior facial and ophthalmic veins.
The nerves for the muscles of the nose are derived from the facial, while
the skin receives branches from the infratrochlear and nasociliary branches of the
ophthalmic, and from the infraorbital of the maxillary.
The Nasal Cavity (Cavum Nasi; Nasal Fossa)—The
nasal chambers are situated one on either side of the median plane. They open in
front through the nares, and communicate behind through the choanae with the nasal
part of the pharynx. The nares are somewhat
pear-shaped apertures, each measuring about
Inside the aperture of the nostril is
a slight dilatation, the vestibule, bounded
laterally by the ala and lateral crus of the greater alar cartilage, and medially
by the medial crus of the same cartilage. It is lined by skin containing hairs and
sebaceous glands, and extends as a small recess toward the apex of the nose. Each
nasal cavity, above and behind the vestibule, is divided into two parts: an olfactory region, consisting of the superior
nasal concha and the opposed part of the septum, and a respiratory region, which comprises the rest of the cavity.
Lateral Wall
On the lateral wall are the superior, middle, and inferior nasal conchae, and below and lateral
to each concha is the corresponding nasal passage or meatus. Above the superior
concha is a narrow recess, the sphenoethmoidal
recess, into which the sphenoidal sinus opens. The superior meatus is a short oblique passage extending about half-way
along the upper border of the middle concha; the posterior ethmoidal cells open
into the front part of this meatus. The middle
meatus is below and lateral to the middle concha, and is continued anteriorly
into a shallow depression, situated above the vestibule and named the atrium of the middle meatus. On raising
or removing the middle concha the lateral wall of this meatus is fully displayed.
On it is a rounded elevation, the bulla ethmoidalis,
and below and in front of this is a curved cleft, the hiatus semilunaris.
The bulla ethmoidalis is caused by the bulging of the middle ethmoidal cells
which open on or immediately above it, and the size of the bulla varies with that
of its contained cells.
The hiatus semilunaris is bounded inferiorly by the sharp concave margin
of the uncinate process of the ethmoid bone, and leads into a curved channel, the
infundibulum, bounded above by the bulla
ethmoidalis and below by the lateral surface of the uncinate process of the ethmoid.
The anterior ethmoidal cells open into the front part of the infundibulum, and this
in slightly over 50 per cent. of subjects is directly continuous with the frontonasal
duct or passage leading from the frontal air sinus; but when the anterior end of
the uncinate process fuses with the front part of the bulla, this continuity is
interrupted and the frontonasal duct then opens directly into the anterior end of
the middle meatus.
Below the bulla ethmoidalis, and partly
hidden by the inferior end of the uncinate process, is the ostium maxillare, or opening from the maxillary sinus; in a frontal
section this opening is seen to be placed near the roof of the sinus. An accessory
opening from the sinus is frequently present below the posterior end of the middle
nasal concha. The inferior meatus is
below and lateral to the inferior nasal concha; the nasolacrimal duct opens into
this meatus under cover of the anterior part of the inferior concha.
Medial Wall—The medial wall or septum
is frequently more or less deflected from the median plane, thus lessening the size
of one nasal cavity and increasing that of the other; ridges or spurs of bone growing
into one or other cavity from the septum are also sometimes present. Immediately
over the incisive canal at the lower edge of the cartilage of the septum a depression,
the nasopalatine recess, is seen. In
the septum close to this recess a minute orifice may be discerned; it leads backward
into a blind pouch, the rudimentary vomeronasal
organ of Jacobson, which is supported by a strip of cartilage, the vomeronasal cartilage. This organ is well-developed
in many of the lower animals, where it apparently plays a part in the sense of smell,
since it is supplied by twigs of the olfactory nerve and lined by epithelium similar
to that in the olfactory region of the nose.
The roof of the nasal cavity is narrow from side to side, except at its
posterior part, and may be divided, from behind forward, into sphenoidal, ethmoidal,
and frontonasal parts, after the bones which form it.
The floor is concave from side to side and almost horizontal antero-posteriorly;
its anterior three-fourths are formed by the palatine process of the maxilla, its
posterior fourth by the horizontal process of the palatine bone. In its anteromedial
part, directly over the incisive foramen, a small depression, the nasopalatine recess, is sometimes seen;
it points downward and forward and occupies the position of a canal which connected
the nasal with the buccal cavity in early fetal life.
