Anatomy of Eye
The eye is the organ of vision. Anatomy of Eye comprises of the eyeball, optic nerve and certain associated structures as orbit, orbital fasciae, muscles, eyelids, conjunctiva and lacrimal apparatus.
The Science dealing with the structure, functions and diseases of the eye is known as Ophthalmology.
The powers of vision and the adaptation of lenses or prisms for the aid of vision is Optometry.
Orthoptics is the treatment of defective visual habits, defects of binocular vision, defects of ocular motivity, etc., by training
Eyelids (palpebrae)
The eyelids or palpebrae are two movable folds of skin situated in front of the eyeball and when closed, they cover the entrance into the orbit.
The upper eyelid is more extensive and movable then the lower eyelid. The interval between the upper and lower eyelids is the palpebral fissure.
It is an oblique slit and the ends are the medial and lateral canthus.
The lateral angle is rounded but the medial one is narrow and drawn into V-shaped recess, the lacrimal recess or lake.
In this lake, there is a round pigmented body called the caruncula lacrimalis.
The external surface of the eyelid is convex and is covered with short hairs.
The mucosal surface is concave, lined by the palpebral conjunctiva and is adapted to the anterior face of the eyeball.
The free border is smooth, black and has a well defined posterior margin where the ducts of tarsal glands open and the anterior margin bears short stiff hairs-the cilia or eye lashes. These are more in the upper than in the lower lid.
The edge of each lid is pierced near its medial angle by a small opening on either side of the caruncle – called puncta lacrimalis that are the entrances into the lacrimal ducts.
The skin is freely movable except at the free edge.
It consists of subcutis with no fat, palpebral muscles, palpebral ligament, fibrous layer constituting the tarsus and palpebral conjunctiva.
The tarsal or Meibomian glands are arranged in parallel rows between tarus and conjunctiva and secrete the palpebral sebum.
Each gland has an independent duct and opening.
Conjunctiva
Conjunctiva of the eye is the mucous membrane, which lines the internal or posterior surface of the eyelids – palpebral conjunctiva.
It is light pink in colour and is reflected on the anterior part of the eyeball as bulbar conjunctiva, the line of reflection being the fornix.
The dorsolateral part of the fornix is pierced by the excretory ducts of the lacrimal gland about 12 to 16 in number.
When the lids are in opposition, the capillary space between the two parts of the conjunctiva constitutes the conjunctival sac.
Palpebral conjunctiva is continuous with the skin of the eyelid at the ciliary margin.
The third eyelid is at the medial angle.
It consists of a semilunar fold of conjunctiva – the membrana nictitatans that covers and encloses a curved plate of hyaline cartilage.
The cartilage is leaf or shovel shaped in outline.
That part which lies in the membrane is thin and wide. The deep part is narrower and thicker and is embedded in fat at the medial side of the eyeball.
The deep part of the cartilage is surrounded by the superficial gland of the third eyelid. It resembles lacrimal gland in structure.
Beneath the third eyelid is a mixed gland called the gland of the third eyelid or Harderian gland.
Blood Supply : Ophthalmic and facial arteries.
Nerve Supply: Ophthalmic and maxillary divisions of the fifth cranial nerve (sensory nerve) and facial nerve.
Lacrimal apparatus
Lacrimal apparatus consists of lacrimal gland, excretory ducts of the gland, two lacrimal ducts or canaliculi, lacrimal sac and nasolacrimal duct.
The lacrimal gland is situated on the dorsolateral surface of the eyeball under the supraorbital process.
It is flattened oval and its superficial face is convex and distinctly lobulated. It resembles the parotid gland in structure.
Its deep face is concave and is separated from the eyeball by the periorbita.
The excretory ducts are 6 to 8 large ones and several small ones and these open into the dorso-lateral part of the fornix conjunctivae.
The puncta lacrimalia are the entrances into the lacrimal ducts that begin here and converge at the medial angle to form a sac, the lacrimal sac that is placed in the lacrimal fossa of the lacrimal bone.
It leads into the nasolacrimal duct that opens into the nasal cavity. The orifice is on internal aspect of the fold of the ventral turbinate and hence not visible externally as in the horse.
Blood Supply: Lacrimal artery
Nerve Supply: Lacrimal nerve (sensory) and secretary fibres from autonomic nervous system.
Refractive media of the eye
Cornea, aqueous humour, lens and vitreous humour are the Refractive media of the eye.
Refractive surfaces of the eye are Anterior surface of the cornea, anterior surface of the lens and posterior surface of lens.
Tunics of the eye
The three coats (tunics) of the eye are-
- The tunica fibrosa (external fibrous tunic) comprising the sclera and cornea.
- The vascular tunic or uvea, consisting of the choroid, ciliary body and iris which provides nourishment to the eyeball, and
- The tunica interna (inner layer) formed by expansion of the optic nerve. Also called the nervous tunic or retina.
Structure of the cornea
A section of the cornea reveals the following histological structure from before backwards: Anterior epithelium, Bowman’s membrane, corneal substance or substantia propria or stroma, Decemet’s membrane and endothelium (ABCDE).
The anterior epitheliums of the cornea and of conjunctiva are continuous with each other.
The precorneal tear film is considered as the physiological anterior most layer of the cornea.
Muscles of the eyeball
There are five straight and two oblique muscles for the eyeball-
- Superior rectus, (retractor oculi)
- Inferior rectus
- External rectus
- Internal rectus
- Posterior rectus (retractor oculi)
- Superior oblique and inferior oblique muscles
All the straight muscles originate around the optic foramen and are inserted to the sclera immediately behind the attachment of the bulbar conjunctiva.
Posterior rectus or retractor oculi muscle is absent in man.
Iris
Iris is a muscular diaphragm between the cornea and the lens, with an opening in its centre. The opening is called the pupil. The pupil of horse, cattle and sheep are horizontally elliptical in shape. In the foal below five years it is more round; in man, dog, monkey and most birds it is circular; and in cat and fox it is vertically elliptical.
The anterior surfaces of the iris as well as the posterior surface of the cornea are covered by endothelium. The posterior surface of the iris is continuous with the pigment layer of retina.
Corpora nigra are small black bodies seen on the papillary border from 11’o clock to 1’o clock angle in equines and is supposed to protect the retina from sun rays while grazing.
Anterior chamber of the eye is the space between the iris and cornea.
Posterior chamber of the eye is the space between the iris and the lens.
Aqueous humour is the clear fluid filling the entire space between the cornea and lens, i.e., the anterior chamber and posterior chamber
The lens is kept in position by the suspensory ligament of the lens. The lens has anterior and posterior poles.
