Anatomy & Refractive States of the Eye
Principal Ocular Structures

 

The Optic Nerve is the special nerve of the sense of sight that carries impulses from the retina to the brain. It’s similar to a cable containing millions of tiny wires which are connected to the rods and cones. Earlier it was mentioned that the aqueous humor helps to maintain an acceptable level of pressure inside the eye and if something were to inhibit the drainage mechanism of the aqueous, it could cause the pressure inside the eye to raise to unacceptable levels possibly causing a condition called glaucoma. But what are the consequences of glaucoma? Sustained intraocular pressure at unacceptably high levels can cause the tiny fibres inside the optic nerve to gradually sever resulting in an irreversible loss of vision. The first nerve endings to sever are the rods which are responsible for peripheral vision. If gone unchecked, eventually the other nerve fibres can sever, resulting in total blindness. In many cases there are no immediate symptoms associated with glaucoma. However, it can generally be detected during a routine eye examination through the use of tonometry and visual fields. Tonometry is a means of measuring the amount of pressure inside the eye, while visual fields can determine the range of peripheral vision available to the patient. Routine eye exams should be recommended, especially for those who are over 40 or who have a history of glaucoma in their family. Even if there is no change in the prescription, a routine eye exam is capable of detecting the possibility of glaucoma and could save the patient’s eyesight.

Visual Pathway
After leaving the eye, the optic nerves continue along what is called the visual pathway. Refer to Figure 4. Here, the optic nerves cross at an X- like structure called the optic chiasm and end up at an area of the brain called the geniculate body.

Visual Pathway

 

 

 

 

 

The information is then transmitted to the visual cortex in another area of the brain called the occipital lobe where conscious recognition of objects occurs. So we don’t actually see with our eyes, we see with our brain. Since we have two eyes, the brain receives two images, one from each eye. The ability to merge these two images into one is called fusion. If, for some reason, the two images cannot be adequately fused, double vision or diplopia results.

Tunics
Certain structures of the eye may be grouped into coverings or tunics and these are summarized in the table below.

 

Tunics

Fibrous tunic	Outer protective coat of the eye. Consiste of the sclera and cornea
Vascular tunic	Also known as the uveal tract. Consists of three parts: the iris, ciliary body and the choroid
Neural tunic	Consists of the retina

The fibrous tunic comprises the outer protective coat of the eye and consists of the sclera and the cornea. The vascular tunic, also known as the uveal tract consists of three parts: the iris, ciliary body and the choroid, while the neural tunic is comprised of the retina.