The simplest eye structure can detect the light and shade of the surrounding environment, and the more complex eye structure can provide vision (compound eyes usually exist in arthropods, such as insects, and usually consist of many simple small eyes and produce an image).
In many vertebrates and some mollusks, the eyes image by projecting light to the photosensitive retina, where the light is received and converted into signals and transmitted to the brain through the optic nerve. Usually the eyes are spherical, filled with transparent gel-like substances, with a lens for focusing and usually an iris to control the amount of light entering the eyes.
The eye is the sensory organ of vision, including eyeball and its appendages. Although the surface area and volume of eyes are small, their functions are very important to life and labor. The eyes are a part of the body, and many systemic diseases can be manifested in the eyes. In order to "capture" the information of light, the eye must be exposed to the body surface, which increases its chances of being attacked by trauma and external pathogens. Eye diseases will eventually affect visual function. The decline of vision not only makes patients suffer, but also brings misfortune to family and society, so the study of ophthalmology is of great significance.
The eye consists of eyeball, orbit, eyelid, conjunctiva, lacrimal apparatus and extraocular muscles.
The eyeball is a spherical organ, which is divided into two parts: the wall of the eyeball and the contents of the eye. The eyeball wall is divided into outer layer, middle layer and inner layer.
The outer layer is called fibrous membrane, including cornea and sclera, which is a connective tissue interwoven by dense collagen fibers and elastic fibers, and the shape of eyeball is determined by this layer.
The sclera is a white spherical membrane with a thickness of 0.3 ~ 1.0 mm and a diameter of 24mm, and there is a hole with a diameter of 1 1mm in front for the cornea to be embedded. An annular Schlemm's canal is buried in the sclera near the hole edge, which is the conduit for aqueous humor outflow. The diameter of the sclera on the nasal side of the posterior pole is 1.5. There are four vortex veins obliquely passing through the sclera wall behind the sclera equator. The posterior ciliary artery and nerve penetrate into the eye at the posterior pole of sclera. Six extraocular muscle tendons are attached to the scleral wall. The sclera wall is also wrapped with the capsule of eyeball fascia, which plays the role of synovium and is beneficial to eyeball rotation.
The cornea is a transparent membrane with a thickness of 0.5 ~ 1.0 mm and a diameter of 1 1mm, which is embedded in the circular hole in front of the sclera. Its overlapping part is a gray translucent ring with a width of 0.3 ~ 1.5 mm, which is called corneal limbus. The transverse diameter of the exposed cornea was 165438+. Its central part has a radius of curvature of 8mm, and its peripheral part is relatively flat. The refractive index of cornea is 1.376, and its interface with air has a diopter of +43D, which is the main refractive medium of eyeball.
Cornea is divided into five layers, the surface is epithelial layer, which is non-keratinized stratified squamous epithelium, and there are rich nerve endings between the epithelium, which is the most sensitive part of body pain and touch. The epithelial surface is covered with a tear film to maintain its moisturizing and respiratory functions. There are still wandering histiocytes, macrophages and lymphocytes in the epithelial layer as the body surface defense system. Below the epithelial layer is the anterior elastic layer, or Bowman's membrane, which is a thickened collagen fiber layer with a thickness of 8 ~14 μ m. It has no regenerative ability and will leave scars once it is destroyed. The third layer of cornea is stroma layer, which accounts for 90% of corneal thickness. It consists of highly differentiated corneal cells, collagen and matrix containing mucin and glycoprotein. They are evenly arranged to make the cornea highly transparent. The back elastic layer, or Dessemer membrane, is attached to the back of the matrix layer, which is the basement membrane of endothelial cells, about 10 ~ 12 micron, and has strong mechanical strength. The back of the cornea is covered with a monolayer of endothelial cells, which has the function of actively transferring ions, continuously pumping water molecules in the corneal stroma layer into the anterior chamber, and keeping the corneal stroma dehydrated and transparent.
The middle layer of the eye wall is the grape membrane, named after its rich blood vessels and deep pigment, such as a peeled purple grape. The uvea is divided into three parts: iris, ciliary body and choroid.
