Anatomical considerations

ANATOMICAL CONSIDERATION

  • 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
  • The eyeball and its surroundings:
    • The eyeball (Oculus bulbi) is situated within the bony cavity known as the orbit and is protected by the eyelids anteriorly. It is surrounded by muscles and a thick padding of retroubular fat posteriorly.
    • The bony orbital rim is complete in some species. The term closed orbits is used when the bony orbital rim surrounding the eyeball is complete. Closed orbit is seen in man, horse, cattle and camel. An open orbit is an orbit with the bony rim incomplete so that part of it is made up of a fibrous ligament. Open orbit is seen in cat, elephant, pig, dog and birds.
    • The anterior segment of the eye is the portion of the eye between the cornea and the lens consisting of the eyelids,, conjunctiva, cornea, iris and pupil, and the anterior capsule of the lens. Posterior segment of the eye is from the lens backwards , namely vitreous and retina .

Eyelids

  • The borders of the two eyelids contain the eye lashes. The third eyelid (membrane nictitans) is a piece of elastic cartilaginous structure situated at the medial canthus of the eye.
  • The deep part of it is embedded in the retroubulbar fat. When the eyeball is forcefully retracted, the resulting pressure in the retrobulbar fat pushes the third eyelid forwards to cover the eye more or less completely, e.g., protrusion of membrane nictitans seen in tetanus due to the contraction of the retractor oculi muscle.

Conjunctiva

  • The conjunctiva has two parts, the palpebral conjunctiva lining the inner surface of the eyelid and the bulbar conjunctiva attached to the eyeball.
  • The epithelial lining of the conjunctiva is continuous with the epithelial lining of the cornea.

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Lacrimal gland

  • It lies in a depression beneath the supra orbital process and secretes tears into the conjunctival fornix by means of small openings. There are numerous accessory lacrimal glands in the conjunctiva.
  • The pre corneal tear film lubricate the epithelial surfaces of the cornea and conjunctiva. It has antibacterial properties and is also concerned with the nutrition of the cornea.
  • The excess tears is drained through the two puncta lacrimalia situated at the inner canthus of the eye into the lacrimal duct.
  • The conjunctival epithelium is continuous with the epithelium of the lacrimal canal and epithelial lining of the cornea. The patency of the lacrimal canal can be tested by instilling a 2% solution of Fluorescein.
  • Nictitans gland (Harderian gland): Resembles lacrimal gland. Situated on the inner surface of the third eyelid close to its outer border.
  • Tarsal glands: These are modified sebaceous glands situated within the tarsal plate. The ducts of these glands open along the free border of the eyelid.

Refractive media of the eye

  • Cornea, aqueous humour, lens and vitreous humour.
  • Refractive surfaces of the eye: 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.

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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 epithelium 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. They are:
    • 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.

Nerve supply

  • Motor nerve supply to the muscles of the eyeball: (3rd,4th,6th cranial nerves)
    • All the muscles except three (the superior oblique, posterior rectus and external rectus), are supplied by the oculomotor nerve.
    • The superior oblique muscle is supplied by the fourth cranial or the trochlear nerve, the posterior and external recti muscles are supplied by the abducent or sixth cranial nerve (SOFT PEAS)
  • Sensory nerve supply to the eye
    • It  derives from the ophthalmic and nasociliary branches.

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Blood supply

  • Blood supply to the eye is from the ophthalmic arteries and veins.

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 surface 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.
Last modified: Thursday, 22 September 2011, 9:40 AM