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SYSTEMATIC ANATOMY

PART IV    SENSORY ORGANS

Chapter 1   Visual Organ

Section 1  Eyeball

I. Shape of Eyeball

II. Structure of Eyeball

Section 2   Accessory Organs of Eye

I.. Eyelids(or palpebra)

II. Conjunctiva

III. Lacrimal Apparatus

IV. Extraocular Muscles

V. Connective Tissue in the Orbit

Section 3  Blood Vessels and Nerves of Eye

I.. Arteries of Eye

II. Veins of Eye

III .Nerves of Eye

 

Chapter 2       Vestibulocochlear Organ

Section 1  General  Discription

Section 2    External Ear

. Auricle

. External Acoustic Meatus

. Tympanic Membrane

Section 3  Middle Ear

. Tympanic Cavity

. Auditory Tube (Eustachian Tube)

. Mastoid Cells

Section 4  Internal Ear

. Bony Labyrinth

. Membranous Labyrinth

. Conduction of Sound

. Internal Acoustic Meatus

PART IV    SENSORY ORGANS

Sensory organs include the receptors and their accessory organs. The receptors receive the stimulationfrom the external or internal environment of the body, and convert it into the nerve impulse According to the position of the receptors and the origin of the stimulation, the receptors may be divided into three kinds.

1.Exteroceptors  They lie in the skin, the mucous membrane of the nasal cavity ,paranasal sinuses and oral cavity, and the visual and auditory organs. They receive stimuli such as touch,pain, temperature, light and sound from the external environment.

       2. Proprioceptors They lie in the muscles, tendons, joints , ligaments and the internal ears, and receive stimuli about the movement of the limbs and trunk, and the stimulation about the equilibrium..

       3. Enteroceptors They lie in the wall of viscera, and blood vessels, and accept the physical and chemical stimulation from the internal environment.

 

Chapter 1   Visual Organ

The visual organ consists of eyeball and accessory organs of the eyeball. The eyeball can receive stimulation of light and transform it into the nerve impulse which is conducted through the visual pathways to the visual center in the brain. The accessory organs include the eyelid, conjunctiva lacrimal apparatus, extraocular muscles, the Connective tissue in the Orbit and so on.

 

 

Section 1  Eyeball

The eyeball lies in the anterior part of the orbit, embedded in the fat tissue of the orbit, and enclosed in a fascial sheath (capsule of Tenon) which separates it from the orbital muscles and fat.

I. Shape of Eyeball

The eyeball is about 2.5 cm in diameter and is spherical in shape, but it is not perfectly spherical, for the anterior part of it has a smaller radius of curvature than the rest of the eyeball. The central points of the anterior and posterior surfaces of  eyeball are known as the anterior and posterior poles. The line joining the two poles is the oculor axis , The line joining the center of the pupil and the fovea centralis of the retina is called optic axis. The optic axis and oculor axis decussate sharply. An imaginary line encircling the eyeball, midway between the anterior and posterior poles, is the equator.

                          FigIV-1

 

II. Structure of Eyeball

The eyeball comprises the wall of the eyeball and the contents enclosed by it (Fig4-1

I. wall of eyeball  It consists of three coats, from outside inwards, which are the fibrous tunic, the vascular tunic and the retina.

1) Fibrous tunic or external tunic  It consists of the cornea in front and the sclera behind.

(1) Cornea  It is the anterior 1/6 of the fibrous tunic of the eyeball . it is transparent and more highly curved , through which the light enters the eye. The cornea is a non-vascular(无血管)structure, but numerous sensory nerve terminals supply it, which  makes it very sensitive to pain and touch stimulation.

(2)Sclera  It is the posterior 5/6 of the outer coat of the eyeball which consists of dense fibrous connective tissue and is white in colour and opaque. It is continous with the cornea anteriorly and connected with the fibrous sheath of the optic nerve  behind.

The external surface of the sclera is in contact with the inner surface of the fascial sheath of eyeball, and the inner surface of it is loosely attached to the choroid  by some delicate pigmented areolar tissue, but firmly adherent to the ciliary body

The point where the optic nerve pierces the sclera is approximately 3mm to the nasal side of the posterior pole of the eyeball, and  presents the cribriform appearance and is named the cribriform plate of sclera, which is a weakened point of the sclera.

               

The junction of the sclera and the cornea is called limbus corneae  and , close to it ,there is a circular canal in the internal part of the sclera ,the sinus venosus sclerae  which drains the aqueous humour.