Iris, an ancient money-shaped membrane tissue, is attached to the peripheral ciliary body, and the central circular hole is the pupil. The color of the iris depends on the pigment content of the individual. The iris of black and yellow people is dark brown, and the iris of white people can be light gray, green blue to khaki. There are many depressions with different depths, sizes and shapes on the front of the iris, and the back is lined with dark black pigment epithelium, which can extend to the edge of the pupil and is black lace-like. Pupillary sphincter is arranged in a ring shape in the iris parenchyma, which is dominated by parasympathetic nerve fibers in the oculomotor nerve, and the pupil shrinks when it contracts. The pupil dilator is located in front of the pigment epithelium and is innervated by sympathetic nerves from the internal carotid artery wall. When contracting, the pupil will expand. The size of the pupil is adjusted by nerve reflex, which can control the intensity of light entering the eye. Iris and lens form iris-lens diaphragm. Separate the anterior chamber from the posterior chamber and vitreous body.
The ciliary body, located behind the iris root, is an annular tissue with a width of 6 ~ 7 mm, which is attached to the inner surface of the posterior sclera of the corneal limbus. There are 70 ~ 80 ciliary processes in front, and the zonules of the lens attach to the ciliary epithelium between the ciliary processes. The posterior part is the flat part of ciliary body, which is 4mm wide and bounded by the serrated edge of choroid. There is smooth muscle in the ciliary body, and its longitudinal fibers are in the outermost layer, and the front end is attached to the spinous process of sclera. Radial and annular fibers are located in longitudinal fibers, which are controlled by parasympathetic nerves. When the muscle fibers contract, the ciliary body folds move forward and toward the central axis, and the suspensory ligament of the lens relaxes. The inside of the ciliary body is lined with two layers of epithelium, namely the ciliary body of the retina. The innermost layer is the pigment-free epithelium, which is the part that produces aqueous humor.
Choroid, attached to the inner surface of most sclera, with an average thickness of 0.25 mm, the outermost layer is the great vascular layer, which is mainly supplied by the posterior ciliary artery, and venous blood is drained by the vortex vein. The middle layer is the vascular middle layer, and the inner layer is the capillary layer. These blood vessels are lobulated. The main function of choroid is to nourish the outer layer of retina, and its blood flow is rich. The oxygen content of venous blood is only 2 ~ 3% lower than that of arterial blood. There are a large number of dendritic melanocytes in choroidal perivascular matrix, which makes the posterior segment of eyeball a dark room to play the visual function of retina. There is a glass membrane (bruch membrane) between choroid and retinal pigment epithelium, which consists of choroidal capillary basement membrane, collagen, elastic fibers and retinal pigment epithelium basement membrane.
The retina, located in the inner layer of the ocular wall, starts from the serrated edge and ends at the optic papilla, and is divided into outer pigment epithelium and inner nerve sensory layer (which can be divided into 9 layers). Pigment epithelium is composed of single-layer cubic cells, which contain pigment and have microvilli at the top. It can swallow and remove the disc membrane of the outer segment of pyramidal rod cells, and the combination of cells constitutes the barrier between choroid and retina. There is a potential gap between pigment epithelium and nerve sensory layer. In some cases, these two parts can be separated to form retinal detachment. The retinal nerve sensory layer consists of three layers of neurons and glial cells. The outermost photoreceptor cells are highly differentiated neuroepithelial cells, called cone cells and rod cells. Their nuclei are located in the outer nuclear layer. Each photoreceptor cell has a terminal foot, an inner segment, a connecting portion, an outer segment, and the like. The outer segment consists of about 65,438+0,000 disks, which are photosensitive micro-organs, and they are adhered to each other by mucopolysaccharide substrate and microvilli of pigment epithelium. The second neuron is bipolar cell and horizontal cell. It receives nerve impulses from photoreceptor cells through axons and axons located in the outer plexiform layer, carries out preliminary analysis and synthesis, and then transmits signals to ganglion cells located in the inner plexiform layer through synapses. Ganglion cells have long axons, which form nerve fiber layers on the surface of retina, concentrate on the scleral cribriform plate on the nasal side of the posterior pole of eyeball, and pass through the eyeball to form optic nerve. The innermost layer of the retina is the internal limiting membrane.
