Eye, the organ of sight is almost spherical in shape with a diameter of about 2.5 cm. In order to understand the diseases and conditions occurring in one’s eye, one must know the Anatomy of eye.
नेत्र शारीर:-
विद्याद् द्व्यङ्गुलबाहुल्यं स्वाङ्गुष्ठोदरसम्मितम् ।
द्व्यङ्गुलं सर्वतः सार्द्धं भिषङ्नयनबुदबुदम् । सुवृत्तं गोस्तनाकारं सर्वभूतगुणोद्भवम् ॥ (सु.उ. 1/10-11)
नयनबुद्बुद (अक्षिगोलक को अपने अङ्गुष्ठ के उदर (मध्य भाग) के प्रमाणानुसार दो अङ्गुल बाहुल्य के (अन्तःप्रवेश प्रमाण = अग्रपश्चात् व्यास) वाला जाने तथा आयाम और विस्तार (लम्बाई और चैड़ाई) में ढाई अङ्गुल प्रमाण जाने। इस तरह इस नेत्रगोलक के सुवृत्त (गोल) तथा गौ के स्तन के आकार।
दृष्टिश्चात्र तथा वक्ष्ये यथा ब्रूयाद्विशारदः ॥
नेत्रायामत्रिभागन्तु कृष्णमण्डलमुच्यते । कृष्णात् सप्तममिच्छन्ति दृष्टिं दृष्टिविशारदाः ॥ (सु.उ. 1/12-13)
नेत्र के आयाम (लम्बाई) का तृतीयांश अर्थात् एक तिहाई भाग कृष्णमण्डल कहा जाता है तथा कृष्णमण्डल का सातवाँ भाग दृष्टि होती है ऐसा नेत्ररोग विशारदों का कथन है ।
मण्डलानि च सन्धींश्च पटलानि च लोचने । यथाक्रमं विजानीयात् पञ्च षट् च षडेव च ॥ (सु.उ. 1/14)
नेत्र में मण्डल, सन्धियाँ और पटल यथाक्रम से 5, 6 और 6 होते हैं ।
नेत्र के मण्डल:-
पक्ष्मवर्त्मश्वेतकृष्णदृष्टीनां मण्डलानि तु । अनूपूर्वन्तु ते मध्याश्चत्वारोऽन्त्या यथोत्तरम् ॥ (सु.उ. 1/15)
पक्ष्म, वर्त्म, श्वेत, कृष्ण और दृष्टि इनके पाँच मण्डल होते हैं जैसे पक्ष्ममण्डल, वर्त्ममण्डल, श्वेतमण्डल, कृष्णमण्डल और दृष्टिमण्डल। उनमें से चार (वर्त्म, श्वेत, कृष्ण तथा दृष्टि) मण्डल पूर्व क्रम से मध्य में रहते हैं। अर्थात् सबसे बाहर वर्त्ममण्डल, उसके भीतर श्वेतमण्डल फिर उसके भीतर कृष्णमण्डल तत्पश्चात् उसके भीतर दृष्टिमण्डल होता है तथा वे ही चार मण्डल यथोत्तर क्रम से अन्त में रहते हैं। अर्थात् सबसे मध्य में दृष्टिमण्डल और उसके अन्त में कृष्णमण्डल, तत्पश्चात् श्वेतमण्डल और उसके भी अन्त में वर्त्ममण्डल होता है ।
- वात से कृष्ण मण्डल की उत्पति होती है, यह मातृज भाव है, रक्त के प्रसाद भाग से उत्पति, नेत्र का ⅓ आयाम का होता हैं।
- शुक्ल मण्डल की उत्पति कफ के प्रसाद भाग से होती है, मातृज भाव है
नेत्र की संधिया:-
पक्ष्मवत्र्मगतः सन्धिवत्र्मशुक्लगतोऽपरः ।
शुक्लकृष्णगतस्त्वन्यः कृष्णदृष्टिगतोऽपरः । ततः कनीनकगतः षष्ठश्चापाङ्गगः स्मृतः ॥ (सु.उ. 1/16)
सन्धियाँ 6 होती है :-
- पक्ष्म तथा वर्त्म की सन्धि,
- वर्त्म और शुक्ल की सन्धि,
- शुक्ल और कृष्णभाग की सन्धि,
- कृष्ण और दृष्टिभाग की सन्धि,
- कनीनकगत सन्धि तथा
- अपाङ्गगत सन्धि
नेत्र के पटल:-
द्वे वत्र्मपटले विद्याच्चत्वार्य्यन्यानि चाक्षिणि जायते तिमिरं येषु व्याधिः परमदारुणः ॥
तेजोजलाश्रितं बाह्यं तेष्वन्यत् पिशिताश्रितम् । मेदस्तृतीयं पटलमाश्रितन्त्वस्थि चापरम् ॥
पञ्चमांशसमं दृष्टेस्तेषां बाहुल्यमिष्यते॥ (सु.उ. 1/17-18)