The Mucous Membrane (membrana mucosa
nasi).—The nasal mucous membrane lines the nasal
cavities, and is intimately adherent to the periosteum or perichondrium. It is continuous
with the skin through the nares, and with the mucous membrane of the nasal part
of the pharynx through the choanae. From the nasal cavity its continuity with the
conjunctiva may be traced, through the nasolacrimal and lacrimal ducts; and with
the frontal, ethmoidal, sphenoidal, and maxillary sinuses, through the several openings
in the meatuses. The mucous membrane is thickest, and most vascular, over the nasal
conchae. It is also thick over the septum; but it is very thin in the meatuses on
the floor of the nasal cavities, and in the various sinuses.
Owing to the thickness of the greater
part of this membrane, the nasal cavities are much narrower, and the middle and
inferior nasal conchae appear larger and more prominent than in the skeleton; also
the various apertures communicating with the meatuses are considerably narrowed.
Structure of the Mucous Membrane—The
epithelium covering the mucous membrane differs in its character according to the
functions of the part of the nose in which it is found. In the respiratory region it is columnar and ciliated.
Interspersed among the columnar cells are goblet or mucin cells, while between their
bases are found smaller pyramidal cells. Beneath the epithelium and its basement
membrane is a fibrous layer infiltrated with lymph corpuscles, so as to form in
many parts a diffuse adenoid tissue, and under this a nearly continuous layer of
small and larger glands, some mucous and some serous, the ducts of which open upon
the surface. In the olfactory region
the mucous membrane is yellowish in color and the epithelial cells are columnar
and non-ciliated; they are of two kinds, supporting cells and olfactory cells. The
supporting cells contain oval nuclei,
which are situated in the deeper parts of the cells and constitute the zone of oval
nuclei; the superficial part of each cell is columnar, and contains granules of
yellow pigment, while its deep part is prolonged as a delicate process which ramifies
and communicates with similar processes from neighboring cells, so as to form a
net-work in the mucous membrane. Lying between the deep processes of the supporting
cells are a number of bipolar nerve cells, the olfactory cells, each consisting of a small amount of granular protoplasm
with a large spherical nucleus, and possessing two processes—a superficial one which
runs between the columnar epithelial cells, and projects on the surface of the mucous
membrane as a fine, hair-like process, the olfactory
hair; the other or deep process runs inward, is frequently beaded, and is continued
as the axon of an olfactory nerve fiber. Beneath the epithelium, and extending through
the thickness of the mucous membrane, is a layer of tubular, often branched, glands,
the glands of Bowman, identical in structure
with serous glands. The epithelial cells of the nose, fauces and respiratory passages
play an important role in the maintenance of an equable temperature, by the moisture
with which they keep the surface always slightly lubricated.
Vessels and Nerves.—The arteries of the nasal cavities are the anterior and posterior ethmoidal
branches of the ophthalmic, which supply the ethmoidal cells, frontal sinuses, and
roof of the nose; the sphenopalatine branch of the $$$ which supplies the mucous
membrane covering the conchae, the meatuses and septum, the septal branch of the
superior labial of the external maxillary; the infraorbital and alveolar branches
of the internal maxillary, which supply the lining membrane of the maxillary sinus;
and the pharyngeal branch of the same artery, distributed to the sphenoidal sinus.
The ramifications of these vessels form a close plexiform net-work, beneath and
in the substance of the mucous membrane.
The veins form a close cavernous plexus beneath the mucous membrane. This
plexus is especially well-marked over the lower part of the septum and over the
middle and inferior conchae. Some of the veins open into the sphenopalatine vein;
others join the anterior facial vein; some accompany the ethmoidal arteries, and
end in the ophthalmic veins; and, lastly, a few communicate with the veins on the
orbital surface of the frontal lobe of the brain, through the foramina in the cribriform
plate of the ethmoid bone; when the foramen cecum is patent it transmits a vein
to the superior sagittal sinus.
The nerves of ordinary sensation are: the nasociliary branch of the ophthalmic,
filaments from the anterior alveolar branch of the maxillary, the nerve of the pterygoid
canal, the nasopalatine, the anterior palatine, and nasal branches of the sphenopalatine
ganglion.