The sclera plays an important role for maintaining the form of the eyeball and protecting the contents within it.

2) Vascular tunic   It is the middle coat, and consists of the iris the ciliary body and the choroid (Fig.12-1). This layer contains numerous blood vessels and pigment cells and is brown in color. Its function is to provide the nutrition for the tissues inside the eyeball.

(1) Iris ( fig.-1- 2  It is the most anterior colored  part of the vascular tunicand is a circular perforated diaphragm with coronary position. Near its center is a round aperture named pupil which controls the amount of light reaching the retina . The free margin of the iris is called papillary margin  and its circumference is continous with the ciliary body.

 

                      FigIV-2 Iris and  pupil

The iris varies greatly in color, it is light blue in white , brown in Chinese and very dark brown in black people. The color of iris is determined by the number of melanophores ,  which contain melanin granules.

The iris contains two sets of smooth muscles. The circularly arranged muscle fibers form the sphincter pupillae  which lie in the papillary margin of the iris. The radially arranged (放射状排列) muscle fibers is called the dilator pupillae  which pass towards the periphery(周围) of the iris. The pupil is decreased in size by the sphincter pupillae and dilated(扩大)by the dilator pupillae. nerve supply: The parasympathetic nerve  innervates the sphincter and the dilator is supplied by the sympathetic nerve.

The iris incompletely divided the space between the cornea and the lens晶状体 into the two parts: the anterior chamber and  the posterior chamber. the peripheral part of the anterior chamber is called the iridocorneal angle or anterior chamber angle. The anterolateral wall of the angle is made up by trabeculae, which is called the trabecular reticulumThe space between trabeculae are termed the space of iridocorneal angle (Fontana's space), which is the very important path of aqueous humor circulation.

(2) Ciliary body  ( fig.-1- 3, -1- 4)   It lies at the inner surface of the junction between the cornea and sclera. It is divisible into the anterior part, ciliary proceees (consisting of a number of60-80radial ridges , and the posterior part ,the ciliary ring  which is more smooth. In the horizontal plane of the eyeball ,the ciliary body presents a triangule in shape, its apex runs posteriorly and continues with the choroids , and its base runs forwards in which the iris inserte .

The ciliary body contains involuntary muscles, the ciliary muscle  which are arranged into three groups, meridionale, radial and circular fibers. they can regulate the degree of the curvature of the lens for close and distal vision.

 The ciliary processes secretes the aqueous humor,which supply nutrition for the cornea and lens and to maintain the normal intraocular pressure.

(3) Choroid  It is the largest part of the middle coat and lies between the sclera and the retina(fig.-1-2,图谱183页图323). It extends as far forwards as the ora serrata  of the retina, and occupies posterior two thirds of the vascular tunic. it is thickest posteriorly, where it is pierced by the optic nerve, and become thinner as it approaches the ciliary body. It is connected with sclera by some delicate areolar tissue. the deep surface of the choroid is fast connected with a layer pigmented cells which belong to the retina.

The choroid contains rich pigment cel1s and vessels, and supplies the nutrition for outer layer of retina.

3) Retina  (the inner tunic )  It is the neural sensory stratum. It lines the internal surface of the vascular tunic, and is divided into three parts from behind forwards:  the optic part(pars optica)the ciliary part(Part ciliaris) and iridial part (part iridica) . The iridial and the ciliary parts are known as the blind part because of lacking light sensitive elements of the retina.

The optic part of retina lines the internal surface of the choroid, and extends from the optic disc to the ora serrata which is the irregular edge of the optic part at the posterior border of the ciliary body. It become thinner gradually from the optic disc to the ora serrata .

Near the center of the posterior part of the retina, there is an oval yellowish area, named the macula lutea (fig.-1-5), it contains a small central pit, the fovea centralis, where visual acuity is highest.

        About 3.5mm to the nasal side of the macula lutea, the optic nerve pierces the retina and forms the optic disc with a diameter of about 1.5mm, which is a light, whitish disk with a central flat depression. The center of the disc is pierced by the central artery and vein of the retina. The optic disc is insensitive to light , and termed theblind spot. On ophthalmoscopic examination the normal disc is seen to be pink during life.

The retina consists of two layers: an outer pigmented layer and the inner nervous layer, only in the region of papilla and the ora serrate are these two layers firmly grown together. hence the nervous layer is readily detached from the pigmented layer even during life and results in the retinodialysis.

