ANATOMY AND PHYSIOLOGY

The orbit is a pear shaped cavity and consist of various bones. 

The roof is formed with frontal bone. 

Lateral wall is formed by 

1. Frontal

2. Zygomatic bone. 

The floor is formed by 

1. Zygomatic, 

2. Maxillary 

3. Palatine bone.

while the medial wall is formed by 

1. Maxillary, 

2. Lacrimal, 

3. Ethmoidal 

4. And sphenoid bone.

It protects eye ball from external injuries.

Roof separates orbit from anterior cranial fossa. 

Floor separates orbit from maxillary sinus. 

Medial wall separates it from ethmoidal sinus.

Question: What is importance of bony relationships of bony orbit

Tears are secreted from lacrimal glands. The lacrimal gland has two parts. The orbital and palpebral.The ducts originate from orbital part passes through palpebral part and empty into lateral part of superior fornix. Tears are also secreted by accessory  lacrimal glands of krause and wolfring over conjunctiva. Tears move over external ocular surface and then drained from puncta. From here they enter canaliculi, lacrimal sac, nasolacrimal duct and finally into nose through opening under inferior turbinate of nose.

Conjunctiva is thin transparent highly vascularized membrane that covers the front of sclera and behind the lids. It has three parts. Bulbar conjunctiva covers sclera. Palpebral conjunctiva covers behind the eyelids and forniceal conjunctiva between the two. It is richly supplied with lymphatic tissue which drains into ipsilateral preauricular and submandibular lymph nodes. It consists of dermis and epidermis. The epithelium is non-keratinized stratified columnar epithelium. The conjunctiva starts at mucocutaneous junction at lid margin.

The human crystalline lens is transparent structure with a refractive power of 20 diopters. It is 10 mm in diameter and 4 mm thick. It has a central nucleus surrounded by epinucleus then cortex and a capsule covering it. It is a biconvex structure. The capsule is thick anteriorly and thinnest ant posterior pole. New cells are formed at equator and move centrally. Thus oldest cell are present centrally and youngest cells are present peripherally. Lens is held in place by suspensory ligament or zonules which originate at cilliry body. 

During accommodation for near, the ciliary muscle contracts. As it is a circular muscle, the traction on zonules decrease and lens assumes more globular shape. Thus its convergence power increases. This ability of the lens to change shape decreases through out life. After 40 years of age it cannot focus light from near and the condition is called presbyopia. Here you need magnifying glasses to help accommodation.

Uvea is intermediate vascular coat of eye ball. It consists of iris, cilliary body and choroid.

The iris consists of two layers of epithelium on posterior surface which are derived from retina embryologically. The stroma is highly vascularized. Iris is thinnest at periphery and is detached easily on trauma. Anteriorly it is lined by an endothelium. From the epithelial cell, two muscle arise. The circular muscle near the pupillary border , sphincter pupillae, which causes constriction of pupil on contraction and is supplied by parasympathetic nerves which come along 3rd nerve. The other muscle near the root of iris is, the dilator pupillae, which cause dilation of pupil on contraction and is supplied by sympathetic nerve fibers.

Ciliary muscle consists of pars plicata and pars plana. The first 2 mm are pars plicata and next 4 mm are pars plana. The Intravitreal injection is usually given at 3.5 mm from limbus in aphakic and pseudophakic patients and 4 mm from limbus in phakic patients. 

The choroid is richly vascular and is adherent to sclera. There is a potential space between these two called suprachoroidal space.

The fluid in anterior segment of eye is called aqueous. It is secreted by epithelium of cilliary body into posterior chamber, passes through pupil into anterior chamber and is drained from angle of anterior chamber. The mechanism of secretion from cilliary body are passive transport, and active transport through Na/K ATPase pump. The raised intraocular pressure is mainly because of aqueous drainage problem. Drainage can be blocked at pupil for example in pupil block glaucoma.The second block is at at anterior chamber angle, where Drainage occurs through two paths. Through trabecular meshwork and uveoscleral outflow from the anterior chamber angle.

The secretion is mediated by sympathetic system. The beta  2 receptor (increase secretion) and alpha 2 receptors (decrease secretion). Enzymes like carbonic anhydrase also play important role in its section.

