Sensory organ: Eye

For clearer vision, the optic nerves, accessory structures, and eyes all work together. Our primary sense is vision.

Location of the Eye

The orbit, a bony structure that almost completely protects the eyeball, houses the eyeball. To allow light to enter the eyeball, a tiny portion in the front has been left relatively unprotected.

Structure of the Eye

The eyeball's three layers are as follows:

• Fibrous layer: (outer coat), It is made up of the cornea and sclera.
• Vascular layer: (middle coat), The choroid, ciliary body, and iris make up this structure.
• Inner layer: (inner coat), It is made up of the retina, which is divided into optical and non-visual components.

Fibrous Layer of Eyeball

The eyeball's fibrous layer serves as both the shape and resistance of the eyeball's external fibrous skeleton. The sclera, which covers the back five-sixths of the eyeball, is the hard, opaque portion of the fibrous layer (coat) of the eyeball. Both the extrinsic (extra-ocular) and intrinsic muscles of the eye can be attached to it. The transparent bulbar conjunctiva allows the anterior portion of the sclera, also known as "the white of the eye," to be seen. The fibrous layer that covers the anterior one-sixth of the eyeball has a transparent portion called the cornea. When viewed laterally, the cornea appears to protrude from the eyeball because its convexity is greater than the sclera's. The two components of the fibrous layer differ from one another mainly in terms of the regularity of the collagen fibers that make them up and the level of hydration in each. The cornea is completely avascular, in contrast to the sclera, which has some vascularity. Fluids on the cornea's exterior and interior surfaces as well as capillary beds around its periphery provide the cornea with nourishment.

Additionally, oxygen from the air is absorbed by the lacrimal fluid. The ophthalmic nerve supplies the cornea's innervations, which are very sensitive to touch (CN-V1). Even very small foreign bodies, like dust particles, can cause blinking, tears to flow, and occasionally very painful sensations. Ulceration may develop as a result of corneal surface drying. The angle created by the sclera and cornea's intersecting curvatures at the corneoscleral junction is known as the corneal limbus. The junction is a 1-mm-wide, gray, translucent circle with numerous capillary loops that provide the avascular cornea with nourishment.

Vascular Layer of Eyeball

The choroid, ciliary body, and iris make up the eyeball's main vascular layer, often known as the uvea or uveal tract. Between the sclera and the retina, the choroid, a dark reddish brown layer, lines most of the sclera and makes up the majority of the vascular layer of the eyeball. Outside of this heavily pigmented and vascularized area are the larger vessels. The innermost, closest to the avascular, light-sensitive layer of the retina, which it supplies with oxygen and nutrients, are the finest vessels (the capillary lamina of the choroid, or choriocapillaris, an extensive capillary bed). This layer is what causes the "red eye" reflection in flash photography because it is engorged with blood in life (it has the highest perfusion rate per gram of tissue of all vascular beds of the body). However, the choroid can be easily removed from the sclera. The choroid is firmly attached to the pigment layer of the retina. The ciliary body and choroid are joined anteriorly.

The ciliary body is a ring-shaped enlargement of the muscular and vascular-rich layer posterior to the corneoscleral junction. It connects the choroid to the iris's border. The lens is attached via the ciliary body. The thickness of the lens, and consequently its focus, is controlled by the contraction and relaxation of the ciliary body's smooth muscle, which is arranged in a circle. Aqueous humor is secreted by ciliary processes, folds on the ciliary body's interior surface. The anterior segment of the eyeball, the area inside the eyeball closest to the lens, the suspensory ligament, and the ciliary body are all filled with aqueous humor.

The iris is a tiny contractile diaphragm with a central opening for light transmission that physically lies on the anterior surface of the lens. To control how much light enters the eye while a person is awake, the pupil's size changes constantly. The pupil's diameter is controlled by two involuntary muscles: the sympathetically stimulated, radially arranged dilator pupillae increases the pupil's diameter and the parasympathetically stimulated, circularly arranged sphincter pupillae reduces it (constricts or contracts the pupil, pupillary miosis) (dilates the pupil). the inner layer of the eye

The Inner Layer of the Eyeball

It is the layer of the eyeball's sensory neural system. The retina is where the visual pathway begins. The retina, in its most basic form, is divided into two functional sections with separate locations: an optic portion and a non-visual retina. The optic portion of the retina has two layers: a neural layer and a pigmented layer, both of which are sensitive to visual light rays.

The neuronal layer is sensitive to light. Prior to transmitting nerve impulses into the axons that make up the optic nerve, it thoroughly analyses visual input. The photoreceptor layer, the bipolar cell layer, and the ganglion cell layer are the three separate layers that make up this layer. Specialized cells called photoreceptors convert light rays into nerve impulses.

The pigmented layer is made up of a single layer of melanin-containing epithelial cells, which supports the choroid's ability to absorb light and lessen light scattering in the eye. The pigmented layer and a layer of supporting cells continue anteriorly into the non-visual retina. The non-visual retina extends from the pupillary margin to the ciliary body, which is the ciliary part of the retina, and the posterior surface of the iris, which is the iridial part of the retina. There are 10 histological layers in the retina.

The fundus of the eyeball is the term used in clinical terminology to describe the inside surface of the posterior portion of the eyeball, which is where light entering the eyeball is focused (ocular fundus). The optic disc (also known as the optic papilla), which is an unique circular region of the retina of the fundus, is where the optic nerve (CN II) leaves the eyeball. The central retinal artery and vein are connected to the optic nerve in a bundle. The optic disc is not photosensitive because it lacks photoreceptors. As a result, this region of the retina is frequently referred to as the blind spot. The macula of the retina, or macula lutea, is located directly lateral to the optic disc (L. yellow spot). The macula's yellow color can only be seen when the retina is examined under red-free light. The macula is a little oval region of the retina that has unique photoreceptor cones that are tailored for visual acuity. Using an ophthalmoscope to observe it usually is not common (a device for viewing the interior of the eyeball through the pupil). The fovea centralis (L. central pit), a depression at the macula's center, is where vision is sharpest. The ora serrata, or irregular posterior border of the ciliary body, marks the termination of the optic portion of the retina anteriorly.

The retinal neural layer, as previously mentioned, contains the photoreceptor layer. Rods and cones are the two different types of photoreceptors. Rods can be stimulated by dim light, whereas cones must be stimulated by rather bright light. In other words, cones are the receptors for daytime vision, while rods are the receptors for night vision. We can see with rods in low light, such as moonlight. Rods do not provide color vision, therefore in low light we can only see in black and white. Cones, however, provide color vision.

Each of the three types of cones—red, green, or blue—is sensitive to a distinct color. These three types of cones, which are dispersed throughout the retina's center region, enable us to distinguish between various hues.

Blood Supply of the Eye

The central retinal artery and the ciliary arteries provide arterial blood to the eye. These arteries are the ophthalmic artery's branches. The central retinal vein is mainly responsible for venous drainage.