नेत्र में 6 पटल होते हैं जिनमें दो वर्त्म पटल तथा चार पटल अक्षिगोलक में होते हैं। इन्हीं नेत्रगोलक के चार पटलों में अत्यन्त दारुण (दुःखदायक) ‘तिमिर’ नामक रोग होता है।
इन चार पटलों में से प्रथम बाह्यपटल तेज व जल के आश्रित है। दूसरा पटल मांस के आश्रित है। तृतीय पटल मेद के आश्रित तथा चौथा अस्थि के आश्रित है। इन चारों की स्थूलता (मोटाई) दृष्टि के पञ्चम भाग के बराबर है ।
नेत्र की सिरा, धमनियां व पेशी:-
- नेत्र मे 8 शिरायें होती है
- 2 धमनियां होती है अश्रुओं का वहन करती है
- 2 पेशियां होती है
मर्म :-
- 2 अपांग
- 2 आवर्त
- स्थपनी
- 4 श्रृंङ्गाटक
नेत्र का पंचभोतिकत्व :-
पलं भुवोऽग्नितो रक्तं वातात् कृष्णं सितं जलात् । आकाशादश्रुमार्गाश्च जायन्ते नेत्रबुदबुदे ॥
पृथिवी से मांसल भाग, अग्नि से पित्तरूप रक्तवर्ण का, वात से कृष्ण भाग, जल से नेत्रगत श्वेत भाग तथा आकाश नामक महाभूत से अश्रुमार्गों की उत्पत्ति होती है।
दृष्टि मसूरदल के समान, पंचमहाभूतों के प्रसाद भाग से बनी होती है
Anatomy of Eye:-
Sense of sight perceived through retina of the eyeball is one of the five special senses.
Its importance is obvious in the varied ways of natural protection. Bony orbit, projecting nose and various coats protect the precious retina. Each and every component of its three coats is assisting the retina to focus the light properly.
The eyeball is the organ of sight. It is almost spherical in shape and has a diameter of about 2.5 cm.
Eyeball is made up of three concentric coats.
- The outer or fibrous coat comprises the sclera and cornea.
- The middle or vascular coat also called the uveal tract consists of the choroid, the ciliary body and the iris.
- The inner or nervous coat is the retina.
Light entering the eyeball passes through several refracting media. From backwards, these are the cornea, the aqueous humour, the lens and the vitreous body.
Outer Coat:
Sclera –
The sclera is opaque and forms the posterior five-sixths of the eyeball. It is composed of dense fibrous tissue which is firm and maintains the shape of the eyeball. It is thickest behind, near the entrance of the optic nerve, and thinnest about 6 mm behind the Sclerocorneal junction where the recti muscles are inserted. However, it is weakest at the entrance of the optic nerve.
The sclera is continuous anteriorly with the cornea at the sclerocorneal junction or limbus. The deep part of the limbus contains a circular canal, known as the sinus venosus sclerae or the canal of Schlemm. The aqueous humour drains into the anterior scleral or ciliary veins through this sinus.
The sclera is fused posteriorly with the dural sheath of the optic nerve. It provides insertion to the extrinsic muscles of the eyeball. The recti in front of the equator, and the oblique muscles behind the equator.
