Anatomy of sensory organs

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 2.5 cm. antero-posteriorly and 1.25 cm. transversely at its widest part. The choanae are two oval openings each measuring 2.5 cm. in the vertical, and 1.25 cm. in the transverse direction in a well-developed adult skull.

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, 3 cm.; breadth, 2.5 cm.; depth from before backward, 2.5 cm. Each opens into the anterior part of the corresponding middle meatus of the nose through the frontonasal duct which traverses the anterior part of the labyrinth of the ethmoid. Absent at birth, they are generally fairly well developed between the seventh and eighth years, but only reach their full size after puberty.

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, 2.2 cm.; transverse breadth, 2 cm.; antero-posterior depth, 2.2 cm. When exceptionally large they may extend into the roots of the pterygoid processes or great wings, and may invade the basilar part of the occipital bone. Each sinus communicates with the sphenoethmoidal recess by means of an aperture in the upper part of its anterior wall. They are present as minute cavities at birth, but their main development takes place after puberty.

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 10 mm. below the level of the floor of the nose; projecting into the floor are several conical elevations corresponding with the roots of the first and second molar teeth, and in some cases the floor is perforated by one or more of these roots. The size of the sinus varies in different skulls, and even on the two sides of the same skull. The adult capacity varies from 9.5 c.c. to 20 c.c., average about 14.75 c.c. The following measurements are those of an average-sized sinus: vertical height opposite the first molar tooth, 3.75 cm.; transverse breadth, 2.5 cm.; antero-posterior depth, 3 cm. In the antero-superior part of its base is an opening through which it communicates with the lower part of the hiatus semilunaris; a second orifice is frequently seen in, or immediately behind, the hiatus. The maxillary sinus appears as a shallow groove on the medial surface of the bone about the fourth month of fetal life, but does not reach its full size until after the second dentition. At birth it measures about 7 mm. in the dorso-ventral direction and at twenty months about 20 mm.

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 3 mm. to the nasal side and a little below the level of the posterior pole. The bulb measures rather more in its transverse and antero-posterior diameters than in its vertical diameter, the former amounting to about 24 mm., the latter to about 23.5 mm.; in the female all three diameters are rather less than in the male; its antero-posterior diameter at birth is about 17.5 mm., and at puberty from 20 to 21 mm.

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.