Optic nerve is a nerve bundle with a diameter of 2mm, and its surface is wrapped by dura mater, arachnoid membrane and pia mater. These sheaths are continuous with the intracranial meninges of the same name. The intraorbital segment of the optic nerve is S-shaped. After penetrating into cranial cavity from optic foramen, optic chiasma is formed in front of pituitary fossa. The nasal fiber crosses the contralateral side, merges with the temporal fiber of the contralateral optic nerve to form an optic nerve pathway, and ends in the lateral geniculate body and the midbrain tetrad. The glial cells in the retina are mainly Miller cells, and their nuclei are located in the inner nuclear layer. As the support of the retina, cell processes are filled between the whole retinal neurons, and condense into the inner limiting membrane of the retina on the inner surface, and form the outer membrane of the retina on the plane equivalent to the inner segment of photoreceptor cells.
The inner layer of retina is mainly supplied by the central retinal artery and accompanying veins, and some areas of a few people are supplied by the ciliary retinal artery, which is a branch of the ophthalmic artery. Blood vessels in the retina and brain lack collateral circulation, and if blood vessels block their distribution areas, ischemia will occur. The arterioles on the retina lack elastic layer and continuous muscle layer, and their regulation is not dominated by sympathetic nerves, forming a unique autonomic system. There is a special macular area in the posterior pole of retina, where cone cells are highly dense, lacking inner core layer and ganglion cells, retinal blood vessels terminate around it, pigment epithelial cells contain more pigment, while photoreceptor layer contains lutein, which is yellow under green light irradiation, hence the name. This is the most acute part of vision. There is an optic disc (optic papilla) with a diameter of 1.5mm on the nasal side of the macula, and a funnel-shaped depression (called physiological depression) in the center. It is the starting point of the optic nerve, from which the central retinal artery and vein enter the eye. There are only nerve fibers and no photoreceptor cells, so there is no visual function, which constitutes a blind area of vision (physiological blind area).
Eye contents include lens, aqueous humor and vitreous body.
The lens is a biconvex transparent tissue, with a thickness of 4 ~ 5 mm and a diameter of 9 ~ 10 mm, which is wrapped by a capsule, and there is a layer of epithelial cells under the anterior capsule. From the embryonic stage, it has been proliferating to the peripheral part. Epithelium transferred to the equatorial part of the lens has been proliferating and elongating, forming lens fibers and overlapping on the surface of fibrils. This growth lasts for life, so the central part of the lens fiber is the oldest and has high density, which is called lens nucleus, and the peripheral part is called lens cortex. The lens hangs behind the pupil through the small ligament. When the ciliary muscle contracts, the small ligaments relax, the lens thickens by its own elasticity, the curvature of the front and rear surfaces increases, and the overall diopter increases, which is beneficial to see the near objects clearly. This is called accommodation.
Aqueous humor is a kind of transparent body fluid, which is produced by ciliary epithelium and enters the posterior chamber first. The posterior chamber is the space between the back of iris, ciliary body and lens periphery, which is filled with aqueous humor. The aqueous humor in the posterior chamber flows into the anterior chamber through the pupillary space. The anterior chamber is the space between the posterior cornea, iris and anterior lens, and it is also filled with aqueous humor. The intersection of the posterior cornea and the anterior iris is called the anterior chamber angle. The anterior chamber angle is filled with annular trabecular meshwork, which is composed of porous thin plate made of collagen fiber as scaffold and covered with endothelial cells. Anterior aqueous humor flows into Schlemm's canal through the gap of the grid, through the endocytosis of endothelial cells, and then flows out of the eyeball through the collecting duct and into the scleral venous plexus. The motive force of aqueous humor (mainly produced by the enzyme system of ciliary epithelium) and the resistance in the outflow channel make the eyeball produce certain pressure, which is called intraocular pressure, which is extremely important for the development and shape maintenance of the eyeball. Normal intraocular pressure is 2 ~ 2.8 kPa, and aqueous humor is extremely important to maintain the metabolism of avascular lens.
Vitreous body is a kind of transparent colloidal tissue, which fills the space in the retina, accounting for about 4/5 of the eyeball volume, about 4.0ml. The water in vitreous body is crosslinked with mucopolysaccharide and hyaluronic acid molecules to form a viscous and elastic colloid, which has the functions of protecting retina and buffering vibration. Vitreous body, retina and ciliary body adhere closely to the front and back of serrated edge, which is called the basal part of vitreous body. There is an S-shaped tube in the center of vitreous body, called crockett tube, which is the remains of primary vitreous body and vitreous blood vessels.
