The Anatomy and Physiology of the Eyeball
Hello everyone, in my previous post, we discussed the Anatomy and Physiology of the Eye, but didn't go in to discuss the Eyeball specifically. In the post, I will be looking at the anatomy and the physiology of the eyeball. I can guarantee you that this post won't be long at all, and it won't be boring in any way. I will do my best to break down every complex word to its simplest form (or give a link to read more about it), I am sure that will make it very easy for you.
The Anatomy and Physiology of The Eyeball Begin
To explain the anatomy of the eyeball, I will be starting with the layers of the eyeball, which are referred to as the eye tunics. The eyeball has three tunics which are the Fibrous Tunic, the Vascular Tunic (Uvea), and the Sensory Tunic. It is important to state that the Fibrous tunic is made up of the Sclera and the Cornea, the Vascular Tunic is made up of the Choroid, the Iris, and the Ciliary Body (Ciliary process and the Ciliaris), and the Sensory Tunic which is the Retina and the Vitreous humor.
In taking them one after the other, we will start with the Fibrous Tunic, which is made up of the Sclera and the Cornea. The Sclera and the cornea are the outermost part of the eyeball. The Sclera is made up of four layers, namely; the Episclera, the stroma, the lamina fusca, and the endothelium.. The Cornea is responsible for protecting the eyes against infection and allowing for light rays to pass through. It is made up of 5 layers which are the epithelial layer which have noisy receptors in them, the Bowman's membrane layer, the stroma layer, Descemet's membrane, and the endothelial Layer..
The next is the Vascular Tunic which is made up of the Choroid, the Iris, and the Ciliary Body (Ciliary process and the Ciliaris), is the next layer after the Fibrous tunic. Starting with the Iris (the colored part of the eye which helps to determine the color of a person's eye), which is a 12 mm in diameter structure after the Cornea, and determines the size of the pupil, and separating the anterior part of the eyeball from the posterior, is responsible for regulating the amount of light that enters into the eyeball. The Iris has two muscles which are the Dilator pupillae (a ring of contractile cells arranged radially in the iris) which when activated, causes the Iris muscle to dilate leading to an expansion of the pupil, and the sphincter pupillae which allows for the iris muscles to contract, constricting the pupil..
The Ciliaris, which receives parasympathetic innervation, and sympathetic nervous innervation, is a part of the Ciliary Body in the eye, is responsible for facilitating the lens by changing its shape when it contracts, responsible for accommodation of near vision and distance vision. When the Ciliary muscle contracts, the Ciliary zonules become loose, causing the lens the bulge to see close visions, and when the ciliary muscle relaxes, the zonules tighten causing the lens the flatten for long vision.. Right above the ciliaris are layers of epithelial cells known as the ciliary process which secretes aqueous humor which passes through anterior eye segment (posterior and anterior chamber), to the pupil to lubricate the external part of the eyeball, after which it collected by the canal of schlemm..
The choroid, which is a posterior extension of the ciliary body and lying between the retina and the sclera, is responsible for supplying the retina with nutrients, as well as maintaining the temperature and volume of the eye by absorbing part of the light rays entering the retina to prevent scattering of light in the retina..
The Lens in the eye, is very important. In fact, if you ask 5 people in your area who know nothing about the eye, they will still be able to tell you that the eye has a lens, in fact, a lot of people refer to the eyeball as the lens, well the lens is located in the eyeball, and it is made up of protein molecule Crystallins, and the lens epithelium (responsible for the development of the lens), is held by the ciliary body is responsible for refracting light rays to the retina.. Between the eye lens and the retina, is the Vitreous humor, which is the starting point of the posterior part of the eyeball allowing the hyaloid canal pass through it. It is responsible for holding the retina together, transmitting light rays from the lens to the retina, and maintaining the intraocular pressure when the extraocular muscles are contracting and relaxing to cause the eye direction to change.,.
The Sensory tunic, which is basically the Retina. The retina is made up of several layers of neurons which makes up the outer pigmented layer, middle, and inner layer of the Retina. It includes the Photoreceptors (The layer of rods and cones), Ganglion cell layer, Bipolar cells, Amacrine cells, horizontal cells. The outer layer which is pigmented is made up of Retinal Pigment Epithelium. The outer layer helps to filter the rays of light that enters into the retina.,.
Let me quickly talk about the rods and cones even though I have a separate topic where I intend to discuss them more. The Rods and Cones are the two photoreceptors in the Retina of the eyes. The Rod cells which consists of Rhodopsin, are responsible for scotopic vision, which functions in dim or dark light sight. The Rod cells are responsible for photopic vision which allows for color visions..
The Optic Nerve (cranial nerve 2) is connected to the eye (posterior of the sclera) via the optic disc. The Optic Disc which is close to the Macula is the region where the ganglion cell axons nerves come together form the optic nerve. This region is usually referred to as the blind spot as there is no rod or cone to respond to light in this region. The Optic disc is responsible for being a point of entry for blood vessels supplying blood to the retina, asides holding the optic nerve.. The optic nerve is the end point of the eyes, which connects to the central nervous system, and transmitting electrical impulses to the brain from the eyes for processing and interpretation.
It is important to know that the eye is very valuable and sensitive organ in the body. It is important to remember that the eyeball is divided into the Fibrous Tunic, the Vascular Tunic, and the Sensory Tunic. Each of these tunics have their role to play. In my next post, I will be looking into the Rod and Cone Cells in details.