Anatomy of the Eye - Eye Health Nepal

Anatomy of the Eye

External Anatomy of the Eye

Eyes are organs of the visual system. They provide animals with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and convert it into electro-chemical impulses in neurons.

Dimensions of an adult eyeball

  • Axial Length (Anteroposterior diameter) 24 mm
  • Horizontal diameter 23.5 mm
  • Vertical diameter 23 mm
  • Circumference 75 mm
  • Volume 6.5 ml
  • Weight 7 gram
Source: Comprehensive Ophthalmology AK Khurana

Parts of the eye:

Diagram of an eyeball
eye anatomy





Parts of the outer eye:

  • EyeBrows
  • Eye Lids
  • ExtraOcular Muscle
  • Lacrimal Glands
  • Lacrimal Apparatus

Outer drawing of eye


Segments of an eyeball

The human eye is divided into 2 segments:

1. Anterior Segment
2. Posterior Segments

Anterior Segments: 

It includes the parts from Cornea to the crystalline lens.
  • Cornea
  • Anterior Chamber
  • Iris
  • Posterior Chamber
  • Lens

Posterior Segments:

It includes the structures posterior to lens,

  • Vitreous humour 
  • Retina,
  • Choroid
  • Optic Disc

Eye parts and functions:

Parts of the outer eye:

EyeBrows:

Our Eyebrows main function is to protect the eyes, however, they also play a critical role in communication through facial expressions. It also helps to catch debris, dust, dandruff or oils from getting into the eyes. The position of eyebrow also helps to shield the eyes from bright sunlight coming from directly above. 
eye lid and eye lashes

EyeLids:

eyelid anatomy diagram

Anatomy of EyeLid

An eyelid is a fold of skin that closes over the eye to protect it. They are curtains placed in front of the eyeballs. It is also known as the shutter of an eye. 

Functions

  • Acts as a shutter which help to protect from trauma and excessive light.
  • It spread tear film over the Cornea and conjunctiva. 
  • Better Cosmetic appearance

Parts of eyelids:
  1. Skin
  2. Layer of Subcutaneous areolar tissues
  3. Layer of Striated muscle (Orbicularis oculi muscle, The levator palpebrae superioris muscle)
  4. Submuscular areolar tissue
  5. Fibrous Layer
  6. Layer of non-striated muscle fibres (Mullers muscle)
  7. Conjunctiva (Palperbral Conjunctiva) 
Glands of Eyelids
  1. Meibomian glands
  2. Glands of Zeis
  3. Glands of Moll
  4. Accessory lacrimal glands of Wolfring

Tear Film 


Tear Film consists of three layers.  The Outer layer is lipid layer, middle aqueous layer, inner mucin layer. It forms the coat over the cornea and conjunctiva and protects it. It also help to remove the small foreign particles from cornea. It has antibacterial properties. 


Conjunctiva


It is Derived from surface ectoderm. Conjunctiva is a translucent mucous membrane which lines the anterior aspect of the eye ball and posterior surface of the eyelid. 

Parts of Conjunctiva

  1. Palpebral conjunctiva 
  2. Bulbar conjunctiva
  3. Conjunctival fornix.

Anatomy of conjunctiva and parts of conjunctiva
Conjunctiva Parts | Source https://quizlet.com/


Anatomy of Cornea

 The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. It is avascular. It covers about one-sixth part of the outer fibrous coat of an eyeball.  The thickness of the cornea in centre is about 0.52 mm and at the periphery about 0.62mm. Refractive power of Cornea is +43 D and refractive index is 1.37.


Layers of Cornea:

  1. Epithelium
  2. Bowman's membrane
  3. Stroma
  4. Pre-Descemet Layer, or Dua Layer
  5. Descemet's membrane 
  6. Endothelium
5 Layers of the human cornearnea
Parts of the Human Cornea



Functions of Cornea:

  • It focuses light on the retina for image formation.
  • Protects the intraocular contents.