The orbit is a conical cavity with an outward opening, and the bone at the edge of the opening is thick and firm, which is called orbital margin. The superior orbital wall is separated from the anterior cranial fossa, the medial wall and the inferior orbit are adjacent to the paranasal sinuses, and the lateral wall is protected by temporal muscle and zygomatic arch. There is a dense fibrous membrane at the anterior orbital opening to form the orbital septum, which is the basis of eyelid. The posterior part of the orbit is called the orbital apex, and there are optic foramen and supraorbital fissure communicating with the cranial cavity. The infraorbital fissure communicates with the sphenomandibular fossa. All nerves and blood vessels in the orbit enter through these holes and seams.
Eyelid, commonly known as eyelid, covers the soft tissue at the opening of orbit and is divided into upper and lower parts by horizontal eyelid fissure. With ptosis, the inner canthus is blunt and the outer canthus is sharp.
Conjunctiva is a thin and transparent mucosa, which combines eyelid and eyeball, and consists of non-keratinized epithelium and underlying lamina propria. The part covering the anterior sclera surface is called bulbar conjunctiva, which starts from the corneal limbus and ends at the eyelid limbus. Both have loose transition parts, called domes. The whole conjunctiva and cornea form a "bag" with an opening at the palpebral fissure, which is called conjunctival sac. There is a semilunar fold on the inner canthus conjunctiva, which is called semilunar fold, and it is the relic of the third eyelid of lower animals. Its lower part has an oval protuberance called lacrimal caruncle, and its surface is covered with squamous epithelium, without keratinization layer, with sebaceous glands and fine hairs.
The lacrimal apparatus is divided into lacrimal gland and lacrimal passage. Lacrimal gland is located in the superior orbital fossa, and is divided into orbital part and small eyelid part by levator palpebrae aponeurosis. It is an exocrine gland, controlled by nerve reflex, secretes serous tears, and the secretory duct is at the opening of the superior temporal vault of the syndesmosis. There are accessory lacrimal glands in the eyelid, which are controlled by sympathetic nerves and provide basic tear secretion. Tears are secreted in the combined membrane capsule, which plays a role in wetting and lubricating. The lacrimal passage consists of lacrimal punctum, lacrimal canaliculus, lacrimal sac and nasolacrimal duct. The lacrimal punctum is located at the inner canthus end of the upper and lower eyelid margin. Due to the siphon effect of lacrimal canaliculus and the negative pressure generated when squeezing the lacrimal sac, the lacrimal punctum enters the lacrimal canaliculus sliding in the eyelid margin of the inner canthus from the lacrimal punctum, and then enters the lacrimal sac through the common lacrimal canaliculus. The lacrimal sac is located between the anterior and posterior attachment edges of the medial canthus tendon, passes down through the membranous nasolacrimal duct located in the bony nasolacrimal duct, and communicates with the inferior nasal passage. Under normal circumstances, some tears evaporate in the conjunctival sac, and the rest are discharged into the inferior nasal passage through the lacrimal passage. Only when the reflective secretion of tears increases or the lacrimal passage is blocked, tears will overflow from the eyelid fissure.
Extraocular muscles are skeletal muscles, and each eye has six muscles. The medial rectus muscle, the inferior rectus muscle, the lateral rectus muscle, the superior rectus muscle and the superior oblique muscle start from the muscle ring at the orbital apex, and the tendon ends of the four rectus muscles are attached to the sclera, and the distances between their attachment edges and the corneal limbus are 5, 6, 7 and 8mm respectively. The superior oblique muscle starts from the orbital apex, goes forward above the orbital nose, hangs on the pulley of the orbital wall through a ligament, then everts backward, passes through the equatorial part of the eyeball under the abdomen of the superior rectus muscle, and attaches to the scleral wall. The inferior oblique muscle starts from the bone wall near the orbital margin of the naso-orbital floor, and its abdominal muscle inclines backward and outward, ending in the sclera on the lower temporal side of the eyeball after the equator. The medial inferior rectus muscle, superior rectus muscle and inferior oblique muscle are innervated by oculomotor nerve, the lateral rectus muscle is innervated by abducens nerve, and the superior oblique muscle is innervated by trochlear nerve. Their coordinated contraction enables the eyeball to rotate at will.
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