Hi, this is Peter from Anatomy Zone and in this tutorial we're going to go through the anatomy of the eyeball. This video is a collaboration between Anatomy Zone and teachmeanatomy.info. Check out the links in the video description below for some useful articles to accompany this video tutorial.
The eyeball is a bilateral and spherical organ, which houses the structures responsible for vision. It lies in a bony cavity within the facial skeleton, known as the bony orbit. Take a look at my previous tutorial to learn about the anatomical features of the bony orbit.
Anatomically, the eyeball can be divided into three parts, the fibrous, vascular and inner layers. These layers have different structures and functions. Let's start with the outermost layer, the fibrous layer. I've switched over now to a view of the isolated eyeball, with some layers dissected away. The fibrous layer of the eye.
is the outermost layer. It consists of the sclera and the cornea, which are directly continuous with each other. You can see the shape and continuity of the cornea with the sclera from this lateral view. Their main functions are to provide shape to the eye and support the deepest structures. The sclera comprises the majority of the fibrous layer, approximately 85%.
and provides attachment to the extraocular muscles. The extraocular muscles are responsible for the movement of the eye. Check out my previous tutorial for more information on these muscles.
In addition, several vessels and nerves penetrate through the sclera, including the optic nerve which you can see here posteriorly. The sclera is visible as the white part of the eye. The cornea is continuous anteriorly with the sclera.
and is transparent and positioned centrally at the front of the eye. Light entering the eye is refracted by the cornea. The vascular layer of the eye lies underneath the fibrous layer.
It consists of three continuous parts, the choroid, the ciliary body and the iris. The choroid is a layer of connective tissue and blood vessels. It provides nourishment to the outer layers of the retina. I've switched over to an anterior view showing the whole eyeball to show you the ciliary body, which I've highlighted in green. You can see how it encircles the eyeball.
The ciliary body itself is comprised of two parts, the ciliary muscle and ciliary processes. I've sliced the eyeball in half and we've rotated around to look at the eyeball in section. Let's just review the structures that we have already discussed so that you are oriented You can see the outer fibrous sclera in continuity with the cornea anteriorly. You can also see the inner vascular choroid and the ciliary body here in green. The ciliary muscle consists of a collection of smooth muscle fibres arranged in three orientations longitudinal, circular and radial.
When these muscles contract the diameter of the circular ciliary body reduces in size. Projecting from the surface of the ciliary body are the ciliary processes. These attach the lens to the ciliary body via these fibres which you can see here, known as zonular fibres. Collectively, the zonular fibres form the suspensory ligament of the lens. The ciliary body therefore controls the shape of the lens.
As I mentioned before, when the ciliary muscle contracts, the circular ciliary body reduces in size. When this happens, The zonular fibres therefore slacken, reducing the tension applied to the outside of the lens, allowing the lens to return to a more rounded shape. This process is known as accommodation and adjusts the lens for near vision.
Conversely, when the ciliary body is relaxed, the diameter of the ciliary body is greater, which means the zonular fibres are held tight, pulling the lens flat. This optimises the lens for long distance vision. In addition to adjusting the lens shape, the ciliary body also contributes to the formation of aqueous humour.
The final structure of the vascular layer is the iris. This is a circular structure with an aperture in the centre called the pupil. The iris is the component of the eye which gives you your eye colour. It has two arrangements of fibres.
which control the size of the pupil, circular fibres and radial fibres. The circular fibres make up the sphincter pupillae muscle, which is innervated by the parasympathetic nervous system. Activation of this system causes constriction of the pupil, which thereby limits the amount of light that can enter the eye.
The radial fibres on the other hand, make up the dilator pupillae muscle, which are innervated by the sympathetic nervous system and cause dilatation of the pupil, thereby increasing the amount of light that can enter the pupil. the amount of light that can enter the eye. I've highlighted the iris in green and sliced the eyeball in half.
Let's rotate around to view the eyeball in section and view the relationship of the iris to the other structures we have covered so far. The inner layer of the eye consists of the retina. the light detecting part of the eye. The retina itself is comprised of two cellular layers, the neural layer and the pigmented layer. The neural layer consists of photoreceptors, the light detecting cells of the retina.
It is located posteriorly and laterally in the eye. The pigmented layer lies underneath the neural layer and is attached to the choroid layer. It acts to support the neural layer and continues around the whole inner surface of the eye.
Anteriorly, the pigmented layer continues, but the neural layer does not. This part is known as the non-visual retina. Posteriorly and laterally, both layers of the retina are present. This is the optic part of the retina.
The optic part of the retina can be viewed during ophthalmoscopy or fundoscopy. The centre of the retina is marked by an area known as the macula lutea. It is yellowish in colour and highly pigmented.
The macula contains a depression called the fovea centralis, which has a high concentration of cones, which are the light-sensitive receptor cells which function best in brightly lit conditions and are responsible for high-acuity colour vision. The area that the optic nerve enters the retina is known as the optic disc. It contains no light-detecting cells and is therefore referred to as the blind spot of the retina.
The central retinal artery enters at this point and its branches are visible at this point when viewed during fundoscopy. There are a few remaining structures to discuss to cover the basic gross anatomy of the eyeball. There are two fluid filled areas in the eye known as the anterior and posterior chambers. The anterior chamber is located between the cornea and the iris.
The posterior chamber is a small chamber located behind the iris and anterior to the lens and suspensory ligaments. The two chambers are continuous with one another via the pupillary opening. The chambers are filled with aqueous humour, which is a clear plasma-like fluid that nourishes and protects the eye. The aqueous humour is secreted first into the posterior chamber and flows into the anterior chamber via the pupil, and is then absorbed into the canal of Schlemm, also known as the scleral venous sinus. This channel encircles the eye, lying at the point between the cornea and the iris.
You can see this highlighted in blue on this model. If the drainage of aqueous humour is obstructed, the intraocular pressure will rise, a condition known as glaucoma. The lens separates these two chambers from the vitreous chamber, which fills the eyeball with a transparent, gel-like substance known as vitreous humour, or vitreous body, extending from behind the lens to the retina posteriorly.
So that's an overview of the gross anatomy of the eyeball. In the next tutorial, we'll take a look at the vascular supply. If you have enjoyed this video, please click the like button below and subscribe for more video tutorials.
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