Microscopic Anatomy of Retina

1. Choroid -
This structure represents the most posterior portion of the vascular layer of the eye, used to provide the eye with nutrients. Number two marks the beginning of the retina.
2. Pigmented Epithelium of Retina -
This demarcates the retina from the choroid. Its dark pigmentation aids in preventing light scattering within the eye.
3. Outer Segments of Rods and Cones -
The rods and cones are the photoreceptors for light. Rods are used primarily for detecting dim light, while the cones are used for detecting color. A detailed description of these are provided in my discussion of basic eye physiology below. Briefly, the outer segments of these structures, which take the shape of discs, contain the molecules used for light absorption, namely rhodopsin. The inner segments of these structures contain the mitochondria, nucleus and all the other standard cell organelles, needed for proper cell funtion. More on rods and cones below in basic eye physiology.
4. Nuclei of Rods and Cones -
Here with proper staining, we are able to see the nuclei of our rods and cones. This region is also referred to as the inner segment of the photoreceptor.
5. Nuclei of Bipolar Neurons -
The bipolar neurons will synapse with the rods and cones. Again, With proper staining, we are able to distinguish between the nuclei of the inner segment of the rods and cones and bipolar cells, with which they will synapse.
6. Nuclei of Ganglion Cells -
Number 6 rests inferior and lateral to ganglion cell nuclei. Again, with proper staining, we are able to differentiate the ganglion cells' nuclei from the bipolar neurons' neuclei. Ganglion cells synapes with the bipolar neurons and will receive or not receive a signal from the bipolar neurons, depending on whether light is present or not. In the presence of light the ganglion cells will receive neurotransmitters from the bipolar neurons. In the absence of light, the ganglion cells will not receive neurotransmitters from the bipolar neurons.
7. Ganglion Cells' Axons -
These axons will merge to form the optic nerve of the eye (see my drawing below).

Gross Anatomy of Eye

Image on right is a laterosagittal view of eye, with cornea anterior and optic nerve posterior (drawing by Orin James).

1. Cornea (anterior white layer) -
This structure bulges anteriorly from its junction with the sclera and forms the "window" for light to enter the eye. The cornea is the first structure light will hit as it makes its way to the retina, therefore making, the cornea a major player in the light-refracting system of the eye.
2. Sclera (posterior blue layer) -
The Sclera forms the posterior portion of the eye, which helps shapes the eyeball and is the bulk of the fibrous layer. Under normal conditions, it is white and opaque. The sclera is also the site for attachment of the extrinsic eye muscles.
3. Iris/Ciliary Body (anterior red) -
This structure is the anterior portion of the uvea or vascular layer. Its void forms the pupil, which allows light to further make its way to the retina. The ciliary body is also comprised of the apparatus that allows for the bending of the lens for proper focusing of light before it hits the retina.
4. Choroid (posterior red) -
This structure, which comprise the posterior portion of the uvea or vascular layer provides the retina with adequate blood supply and nutrients.
5. Retina (posterior yellow layer) -
This layer contains the photoreceptors needed to detect light. These photoreceptors include rods, cones, bipolar cells, ganglion cells and other cell types needed to detect light. The ganglion cell, however, will fuse to form the optic nerve.
6. Optic Nerve (posterior terminal yellow) -
The optic nerve receives electrochemical signals from the ganglion cells and transmits this information to the brain.
7. Lens (anterior yellow oval) -
The lens, a transparent structure comprised of crystalline proteins, resides posterior to the cornea and is held in place with suspensory ligaments found on the ciliary body. These ligaments will facilitate bending of the lens in order to better focus the image onto the retina.
8. Vitrous Humor -
The vitrous humor can be found in the posterior region of the eye (behind lens). It is a gelatinous mass needed to maintain the shape of the eye.
9. Aqueous Humor -
The aqueous humor can be found tin the anterior region of the eye (in front of lens). It circulates throughout the eye and serves as a nutrient and waste removal source. In the case of waste removal, it will drain into the canal of schlemm.
10. Canal of Schlemm (Sclera Venous Sinus) -
This structure will drain the aqueous humor into the veins of the sclera. The aqueous humor is then removed and recycled within a few hours of its formation.

Extrinsic Muscles of Eye

To the right are images of a model of a right eye, its extrinsic eye muscles (red) and tendons (white). Image (a) represents the lateral view of the eye, while image (b) represents the medial view of the same eye (photos by Orin James). Here is a brief description of the roles of each eye muscle and the cranial nerve responsible for its control.

1. Inferior Oblique Muscle -
This muscle is responsible for raising the eye and turning it laterally. It is controlled by cranial nerve III (oculomotor nerve).
2. Inferior Rectus Muscle -
This muscle is responsible for lowering the eye and turning it medially. It is controlled by cranial nerve III (oculomotor nerve).
3. Lateral Rectus Muscle -
This muscle is responsible for moving the eye laterally. It is controlled by cranial nerve VI (abducens)
4. Superior Rectus Muscle -
This muscle is responsible for raising the eye and turning it medially. It is controlled by cranial nerve III (oculomotor).
5. Medial Rectus Muscle -
This muscle is responsible for moving the eye medially. It is controlled by cranial nerve III (oculomotor).
6. Superior Oblique Muscle -
This muscle is responsible for lowering the eye and turning it laterally. It is controlled by craninal nerve IV (trochlear).
7. Superior Oblique Tendon -
This tissue serves as the insertion point for the superior oblique muscle. Together they form a pully system with the trochlea (not shown) to aid in rotation and securing the location of the eye as it rotates.
8. Common Tendinous Ring -
This tissue serves as a "wrap" for all extrinsic eye muscles and the optic nerve (shown in figure (b) extending posteriorly from number 8).

-O. James