The nervous layer can be divided into three cell layers: the outmost are the light-sensitive rods and cons (photoreceptors), the innermost are the ganglion cells whose axons form the optic nerve, and the middle layer are the bipolar cells which connect the rods and cons to the ganglion cellsfig.-1- 6.

          2. Contents of the eyeball

The contents of the eyeball include the aqueous humor, the lens and the vitreous body (fig.-1-2,图谱183页图323). They are all transparent and avascular, and with the cornea altogether form the refractive media . Each plays a part in refracting the light entering the eye.

1) Chamber of eye  It is the space between the cornea and the lens, it is filled with aqueous humor and is divided incompletely into the anterior and the posterior chambers by the iris. The anterior and the posterior chambers communicate through the pupil

2) Aqueous humor  It is colorless, transparent and watery fluid. It fills the chambers of the eye. It is formed by active transport and diffusion from the capillaries of the ciliary processes, from which it inters the posterior chamber, then it passes into the anterior chamber through pupil and escapes from the iridocorneal angle into sinus venosus sclerae fig.-1-3, 图谱183页图324through the space of iridcorneal angle, and finally, draines into the ophthalmic veins via anterior ciliary veins .

3) Lens   It lies between the iris and the vitreous body . It is a transparent and elastic, biconvex body lacking blood vessels and nerves, and the convexity of its anterior surface is less than that of its posterior surface.

A elastic membrane which surrounded the lens closely is known as the lens capsule. The 1ens is attached to the inner side of the ciliary processes by the ciliary zonule ( or suspensory ligament . The shape of the lens is maintained by the tension of the ciliary zonule. During near vision , the ciliary muscle contracts, it pulls the ciliary body anteromedially , relaxes the ciliary zonule, and allows the elastic lens to increase its convexity, thus shortening the focal length and bringing near objects into sharp focus on the retina . While during far vision, the condition is reversed.

The elasticity of the lens decrease with age and results in decrease of dioptric range(屈光力的降低), thus focusing on near objects becomes progressively more difficult, which is known as presbyopia . In the aged, the lens may also becomes opaque termed senile cataract.

4) Vitreous body  fig.-1- 2, 图谱183页图323  It is a colorless, transparent and jelly-like body which fills the posterior four-fifths of the eyeball(the vitreous chamber. The vitreous body is condensed superficially to form a transparent envelope, the vitreous membrane . A minuter canal runs through it from the optic disc to the posterior surface of the lens, and represents the remains of a branch of the central artery of the retina (hyaloid artery玻璃体动脉) which supplied the developing lens in the fetus(胎儿), but which subsequently disappeared.

The vitreous body abuts on  the retina, ciliary body and the lens which forms the deep hyaloid fossa on the anterior surface of the vitreous. The vitreous body plays the supporting role for the retina. The retinodialysis( may occur, when this supporting role is weakened.

 

Section 2   Accessory Organs of Eye

 

The accessory organs of the eye include the eyelids , conjunctiva, lacrimal apparatus , extraocular muscles  and the fasciae within orbit.

   I. Eyelids(or palpebra)

They consist of the upper and lower eyelids (fig.-1- 7). They meet at the medial and lateral angles of the eye. The gap space between the two eyelids is termed the palpebraI fissure, whose medial and lateral end are respectively termed the medial and lateral angle of the eye .the medial angle of the eye presents wider prolongation called lacrimal lacus surrounding the lacrimal caruncle . On the medial end of the margin of each eyelid, there is a small concial elevation termed the lacrimal papilla. A minute opening on the summit of the papilla is known as the lacrimal punctum (fig.-1- 8).

 

The eyelashes  are attached to the free edge of the eyelids. The ciliary glands(g1and of Zeis)are arranged in rows inside the free margin of each eyelid and open near the attachments of the eyelashes. Inflammation of these glands is known as the sty .

The eyelids consist of the following layers from without inwards( Fig.-1- l): skin, superfcial fscia, muscular layer, tarsus , and conjunctiva . The skin of the eyelids is very thin, the superfiacial fascia is loose and devoid  of fat, It allows the skin to move freely over the eyelids , and can become greatly swollen with fluid or blood after an injury.