The nasociliary branch of the ophthalmic
distributes filaments to the forepart of the septum and lateral wall of the nasal
cavity. Filaments from the anterior alveolar nerve supply the inferior meatus and
inferior concha. The nerve of the pterygoid canal supplies the upper and back part
of the septum, and superior concha; and the upper nasal branches from the sphenopalatine
ganglion have a similar distribution. The nasopalatine nerve supplies the middle
of the septum. The anterior palatine nerve supplies the lower nasal branches to
the middle and inferior conchae.
The olfactory, the special nerve of the sense of smell, is distributed to
the olfactory region. Its fibers arise from the bipolar olfactory cells and are
destitute of medullary sheaths. They unite in fasciculi which form a plexus beneath
the mucous membrane and then ascend in grooves or canals in the ethmoid bone; they
pass into the skull through the foramina in the cribriform plate of the ethmoid
and enter the under surface of the olfactory bulb, in which they ramify and form
synapses with the dendrites of the mitral cells.
The accessory sinuses or air cells
of the nose are the frontal, ethmoidal,
sphenoidal, and maxillary; they vary
in size and form in different individuals, and are lined by ciliated mucous membrane
directly continuous with that of the nasal cavities.
The Frontal Sinuses (sinus frontales),
situated behind the superciliary arches, are rarely symmetrical, and the septum
between them frequently deviates to one or other side of the middle line. Their
average measurements are as follows: height,
The Ethmoidal Air Cells (cellulae
ethmoidales) consist of numerous thin-walled cavities situated in the ethmoidal
labyrinth and completed by the frontal, maxilla, lacrimal, sphenoidal, and palatine.
They lie between the upper parts of the nasal cavities and the orbits, and are separated
from these cavities by thin bony laminae. On either side they are arranged in three
groups, anterior, middle, and posterior. The anterior and middle groups
open into the middle meatus of the nose, the former by way of the infundibulum,
the latter on or above the bulla ethmoidalis. The posterior cells open into the
superior meatus under cover of the superior nasal concha; sometimes one or more
opens into the sphenoidal sinus. The ethmoidal cells begin to develop during fetal
life.
The Sphenoidal Sinuses (sinus sphenoidales)
contained within the body of the sphenoid vary in size and shape; owing to the lateral
displacement of the intervening septum they are rarely symmetrical. The following
are their average measurements: vertical height,
The Maxillary Sinus (sinus maxillaris;
antrum of Highmore), the largest of the accessory sinuses of the nose, is a
pyramidal cavity in the body of the maxilla. Its base is formed by the lateral wall
of the nasal cavity, and its apex extends into the zygomatic process. Its roof or
orbital wall is frequently ridged by the infra-orbital canal, while its floor is
formed by the alveolar process and is usually 1/2 to
The Organs of Taste — (Organon Gustus)
The peripheral gustatory or taste
organs consist of certain modified epithelial cells arranged in flask-shaped
groups termed gustatory calyculi (taste-buds), which are found on the tongue
and adjacent parts. They occupy nests in the stratified epithelium, and are present
in large numbers on the sides of the papillae vallatae and to a less extent on their
opposed walls. They are also found on the fungiform papillae over the back part
and sides of the tongue, and in the general epithelial covering of the same areas.
They are very plentiful over the fimbriae linguae, and are also present on the under
surface of the soft palate, and on the posterior surface of the epiglottis. Structure.—Each taste bud is flask-like in shape its broad base resting on the corium,
and its neck opening by an orifice, the gustatory
pore, between the cells of the epithelium. The bud is formed by two kinds of
cells: supporting cells and gustatory cells. The supporting cells are mostly arranged like the staves of a cask, and
form an outer envelope for the bud. Some, however, are found in the interior of
the bud between the gustatory cells. The gustatory
cells occupy the central portion of the bud; they are spindle-shaped, and each
possesses a large spherical nucleus near the middle of the cell. The peripheral
end of the cell terminates at the gustatory pore in a fine hair-like filament, the
gustatory hair. The central process passes
toward the deep extremity of the bud, and there ends in single or bifurcated varicosities.