Cornea –
The cornea is transparent. It replaces the sclera over the anterior one-sixth of the eyeball. Its junction with the sclera is called the sclerocorneal junction or limbus.
The cornea is more convex than the sclera, but the curvature diminishes with age. It is separated from the iris by a space called the anterior chamber of the eye.
The cornea is avascular and is nourished by lymph which circulates in the numerous corneal spaces and by the lacrimal fluid.
It is supplied by branches of the ophthalmic nerve and the short ciliary nerves (through the ciliary ganglion). Pain is the only sensation aroused from the cornea.
Middle Coat:
Choroid –
Choroid is a thin pigmented layer which separates the posterior part of the sclera from the retina. Anteriorly, it ends at the ora serrata by merging with the ciliary body. Posteriorly, it is perforated by the optic nerve to which it is firmly attached.
Its outer surface is separated from the sclera by the supra choroidal lamina which is traversed by the ciliary vessels and nerves. Its attachment to the sclera is loose, so that it can be easily stripped. The inner surface is firmly united to the retina.
Ciliary Body –
Ciliary body is a thickened part of the uveal tract lying just posterior to the corneal limbus. It is continuous anteriorly with the iris and posteriorly with the choroid. It suspends the lens and helps it in accommodation for near vision.
- The ciliary body is triangular in cross-section. It is thick in front and thin behind. The scleral surface of this body contains the ciliary muscle. The posterior part of the vitreous surface is smooth and black (pars plana). The anterior part is ridged anteriorly (pars plicata) to form about 70 ciliary processes. The central ends of the processes are free and rounded.
- Ciliary zonule is thickened vitreous membrane fitted to the posterior surfaces of ciliary processes. The posterior layer lines hyaloid fossa and anterior thick layer form the suspensory ligament of lens.
- The ciliary muscle is a ring of unstriped muscle which are longitudinal or meridional, radial and circular. The longitudinal or meridional fibres arise from a projection of sclera or scleral spur near the limbus. They radiate backwards to the supra choroidal lamina. The radial fibres are obliquely placed and get continuous with the circular fibres.
Iris –
- This is the anterior part of the uveal tract. It forms a circular curtain with an opening in the centre, called the Pupil. By adjusting the size of the pupil, it controls the amount of light entering the eye, and thus behaves like an adjustable diaphragm.
- It is placed vertically between the cornea and the lens, thus divides the anterior segment of the eye into anterior and posterior chambers, both containing aqueous humour. Its peripheral margin is attached to the middle of the anterior surface of the ciliary body and is separated from the cornea by the iridocorneal angle or angle of the anterior chamber. The central free margin forming the boundary of the pupil rests against the lens.
Inner Coat/ Retina:
- This is the thin, delicate inner layer of the eyeball. It is continuous posteriorly with the optic nerve. The outer surface of the retina (formed by pigment cells) is attached to the choroid, while the inner surface is in contact with the hyaloid membrane (of the vitreous). Opposite the entrance of the optic nerve (inferomedial to the posterior pole), there is a circular area known as the optic disc. It is 1.5 mm in diameter.
- The retina diminishes in thickness from behind forwards and is divided into optic, ciliary and iridial parts. The optic part of the retina contains nervous tissue and is sensitive to light. It extends from the optic disc to the posterior end of the ciliary body. The anterior margin of the optic part of the retina forms a wavy line called the ora serrata. Beyond the ora serrata, the retina is continued forwards as a thin, non-nervous insensitive layer that covers the ciliary body and iris, forming the ciliary and iridial parts of the retina. These parts are made up of two layers of epithelial cells.
- The rods and cones are the light receptors of the eye. The rods contain a pigment called visual purple. They can respond to dim light (scotopic vision). The periphery of the retina contains only rods, but the fovea has none at all. The cones respond only to bright light (photopic vision) and are sensitive to colour. The fovea centralis has only cones. Their number diminishes towards the periphery of the retina.
Aqueous Humour –
- This is a clear fluid which fills the space between the cornea in front and the lens behind the anterior segment.
- This space is divided by the iris into anterior and posterior chambers which freely communicate with each other through the pupil.