 The muscular layer is the palpebral part of orbicularis oculi, which may close the palpebraI fissure when contracting. The upper eyelid is elevated by the levator palpebrae superioris whose origin is at the upper margin of the optic canal. Its tendon expands into a wide, thin sheet and  inserts into the skin and upper margin of the superior tarsus. The deep lamella of the levator palpebrae superioris contains the involuntary muscle fibers termed the superior tarsalis(also called Muller muscle), which is attached to the upper margin of  superior tarsus. The tarsi are formed by dense connective tissue which  extend from the free margins of the eyelids and stiffen them(fig.-1- 8). The tarsal glands  (of Meibom) are embedded in deep of tarsus, and open on the margin of eyelids. Their secretion ensures the airtight closure of the eyelids. A swelling of the eyelid because of the retained secretion of these glands is known as the chalazion .

              FigIV- Accessory Organs of Eye

II.  Conjunctiva

 The Conjunctiva  is a thin and transparent mucous membrane which covers the anterior surface of the eyeball (bulbar conjunctiva  and lies the deep surface of the eyelids (palpebral conjunctiva , The reflected parts of the conjunctiva from the superior and inferior eyelids onto the eyeball are respectively named the superior and inferior conjunctival fornices ( Fig.-1- l). The palpebral conjunctiva is firmly attached to the tarsi(睑板) but the bulbar conjunctiva is loosely attached to the sclera. The saccular space formed by the conjunctiva is named the conjunctival sac, which is closed when the eye is shut, it open to the exterior through the palpebral fissure and into the lacrimal sac through the canaliculi.

III. Lacrimal Apparatus

The Lacrima apparatus  consists of the lacrimal gland  and the lacrimal passage (Fig.-1- 8).

1. Lacrimal gland

 It lies in the lacrimal fossa of the orbit, and its excretory ducts open into the superior fornix of the conjunctiva sac. The secretion of lacrimal gland (tears )flows over the anterior surface of the eyeball, and collects eventually in the lacrimal lacus  of the medial angle of the eye to the lacrimal puncta and then enter the Lacrimal Passages. The tears contain special enzyme dissolving bacteria, help to remove the foreign material and moist the front surface of eyeball.

       2. Lacrimal Passages

       1) Lacrimal puncta   They lie on the summit of the lacrimal papilla which lie on the medial ends of the free margin of each eyelid. It leads into the lacrimal ductile.

       2) Lacrimal ductuli (canaliculi)  They first run upward or downward respectively and then turn to the medial at a right angle, join and open into the lacrimal sac. Each ductulus is about 1.0cm long.

    3) Lacrimal sac  It is lodged in the fossa for lacrimal sac situated in the anterior part of the medial wall of the orbit. The medial palpebral ligament lie anterior to its upper part. The superior end of the lacrimal sac is blind(盲端), and its inferior end continues with the nasolacrimal duct. The contraction of orbicularis oculi may dilate this sac, thus the tears is sucked into the lacrimal sac via the lacrimal ductu1i.

       4)  Nasolacrimal duct  It is a membranous canal about 18 mm long, It extends from the lower part of the lacrimal sac, and descends in the nasolacrimal canal to the anterior part of inferior nasal meatus.

 

IV. Extraocular Muscles

The extraocular muscles include the four rectus two oblique muscles and the levator palpebra superioris ( Fig.-1-1, -1-9).

I.  Four rectus muscles  They arise from a common tendinous ring which surround the optic canal. The suterior rectus passes anterolaterally above the optic nerve and the eyeball; the inferior rectus runs in the same direction below the optic nerve . The medial rectus runs forwards along the medial to the optic nerve, and the lateral rectus  passes anterolaterally lateral to the optic nerve. Their flat tendon are inserted into the sclera about 0.51cm from the margin of the cornea. The medial rectus turns the anterior pole of the eyeball medially and the lateral rectus turns it laterally, the superior rectus turns the eyeball superomedially, and the inferior rectus turns the eyeball inferomedially.

2. Two oblique muscles  The superior obliquus arises from the common fibrous ring, runs forwards along the upper part of the medial orbital wall, then it ends in a slender tendon which enters the trochlea formed by a ligamentous ring which is attached to the upper margin of the orbit and turns posterolaterally to pass between the superior rectus and the eyeball, and is inserted into the superior surface of the sclera behind the equator of the eyeball. Contraction of this muscle turns the anterior pole of the eyeball inferolatarally. The inferior obliquus arises from the floor of the orbit just lateral to the opening of nasolacrimal canal, and passes latera1ly and backwards and is inserted into lateral surface of the sclera behind the equator.  Its action is to turn the eyeball superolaterally.