The nerve fibrils after losing their medullary sheaths enter the taste bud, and
end in fine extremities between the gustatory cells; other nerve fibrils ramify
between the supporting cells and terminate in fine extremities; these, however,
are believed to be nerves of ordinary sensation and not gustatory. Nerves
of Taste.—The chorda tympani nerve, derived from the
sensory root of the facial, is the nerve of taste for the anterior two-thirds of
the tongue; the nerve for the posterior third is the glossopharyngeal.
The Organ of Sight (Organon Visus; The Eye)
The bulb of the eye (bulbus oculi;
eyeball), or organ of sight, is contained
in the cavity of the orbit, where it is protected from injury and moved by the ocular
muscles. Associated with it are certain accessory structures, viz., the muscles,
fasciae, eyebrows, eyelids, conjunctiva, and lacrimal apparatus. The bulb of the eye is imbedded in the fat
of the orbit, but is separated from it by a thin membranous sac, the fascia bulbi. It is composed of segments
of two spheres of different sizes. The anterior segment is one of a small sphere;
it is transparent, and forms about one-sixth of the bulb. It is more prominent than
the posterior segment, which is one of a larger sphere, and is opaque, and forms
about five-sixths of the bulb. The term anterior
pole is applied to the central point of the anterior curvature of the bulb,
and that of posterior pole to the central
point of its posterior curvature; a line joining the two poles forms the optic axis. The axes of the two bulbs are
nearly parallel, and therefore do not correspond to the axes of the orbits, which
are directed forward and lateralward. The optic nerves follow the direction of the
axes of the orbits, and are therefore not parallel; each enters its eyeball
Development.—The eyes begin to develop
as a pair of diverticula from the lateral aspects of the forebrain. These diverticula
make their appearance before the closure of the anterior end of the neural tube;
after the closure of the tube they are known as the optic vesicles. They project toward the sides of the head, and the peripheral
part of each expands to form a hollow bulb, while the proximal part remains narrow
and constitutes the optic stalk. The
ectoderm overlying the bulb becomes thickened, invaginated, and finally severed
from the ectodermal covering of the head as a vesicle of cells, the lens vesicle, which constitutes the rudiment
of the crystalline lens. The outer wall of the bulb becomes thickened and invaginated,
and the bulb is thus converted into a cup, the optic cup, consisting of two strata of cells. These two strata are continuous
with each other at the cup margin, which ultimately overlaps the front of the lens
and reaches as far forward as the future aperture of the pupil. The invagination
is not limited to the outer wall of the bulb, but involves also its postero-inferior
surface and extends in the form of a groove for some distance along the optic stalk,
so that, for a time, a gap or fissure, the choroidal
fissure, exists in the lower part of the cup. Through the groove and fissure
the mesoderm extends into the optic stalk and cup, and in this mesoderm a bloodvessel
is developed; during the seventh week the groove and fissure are closed and the
vessel forms the central artery of the retina. Sometimes the choroidal fissure persists,
and when this occurs the choroid and iris in the region of the fissure remain undeveloped,
giving rise to the condition known as coloboma
of the choroid or iris.
The retina is developed from the optic cup. The outer stratum of the cup
persists as a single layer of cells which assume a columnar shape, acquire pigment,
and form the pigmented layer of the retina; the pigment first appears in the cells
near the edge of the cup. The cells of the inner stratum proliferate and form a
layer of considerable thickness from which the nervous elements and the sustentacular
fibers of the retina, together with a portion of the vitreous body, are developed.
In that portion of the cup which overlaps the lens the inner stratum is not differentiated
into nervous elements, but forms a layer of columnar cells which is applied to the
pigmented layer, and these two strata form the pars ciliaris and pars iridica
retinae.
The cells of the inner or retinal layer
of the optic cup become differentiated into spongioblasts and germinal cells, and
the latter by their subdivisions give rise to neuroblasts. From the spongioblasts
the sustentacular fibers of Müller, the outer and inner limiting membranes,
together with the groundwork of the molecular layers of the retina are formed. The
neuroblasts become arranged to form the ganglionic and nuclear layers. The layer
of rods and cones is first developed in the central part of the optic cup, and from
there gradually extends toward the cup margin. All the layers of the retina are
completed by the eighth month of fetal life.
The optic stalk is converted into the
optic nerve by the obliteration of its
cavity and the growth of nerve fibers into it. Most of these fibers are centripetal,
and grow backward into the optic stalk from the nerve cells of the retina, but a
few extend in the opposite direction and are derived from nerve cells in the brain.
