Anatomy of Eye

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Anatomical Dimensions of Eye

Ocular muscles and motility:-

There are about 33 muscles present in human eye. Some extraocular and some intraocular. 

Extraocular muscles: there are total 9 extra ocular muscle present in each eye -

  • Levator palpebrae superioris 
  • Orbicularis oculi
  • Mullar's muscle
  • Superior rectus (7.7mm) 
  • Inferior rectus (6.6mm) 
  • Medial rectus (5.5mm) 
  • Lateral rectus (7.0mm) 
  • Superior oblique (40mm) 
  • Inferior oblique (33mm) 

Eye diamensions :

Radius of sclera: about 12mm
Horizontal diameter of eyeball -23. 5mm
Anterio-posterior diameter -24. 2mm
Circumference of eye- 75mm
Anterior chamber death 2.5mm to 4.0mm

Lacrimal apparatus 

Canaliculi 0.5 mm in diameter l, length (vertical 2mm, horizontal 8mm) 
Lacrimal sac volume - 20 c.mm
         Tear film 
Discribed in 1946 by wolff
Thickness 4 to 8 um 
After blink thickness 9um , decreased within 30 sec. 
Volume of tear film 7um
Temp. 35°c at limbus and 30°c at anterior cornea. 
Glucose present about 3 to 10 mg/ml.

Lid

  • Lid margin - 2 mm in width 
  • Junction of skin and conjunctiva - grey line
  • Eye lashes arranged in 2-3 rows 
  • Upper lid (100-150) 
  • Lower lid (50-70)
  • Diameter of eye lashes 20 - 120 um and 6-12mm in length. 
  • Palpebral fissure 18-21mm at birth ( horizontal 18-21mm, vertical 8 mm) 
  • Caruncal body 5mm×3mm in size. 
  • Lateral canthe - 2mm approx. 
  • Tarsal plate - 29 mm long and 1mm thick ( upper tarsus is about 10-11mm in hight, lower tarsus is 4-5 mm in hight. 
  • Tarsal gland present 20-30 in each lid vertically. 
  • Tear film is oily layer. 
  • Zeis gland secretes (sebum) prevents eye lashes from becoming dry and brittle. 
  • Moll gland about 1.5 to 2 mm long and unbranched spiral shape. 
  • Accessary lacrimal gland 2 to 5 in upper lid and 2 to 3 in lower lid. 
  • During blink, the upper lid covers the center of the pupil for a periods 0.10 sec. 
  • Blink rate in lion 1 blink/min.,
In rabbit - 1-2 blink/ min. 
In monkey 45blink/min.
In human normal blink rate range  12 - 20 times/min. 
The duration of blinking usually less than 360 msec. 

  • Duration closing phase - 83 msec. 
  • Duration opening phase - 175msec. Approx. 
  • The vision is obviously interrupted during the blinking for about a period of 130msec. The loss of Vn for even 130msec. may be serious for occupations that requires close observation of rapid motion e.g. an Airplane pilot.

Orbit

Volume of the orbit 30 cc. 

Cornea, limbus,& sclera:

5 mm area of the cornea forms the powerful refracting surfaces of eye. 
Anterior surface power : +48D
Posterior surface power: -5D
The posterior corneal radius of curvature averages 1.2mm less than anterior surface. 
Layers of cornea:
1.epithelium- it is about 50-90um thick. 
It acts as the barrier to chemicals and microbs. 
2.Bowman's membrane- 
It is about 8-14um thick. 
It maintains the shape of cornea. 
3.Stroma-it is about 0.5 mm thick. 
It maintains the transparency of cornea. 
4.Descemet's membrane-it is about 10-12 mm thick. 
It prevents the cornea from trauma because of it's ilastic nature. 
5.Dua's layer:it's about 15 um thick. 
6.Endothelium: it is about 18-20um thick. 
It maintains the corneal clarity by removing water from stroma. 
And also helps the cornea for respiration by pumping action of it's cells. 

Uvea: 

Iris-
Diameter : 12 mm about
0.5 mm thick 
It defines the colour of iris, the anterior limiting layer of the iris is responsible for the colour of iris
Ciliary body- 
It is like an isoscele triangle. 
About 6 mm in size 
Pars plicata (corona ciliaris) (2 - 2.5 mm) 
Pars plana(orbicuris ciliaris) 3 - 5 mm 
Ciliary processes - 70 - 80 present , each about 2mm long & 0.5 mm in diameter. 

Choroid
Thickness  : posteriorly - 0.22mm
                      Anteriorly - 0.1 mm
Suprachoroidal lamina - 10 - 34um
Choriocapillaris - 18 to 50um

Aqueous humar 
Secreted from pars plicata part of the ciliary body 
Secretion rate - 2.3ul/min.
Density- 1.025 to 1.040
Osmotic pressure - 3 - 50 mo sm /l.
Ph - 7.2

Lens

Diameter at birth - 6.5 mm 
In adult - 9 - 10 mm
Zonular diameter - 0.35 to 1.0 um
Nucleus refractive index 1.40
Cortex refractive index 1.38

Vitreous & Retina

Weight of vit. 4 gm
Volume of vit. 4 cc ( 2/3rd of globe) 
  • Retina
  • Surface area of retina  266mm²
  • Macula lutea diameter 5.5 mm 
  • Fovea is situated slightly temporal to the anatomical axis of the eye. 
  • Diameter of fovea 1.85 mm and 0.25 mm thick correspondence to 5° visual field. 
Foveola 0.35mm in diameter 
The foveal avascular zone (FAZ) is about 0.5mm
Rod free zone (RFZ) 0.35mm
  • There are 120 million rods and 6.5 million cones present in retina. 
  • The highest density of cones is at fovea with a avg. Of 199000 cones/mm².
  • The density of rod 160000 rods/mm².
  • The rod free zone 0.35mm which correspondence 1.25° of visual field. 
  • Rod cells is about 40 - 60 um long and purple in colour . 
  • Cone cells is about 40 - 80um long and there are three types acco. To colour ( red, green and blue) . 

  •             Thanks

Location: India

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