Anatomy of Eye

Accessory structures


• Extrinsic muscles of eye

         

          The following are six extrinsic muscles of the eye.

Levator palpebrae superioris muscle (Oculomotor nerve)

Superior rectus muscle (Oculomotor nerve)

Inferior rectus muscle (Oculomotor nerve)

Medial rectus muscle (Oculomotor nerve)

Inferior oblique muscle (Oculomotor nerve)

Superior oblique muscle (Trochlear nerve)

Lateral rectus muscle (Abducent nerve)

• Eyelids (palpebrae)

         There are two eyelids: upper and lower. Upper eyelid is more mobile due to presence of levator palpebrae superioris muscle. The angles (2) at which the two eyelids meet are called commisures (medial and lateral). The medial commisure contains an elevation called as commissural carbuncle which secretes whitish color material when we are asleep.

Layers of eyelids

          From superficial to deep: skin→ subcutaneous tissue→ fibers of levator palpebrae superioris (upper eyelid only) muscle→ tarsal plate→ tarsal (meiobiam) glands→ conjunctiva (Palpebral)

          Reflection of the conjunctiva onto the surface of the eye is called bulbar conjunctiva. The portion where the Palpebral conjunctiva transforms into bulbar conjunctiva is called fornix of the eye.

• Eye lashes

          They are located at the margins of both the eyelids.

• Eye brows

          Each eye brow is located above an eye.

• Lacrimal apparatus

          It includes lacrimal glands, lacrimal ducts, lacrimal puncta, lacrimal canals, lacrimal sac and nasolacrimal duct.

Anatomy of eye

          Each eye is 2.5cm in diameter having the following three layers.

1. Fibrous tunic (outer layer) anterior one sixth is cornea and posterior five sixth is sclera.

2. Vascular tunic (middle layer)

3. Retina

Fibrous tunic

Cornea

         It is transparent avascular (no vessel) layer which is subdivided into three layers.

External is made up of non-keratinized stratified squamous epithelium.

Middle is of collagen fibers.

Internal layer is of simple squamous epithelium.

Sclera

         This portion of fibrous tunic is made up of dense connective tissue (collagen fibers and fibroblasts). It gives shape to the eye.

         There is a canal at junction of sclera and cornea called as canal of schlemm through which the aqueous humor drains to the venous sinus.

Vascular tunic (uvea)

          Choroid, ciliary body and iris are the three parts of the vascular tunic. Posterior two third is the choroid which is highly vascularized and helps in the nourishment of both sclera (outside) and retina (inside). Slightly posterior to the junction of sclera with cornea or at the level of ora serrata of the retina, choroid transforms in a slightly expanded part called ciliary body. Projections of ciliary body are called ciliary processes.  
          Ciliary processes secrete aqueous humor which helps in the nourishment of the lens and internal portion of cornea. They have thread like structures called as zonular fibers (suspensory ligaments) which attaches to the lens and helps in the accommodation of lens.

         Ciliary body has muscle called ciliary muscle which on contraction causes stretching of zonular fibers (suspensory ligaments) helping in accommodation phenomenon of the lens. Long extensions from ciliary body which surrounds the anterior part of the lens is called iris. Iris contains two types of muscles: circular and radial muscle.

         Parasympathetic innervation causes contraction of circular muscle and hence constricts the pupil. Pupil is the central hole in the iris. Sympathetic innervation causes contraction of the radial muscle and hence causes dilation of pupil.

Retina

         Retina is the innermost layer of the eye. It covers the posterior 2/3 circumference of eye. Retina is further divided into the following layers.

Pigmented layer

Neural layer

• Pigmented layer

          Pigmented layer is made up of melanin containing epithelium cells. This layer prevents scattering of light within the eye and prevent reflection of light. It is located in between the choroid of the vascular tunic and neural layer of the retina.
         Neural layer has three types of cells divided by two synaptic layers. The most external layer of cells is rods and cones. There are 120 million rods and 6 million cons in one eye. Rods are responsible for dim light. These cells are mostly active at night. Decrease number of rods will create difficulty night vision (black difficulty in night vision or black and white).
         Rods are concentrated towards the periphery. Cones are responsible for bright day light and color vision. Cones are mainly concentrated towards the center. Just opposite to the central axis of the eye in the retina is a yellow spot called macula lutea. In the center of macula lutea is a depression called fovea centralis. It contains only cones and no rods and is responsible for acuity (sharpness or resolution) of vision.
         Both rods and cones form the photoreceptors part of the neural layer o f the retina. From the photoreceptor cells (rods and cons) the visual impulses flow through the outer synaptic (plexiform) layer into the middle part of the neural layer and synapses with the bipolar cells. Bipolar cells in turn synapses with ganglion cells through the inner synaptic (plexiform) layer. Axons of ganglion cells then converge at the optic disc forming the optic nerve which then leaves the orbit of the eye. Optic disc is also called blind spot of the eye. If the light falls here no image will be formed.

Lens

          The cavity of the eye is divided into 2 parts: anterior and posterior cavity by the presence of lens. Lens is made up of crystalline cells which are arranged in the form of an onion (layers). Externally the lens is covered by a capsule. Lens like cornea is transparent and devoid of blood and lymphatic vessels. The anterior cavity of the eye is further divided into 2 chambers: anterior and posterior by the iris.
          Aqueous humor produced by the ciliary processes, first enters the posterior chamber and then flow into the anterior chamber through the pupil. From the anterior chamber aqueous humor enter the canal of schlemm and drain into the venous system. Posterior cavity is filled with vitrous body.
         Vitrous body is a gel like structures which is formed during embryological development. It helps to maintain the position of the retina against the choroid. Any damage or leakage of the vitrous body will cause displacement of the retina. Vitrous body along with the aqueous humor helps in the formation of intra ocular pressure (16mmhg).

Posted in: Anatomy; Sense Organs