The Human eye is a complex machine, which performs the most important function of visualizing the surroundings. It not only helps in recognizing images but also helps in understanding the distance to an object and its depth. At the same time, in order to perform these functions and gain the much-needed 3D vision, the two eyes must work together in a precise manner.
The eyes are located in the eye sockets of the skull bone and therefore are able to withstand any outside pressures enforced over the skull. Within the eye socket, muscle attachments and other soft tissues cushion the eyeballs from even the most violent vibration. At the same time, this anatomical arrangement allows the eyes to move while maintaining a firm grounding within the eye socket.
The eyeballs can be described as a slightly asymmetrical globe, which measures 1 inch in diameter. Its shape makes it possible for the attached small muscles to manipulate the position of the eyeball without much effort.
Sclera and the cornea:
Sclera is a thick fibrous tissue layer, which forms the white portion of the eye visible from the front. The cornea is a dome shaped structure made out of transparent material appearing like glass and covers the central portion of the sclera in front of the eye. The cornea allows light to pass onto the lens of the eye and aids in focusing the same onto the light sensitive retinal layer in the back of the eye.
The choroid, ciliary body, iris and the lens:
These first three structures can be considered as a continuum of the same layer, which lies between the sclera and the retinal layer in the back of the eye. However, they have distinct functions and structural arrangements when compared to each other.
The choroid forms the middle layer in the covering of the eye and contains many blood vessels to provide nutrition and oxygen to the retinal layer. It is highly pigmented to prevent the scattering of light aimed at the retinal layer in the back. The iris is the continuation of the choroid in the front of the eye and gives rise to the ‘pupil’, which allows light to go through. Thus, the iris controls the size of the pupil and has the ability to adjust in response to the outside illumination. The iris can appear in various colors such as black, brown, blue…etc, depending on the genetic inheritance of a particular person.
The ciliary body is a unique structure in its function as it provides attachment to the suspensory ligament of the lens and also produce a secretion (aqueous humor) which fills the anterior and posterior chambers of the eye, on either side of the iris. The contractions of the ciliary muscles will increase or decrease the tension within the lens and therefore change its curvature depending on the object visualized by the eye.
The lens is a biconvex structure, which has the ability to increase or decrease its curvature depending on the force exerted by the ciliary muscles. It is made out of a lens capsule from outside and is packed with lens fibers in the center.
The retinal layer forms the inner most surface in the back of the eye and it contains the light sensitive cells known as the ‘cones’ and the ‘rods’. There are millions of cones and rods in the retina and they can convert the light into recognizable electric signals, which can be sent via the optic nerve to the brain centers. The central portion of the retina is the most sensitive towards light and it is called the fovea, which gives the central vision of a person.
The optic nerve:
As mentioned before, the electrical signals produced by the retinal photoreceptor cells travels through the optic nerve to the relevant area of the brain before it is processed to recognize as an image. Many nerve fibers get together in forming the optic nerve and the place where all nerve fibers converge forms the ‘blind spot’ of the retina which is also known as the ‘optic disk’. Thus, certain changes taking place in the brain or in the nerves can alter the appearance of the optic disk, which is used by clinicians in making certain diagnosis.