The fibers of the optic nerve receive their medullary sheaths about the tenth week
after birth. The optic chiasma is formed
by the meeting and partial decussation of the fibers of the two optic nerves. Behind
the chiasma the fibers grow backward as the optic tracts to the thalami and mid-brain.
The crystalline lens is developed from the lens vesicle, which recedes within
the margin of the cup, and becomes separated from the overlying ectoderm by mesoderm.
The cells forming the posterior wall of the vesicle lengthen and are converted into
the lens fibers, which grow forward and fill up the cavity of the vesicle. The cells
forming the anterior wall retain their cellular character, and form the epithelium
on the anterior surface of the adult lens. By the second month the lens is invested
by a vascular mesodermal capsule, the capsula
vasculosa lentis; the bloodvessels supplying the posterior part of this capsule
are derived from the hyaloid artery; those for the anterior part from the anterior
ciliary arteries; the portion of the capsule which covers the front of the lens
is named the pupillary membrane. By the
sixth month all the vessels of the capsule are atrophied except the hyaloid artery,
which disappears during the ninth month; the position of this artery is indicated
in the adult by the hyaloid canal, which reaches from the optic disk to the posterior
surface of the lens. With the loss of its bloodvessels the capsula vasculosa lentis
disappears, but sometimes the pupillary membrane persists at birth, giving rise
to the condition termed congenital atresia
of the pupil.
The vitreous body is developed between the lens and the optic cup. The lens
rudiment and the optic vesicle are at first in contact with each other, but after
the closure of the lens vesicle and the formation of the optic cup the former withdraws
itself from the retinal layer of the cup; the two, however, remain connected by
a network of delicate protoplasmic processes. This network, derived partly from
the cells of the lens and partly from those of the retinal layer of the cup, constitutes
the primitive vitreous body. At first these protoplasmic processes spring from the
whole of the retinal layer of the cup, but later are limited to the ciliary region,
where by a process of condensation they appear to form the zonula ciliaris. The
mesoderm which enters the cup through the choroidal fissure and around the equator
of the lens becomes intimately united with this reticular tissue, and contributes
to form the vitreous body, which is therefore derived partly from the ectoderm and
partly from the mesoderm.
The anterior chamber of the eye appears as a cleft in the mesoderm separating
the lens from the overlying ectoderm. The layer of mesoderm in front of the cleft
forms the substantia propria of the cornea, that behind the cleft the stroma of
the iris and the pupillary membrane. The fibers of the ciliary muscle are derived
from the mesoderm, but those of the Sphincter and Dilatator pupillae are of ectodermal
origin, being developed from the cells of the pupillary part of the optic cup.
The sclera and choroid are derived
from the mesoderm surrounding the optic cup.
The eyelids are formed as small cutaneous folds, which about the middle
of the third month come together and unite in front of the cornea. They remain united
until about the end of the sixth month.
The lacrimal sac and nasolacrimal
duct result from a thickening of the ectoderm in the groove, nasoöptic furrow, between the lateral
nasal and maxillary processes. This thickening forms a solid cord of cells which
sinks into the mesoderm; during the third month the central cells of the cord break
down, and a lumen, the nasolacrimal duct, is established. The lacrimal ducts arise
as buds from the upper part of the cord of cells and secondarily establish openings
(puncta lacrimalia) on the margins of
the lids. The epithelium of the cornea
and conjunctiva, and that which lines the ducts and alveoli of the lacrimal gland,
are of ectodermal origin, as are also the eyelashes
and the lining cells of the glands which open on the lid-margins.
The Organ of Hearing (Organon Auditus; The Ear)
The ear, or organ of hearing,
is divisible into three parts: the external
ear, the middle ear or tympanic cavity, and the internal ear or labyrinth.
