The retina is composed of several layers and contains specialized cells for the initial processing of visual stimuli. The posterior cavity (vitreous chamber) is filled with a more viscous fluid called the vitreous humor. This opening allows for the drainage of aqueous humor from the anterior compartment of the eye and, therefore, reducing intraocular pressure related to glaucoma.The posterior cavity is the space behind the lens that extends to the posterior side of the interior eyeball, where the retina is located. The aqueous humor then circulates into the anterior chamber through the pupil and is then returned to the blood through the scleral venous sinus, that is a small opening found at the junction of the cornea and sclera. It is at the ciliary bodies that the secretion of the aqueous humor take places derived from capillaries in their walls. The posterior chamber contains the lens and the ciliary bodies. The anterior cavity is also divided up into two chambers: the anterior chamber and the posterior chamber. It is filled with a watery fluid called the aqueous humor. The anterior cavity is the space between the cornea and lens, including the iris and ciliary body. The eye is also divided into two cavities: the anterior cavity and the posterior cavity. The innermost layer of the eye is the neural tunic, or retina, which contains the nervous tissue responsible for photoreception. The iris constricts the pupil in response to bright light and dilates the pupil in response to dim light. The iris is a smooth muscle that opens or closes the pupil, which is the hole at the center of the eye that allows light to enter. Overlaying the ciliary body, and visible in the anterior eye, is the iris-the colored part of the eye. These two structures bend the lens, allowing it to focus light on the back of the eye. The choroid is posterior to the ciliary body, a muscular structure that is attached to the lens by zonule fibers (also called suspensory ligaments). The choroid is a layer of highly vascularized connective tissue that provides a blood supply to the eyeball. The middle layer of the eye is the vascular tunic, which is mostly composed of the choroid, ciliary body, and iris. This cornea is one of two major structures, along with the lens, in the eye that will cause light to bend and be focused on the posterior or back of the eye, where special types of receptors called photoreceptors are located. The transparent cornea covers the anterior tip of the eye and allows light to enter the eye. The sclera accounts for five sixths of the surface of the eye, most of which is not visible, though humans are unique compared with many other species in having so much of the 'white of the eye' visible (Figure 3). The outermost layer is the fibrous tunic, which includes the white sclera and clear cornea. The eye is composed of three main layers or coats of tissues called tunics.

Vascular tunic