There are 3 parts of trabecular mesh work. The uveal, corneoscleral and juxtacanalicular. The last part play major part in resistance to aqueous outflow.

The shlemm canal is a perilimbal canal which drain aqueous and send it to episcleral veins.

Cornea is transparent structure in front of eye. It consists of 5 layers: epithelium, bowman layer, stroma, descemets membrane and endothelium. It has refractive power of 43 diopters almost 2/3rd of eye power. It is most densely innervated tissue of body. The nutrients are supplied and wast products are removed by aqueous from inner surface and tears from outer surface.. It is non-vascularized.

The epithelium is non keratinized and stratified squamous. Basal cells are columnar, followed by two layers of wing cells and two layers of squamous cells. Epithelium has regenerative potential. 

The bowman layer is thickened layer of anterior stroma. It does not regenerate.

Stroma constitutes 90% thickness of cornea and scar in response to trauma. It does not regenerate

Descemet membrane is thick part of posterior stroma and has regenerative potential

The endothelium has pump mechanism which remove water from cornea and is important in its transparency. It does not regenerate and continuously loose cells. The adult cell density is 3000 cm2. 

Cornea is 12 mm diameter. It is 540 um thick in center and increases thickness peripherally.

Factors responsible for transparency of cornea

1. regular arrangement of stromal collagen fiber resulting in diffractive interference of light rays

2. endothelium pump mechanism that keeps water out of cornea

Vitreous is gelly like fluid that fills the posterior segment of eye. It provides clear fluid through which light rays passes to retina. It is 99% water.

The vitreous is attached anteriorly with lens through ligament of weigert. It is attached strongly at ora serrata which lines retina and pars plana. This place is called vitreous base. It is attached posteriorly to optic disc and macula in circular fashion. It also attaches to large blood vessels here.

Retina is the inner most neurological coat of eye ball. Retina consists of 10 layers. It mainly consists of two layers. The neurosensory retina and retinal pigment epithelium. The neurosensory retina consists 9 layers which are following from inside of eye to outside

1. internal limiting membrane

2. nerve fiber layer

3. ganglion cell layer

4. inner plexiform layer

5. inner nuclear layer

6. outer plexiform layer

7. outer nuclear layer

8. external limiting membrane

9. photoreceptor layer

10. retinal pigment epithelium

It is misalignment of eyes. Six muscles control eye ball. Four recti and two obliques. 

The lateral rectus mainly move the eye ball laterally or outwards. This is also called abduction of eye. It arises from annulus of zinn. It inserts anteriorly 6.9 mm near temporal limbus.

The medial rectus mainly moves eye medially or inward. This is also called adduction of eye. It arises from annulus of zinn and moves anteriorly to insert 5.5 mm behind nasal limbus.

The superior rectus muscle also arises from annulus of zinn and inserts 7.7 mm behind superior limbus. It  mainly moves eye superiorly or elevates eye. The secondary actions are adduction and intorsion.

The inferior rectus muscle arises from annulus of zinn 6.5 mm behind inferior limbus. It moves the eye inferiorly or depresses the eye in primary position. While its secondary actions are adduction and extorsion

The superior oblique arises near optic foramen and moves anteriorly towards trochlea at the superomedial part of orbit. It passes from trochlea and then is reflected back and laterally to be inserted at the posterior upper temporal quadrant of the globe. Its main action is intorsion of globe. It secondary actions are depression and abduction

The inferior oblique arises just behind orbital margin lateral to lacrimal sac. It moves posteriorly to attach at posterior inferior temporal quadrant of globe near macula. Its main actions are extorsion and secondary actions are elevation and abduction.

The nerve supply to these muscle are 

LR6 SO4 O3 that means lateral rectus 6th nerve, superior oblique 4rth nerve and others by 3rd nerve. Also remember that superiors are intorters and inferiors are extorters. Intorsion is rotation of superior limbus medially and extorsion is movement of superior limbus laterally.

Optic nerve starts at the posterior pole of eye and moves back, It is covered by three meningeal layers. These are pia, arachnoid and dura. Its diameter inside eye is 1.5 mm. The intraorbital part is 25 mm long, the intracanalicular part is 6mm and intracranial part is 10mm long.