3. Levator palpebrae superior It is a thin, triangular-shaped muscle. It arises from the inferior surface of the lesser wing of the sphenoid bone near the optic canal, its wide aponeurosis inserts into the skin and tarsal plate of the upper lid, it can elevate the superior eyelid.

The movement of the eyeball results from the cooperation of the extraocular muscles of both eyes. For example, when two eyes are looking upwards, the superior rectus and the inferior oblique muscle of two eyes must contract simultaneously; when two eyes are looking to the right side, the lateral rectus of the right eye and medial rectus of the left eye must contract simultaneously and their opponent muscles(拮抗肌) must be relaxed at the same time.

 

V. Connective Tissue in the Orbit

 

The orbital cavity contains a lot of fat tissues, named the adipose body of orbit. It fills the space around the organs within the orbit( Fig.-1-1), and plays a supporting and protective role for these organs. At the anterior margin of the orbit, the adipose body is bordered by the orbital septum which is stretched from the tarsi to the margins of the orbit. The eyeball is embedded in this adipose tissue but is separated from it by a thin membranous sac, named the sheath of eyeball ( or capsule of Tenon). This sheath envelops the eyeball but is deficient in front over the cornea. It is seperated from the eyeball by the episcleral space. the space contains some delicate, soft areolar tissue which allows the eyeball to rotate freely inside the sheath.

The extraocular muscles pierces the sheath of eyeball at the equator of the eyeball, and receives a covering sleeve from the sheath of eyeball, which fades out posteriorly in continuity with the epimysium.

 

Section 3  Blood Vessels and Nerves of Eye

I.Arteries of Eye

The eye is supplied by the ophthalmic artery(Fig. -1-10).It arises from the internal carotid artery inside the cranial cavity and enters the orbital cavity through the optic canal. In the orbital cavity, at first it lies below the optic nerve, then winds round the lateral side of the nerve, crosses above it to reach the medial wall of the orbit, and runs forwards below the superior obliquus ,and ends by dividing into the supratrochlear and dorsal nasal arteries. Its branches are as follows:

I. Central artery of retina  It is the first and most important branch of the ophthalmic artery (Fig. -1-5,10).It pierces the inferior surface of the optic nerve about 1.0-1.5cm behind eyeball, and runs forwards to the optic disc of the retina along the center of the optic nerve. In the optic disc, it divides into superior and inferior branches, each of these divides into a large temporal and a smaller nasal arteriole, which run on the internal surface of the retina to the ora serrata, and supply the internal lamina of the retina. The branches of the central artery of retina can be seen through the pupil by the ophthalmoscopy. If this artery occluded, blindness of the eye results.

2. Short posterior ciliary arteries(Fig. -1-11) They are severa1 in number and pass forwards around the optic nerve. They pierce the sclera close to the optic nerve and supply the choroid coat,so they are called choroidal arteries.

3.  Long posterior ciliary arteries or iridial arteries They are two in number. They pierce the posterior part of the sclera some distance from the optic nerve, and run forwards, on two sides of the eyeball, between the sclera and choroid, to reach the root of the iris, where they divide into two branches which anastomose with the anterior ciliary arteries to form the greater arterial circle of iris, from which radial vessels extend to the lesser arterial circlc around the pupil.

In addition, the ophthalmic artery also sends the anterior and posterior ethmoidal arteries the lacrimal artery, and muscular branches which send out the anterior ciliary arteries. these arteries distribute to the corresponding structures.

II. Veins of Eye

I. Ophthalmic veins  They are two in number, the superior and inferior, and are devoid of valves .

The superior ophthalmic vein It is formed behind the medial part of the upper eyelid and communicates anterior1y with the facial and supraorbital veins. It runs with the ophthalmic artery in the orbit. inferior ophthalmic vein  is smaller, and begins from a venous network near the anterior part of the floor of the orbit and runs backwards below the optic nerve. Both veins receive numerous tributaries in the orbit, and passing through the superior orbital fissure, open into the cavernous sinus either separately or by a common trunk.

2. Central vein of retina  It runs with corresponding artery and ends in the superior ophthalmic vein.

       3. Vorticose veins  The veins of the iris, ciliary body and choroid converge to form 4-5 vorticose vein which end in the superior and inferior ophthalmic veins (Fig. -1-11).

 

III .Nerves of Eye

The nerves of eye include optic nerve, oculomotor nerve, trochlear nerve, abducent nerve, Ophthalamic nerve, sympathetic and parasympathetic nerves. These nerves will be described in nervous system.

 

Bi Yushun: Shandong University School of Medicine