- The aqueous humour is secreted into the posterior chamber from the capillaries in the ciliary processes. It passes into the anterior chamber through the pupil. From the anterior chamber, it is drained into the anterior ciliary veins through the spaces of the iridocorneal angle or angle of anterior chamber (located between the fibres of the ligamentum pectinatum) and the canal of Schlemm.
- Interference with the drainage of the aqueous humour into the canal of Schlemm results in an increase of intraocular pressure (glaucoma).
- This produces cupping of the optic disc and pressure atrophy of th retina causing blindness. [The intraocular pressure is chiefly due to the aqueous humour which maintains the constancy of the optical dimensions of the eyeball.]
- The aqueous is rich in ascorbic acid, glucose and amino acids, and nourishes the avascular tissues of the cornea and lens.
Lens –
- The lens is a transparent biconvex structure which is placed between the anterior and posterior segments of the eye.
- It is circular in outline and has a diameter of 1 cm.
- The central points of the anterior and posterior surfaces are called the anterior and posterior poles. The line connecting the poles constitutes the axis of the lens, while the marginal circumference is termed the equator.
- The chief advantage of the lens is that it can vary its dioptric power. It contributes about 15 dioptres to the total of 58 dioptric power of the eye.
- The posterior surface of the lens is more convex than the anterior.
- The anterior surface is kept flattened by the tension of the suspensory ligament. When the ligament is relaxed by contraction of the ciliary muscle, the anterior surface becomes more convex due to elasticity of the lens substance.
Viterous Humour –
- It is a colourless, jelly-like transparent mass which fills the posterior segment (posterior four-fifths) of the eyeball.
- It is enclosed in a delicate homogeneous hyaloids membrane.
- Behind, it is attached to the optic disc, and in front to the ora serrata; in between it is free and lies in contact with the retina.
- The anterior surface of the vitreous body is indented by the lens and ciliary processes.
Blood Vessels and Nerves:-
Arteries –
Ophthalmic artery, a branch of internal carotid artery, constitutes the main source of blood supply for the eyeball and other orbital structures.
Veins –
Veins draining blood from the eyeball include:
- Central retinal vein which drains blood from the retina; and
- Anterior ciliary veins, short posterior ciliary veins and venae verticosae which drain blood from the uveal tissue.
Main venous channels which ultimately get tributaries from various orbital structures include:-
- Superior ophthalmic vein,
- Inferior ophthalmic vein,
- Middle ophthalmic vein,
- Medial ophthalmic vein,
- Angular vein, and
- Cavernous sinus.
Nerves –
Ophthalmic nerve, smallest of the three divisions of trigeminal (5th cranial) nerve, supplies the various ocular structures through its three branches:
- Lacrimal nerve= It lies in the lateral part of the orbit and supplies lacrimal gland, conjunctiva and lateral part of upper eyelid.
- Frontal nerve= It divides into two branches in the middle of orbit:-
- Supratrochlear nerve supplies the conjunctiva, middle part of upper eyelid, and skin of the forehead above the root of nose.
- Supraorbital nerve supplies the conjunctiva, central part of upper eyelid, and part of the skin of forehead and scalp.
- Nasociliary nerve= It has following branches:
- Long ciliary nerves two in number, pierce the sclera on either side of optic nerve, run forward between sclera and choroid and supply sensory nerves to the ciliary body, iris and cornea.
- Communicating branches to ciliary ganglion form its sensory root and their fibres pass along the short ciliary nerves, to supply the ciliary body, iris and cornea.
- Posterior ethmoidal nerve supplies the ethmoidal and sphenoidal air sinuses.
- Anterior ethmoidal nerve is a terminal branch of nasociliary nerve which leaves the orbit through the anterior ethmoidal foramen.
- Infratrochlear nerve is the other terminal branch of nasociliary nerve. It runs forward and supplies the conjunctiva, lacrimal sac, caruncle, and medial part of the eyelids.
Lymphatic drainage:-
Lymphatics draining the eyelids, conjunctiva and orbital tissues are arranged in two groups:
• Medial group of lymphatics drain into the submandibular lymph nodes, and
• Lateral group of lymphatics drain into the subauricular lymph nodes.