The Development of the Ear.—The first rudiment
of the internal ear appears shortly after that of the eye, in the form of a patch
of thickened ectoderm, the auditory plate,
over the region of the hind-brain. The auditory plate becomes depressed and converted
into the auditory pit. The mouth of the
pit is then closed, and thus a shut sac, the auditory vesicle, is formed, from it the epithelial lining of the membranous
labyrinth is derived. The vesicle becomes pear-shaped, and the neck of the flask
is obliterated. From the vesicle certain diverticula are given off which form the
various parts of the membranous labyrinth. One from the middle part forms the ductus
and saccus endolymphaticus, another from the anterior end gradually elongates, and,
forming a tube coiled on itself, becomes the cochlear duct, the vestibular extremity
of which is subsequently constricted to form the canalis reuniens. Three others
appear as disk-like evaginations on the surface of the vesicle; the central parts
of the walls of the disks coalesce and disappear, while the peripheral portions
persist to form the semicircular ducts; of these the superior is the first and the
lateral the last to be completed. The central part of the vesicle represents the
membranous vestibule, and is subdivided by a constriction into a smaller ventral
part, the saccule, and a larger dorsal and posterior part, the utricle. This subdivision
is effected by a fold which extends deeply into the proximal part of the ductus
endolymphaticus, with the result that the utricle and saccule ultimately communicate
with each other by means of a Y-shaped canal. The saccule opens into the cochlear
duct, through the canalis reuniens, and the semicircular ducts communicate with
the utricle.
The mesodermal tissue surrounding the
various parts of the epithelial labyrinth is converted into a cartilaginous ear-capsule,
and this is finally ossified to form the bony labyrinth. Between the cartilaginous
capsule and the epithelial structures is a stratum of mesodermal tissue which is
differentiated into three layers, viz., an outer, forming the periosteal lining
of the bony labyrinth; an inner, in direct contact with the epithelial structures;
and an intermediate, consisting of gelatinous tissue: by the absorption of this
latter tissue the perilymphatic spaces are developed. The modiolus and osseous spiral
lamina of the cochlea are not preformed in cartilage but are ossified directly from
connective tissue.
The middle ear and auditory tube
are developed from the first pharyngeal pouch. The entodermal lining of the dorsal
end of this pouch is in contact with the ectoderm of the corresponding pharyngeal
groove; by the extension of the mesoderm between these two layers the tympanic membrane
is formed. During the sixth or seventh month the tympanic antrum appears as an upward
and backward expansion of the tympanic cavity. With regard to the exact mode of
development of the ossicles of the middle ear there is some difference of opinion.
The view generally held is that the malleus
is developed from the proximal end of the mandibular (Meckel’s) cartilage, the incus in the proximal end of the mandibular
arch, and that the stapes is formed from
the proximal end of the hyoid arch. The malleus, with the exception of its anterior
process is ossified from a single center which appears near the neck of the bone;
the anterior process is ossified separately in membrane and joins the main part
of the bone about the sixth month of fetal life. The incus is ossified from one
center which appears in the upper part of its long crus and ultimately extends into
its lenticular process. The stapes first appears as a ring (annulus stapedius) encircling a small vessel,
the stapedial artery, which subsequently undergoes atrophy; it is ossified from
a single center which appears in its base.
The external acoustic meatus is developed from the first branchial groove.
The lower part of this groove extends inward as a funnel-shaped tube (primary meatus)
from which the cartilaginous portion and a small part of the roof of the osseous
portion of the meatus are developed. From the lower part of the funnel-shaped tube
an epithelial lamina extends downward and inward along the inferior wall of the
primitive tympanic cavity; by the splitting of this lamina the inner part of the
meatus (secondary meatus) is produced, while the inner portion of the lamina forms
the cutaneous stratum of the tympanic membrane. The auricula or pinna is developed
by the gradual differentiation of tubercles which appear around the margin of the
first branchial groove. The rudiment of the acoustic nerve appears about the end of the third week as a group of
ganglion cells closely applied to the cephalic edge of the auditory vesicle. Whether
these cells are derived from the ectoderm adjoining the auditory vesicle, or have
migrated from the wall of the neural tube, is as yet uncertain. The ganglion gradually
splits into two parts, the vestibular ganglion
and the spiral ganglion. The peripheral
branches of the vestibular ganglion pass in two divisions, the pars superior giving
rami to the superior ampulla of the superior semicircular duct, to the lateral ampulla
and to the utricle; and the pars inferior giving rami to the saccule and the posterior
ampulla. The proximal fibers of the vestibular ganglion form the vestibular nerve;
the proximal fibers of the spiral ganglion form the cochlear nerve.