Sclera 

Sclera covers 5/6th part of the eyeball. Sclera in the Greek Word mean Hard. So it is dense and opaque. The sclera is covered by a thin layer of loose tissue called episclera. It is separated from the choroid by the suprachoroidal space. The extraocular muscles are inserted in the sclera.  Thickness of sclera is different in different parts, the most thickest part of sclera is at posterior pole and the thinnest part at underneath of Extraocular Muscle. 

It has three layers 
  • Episclera 
  • Sclera proper 
  • Lamina fusca 

Blood supply – episcleral and choroidal vessels
Nerve supply – Short ciliary and long ciliary nerves.



Anatomy of the UVEA 

Uvea is the middle vascular layer of the eyeball. It consists of 
  • Iris 
  • Ciliary Body 
  • Choroid

Iris 

Iris is a free circular diaphragm with central opening called Pupil. The iris is attached at its periphery (root) to the middle of the anterior surface of the ciliary body. It is responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. The color eye of  is defined by that of the iris color. The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, and blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculocutaneous albinism.


Muscle of Iris 

It has two muscles. They are  Sphincter pupillae (Circular muscle) helps for pupillary constriction
and Dilator pupillae ( Radial muscle) which helps for pupillary dilatation.


Ciliary Body 

Ciliary body is the middle portion of uvea. It consists Ciliary muscle, which is ring shaped muscle that controls the shape of the lens (accommodation) The ciliary body is composed of unstriated ciliary
muscle fibers, stroma and blood vessels. The inner surface of the ciliary body has two distinct zones.
- Pars plicata: It is about 70 plication, has ciliary process responsible for production of aqueous 
- Pars plana:  It is the posterior smooth part, safe and avascular zone for pars plana lensectomy and vitrectomy procedures.


Choroid 

The choroid is a thin vascular membrane extending from the optic disk to the ora serrata. It is largely
composed of layers of large and small vessels, Pigment cells, wandering cells, smooth muscle fibers, and nerves in the intravascular spaces from the stroma of the choroid.
  • It is posterior part of uvea. 
  • This is a vascular sheet which separates sclera from retina.


Crystalline Lens


It is transparent biconvex structure derived from surface ectoderm. 
Dioptric power : 15-16 D. Diameter is about 9-10 mm. Thickness varies with age. 

 Structure of Lens: 

  • Anterior lens capsule 
  • Anterior lens epithelium 
  • Lens Nucleus – consists of elongated lens fibres 
  • Contains 64% water, 35% Protein (crystallin), 1% Lipids (Cholesterol, sphingomyelin, lipoproteins)

Vitreous Cavity 

Contain vitreous humor which is transparent gel like structure. It is Composed of collagen fibers and hyaluronic acid. Vitreous Humor mainly serves the optical medium. In addition, it mechanically stabilizes the volume of the globe and is a pathway for nutrients to reach the lens and retina. The vitreous humor (also known simply as the vitreous) is a clear, colourless fluid that fills the space between the lens and the retina of your eye.


Retina 

The retina is a thin, semitransparent, multilayered sheet of neural tissue that lines the inner aspect of the posterior two-thirds of the wall of the globe. The retina comprises photoreceptor cells, a relay layer of bipolar cells and ganglion cells and their axons that run into the central nervous system. 

Layers of Retina 

Microscopically, the retina from without inwards is made up of following ten layers

1. Retinal pigment epithelium
2. Layers of rods and cones
3. External limiting membrane
4. Outer nuclear layer
5. Outer plexiform layer
6. Inner nuclear layer
7. Inner plexiform layer
8. Ganglion cell layer
9. Nerve fiber layer, and
10. Internal limiting membrane.

The retina receives its blood supply from two sources: the choriocapillaris immediately outside Bruch’s membrane, which supplies the outer third of the retina, including the outer plexiform and outer nuclear layers, the photoreceptors, and the retinal pigment epithelium; and branches of the central retinal artery,
which supply the inner two-thirds. 

Layer of The Retina
Layers of Retina | Source: www.memorangapp.com