The sensory system is responsible for perceiving stimuli from the environment by the sense organs, sending this information to the central nervous system in order to be interpreted.
The sensory system is made up of five sense with their respective sense organs: sight, hearing, smell, taste and touch.
This is the sense responsible for perceiving electromagnetic radiation from the environment or, in other words, light. Its sense organ is the eye. We are able to perceive electromagnetic radiation with wave-lengths from 400 nm to 700 nm, known as the visible light spectrum (lower radiation, such as ultraviolet rays, or higher radiations such as infrared rays are not visible).
Sight is, in human beings, the predominant sense.
Eyes are the main organs of sight. Their receptor structure is made up of a layer of cells that perceive light, surrounded and protected by a group of accessory structures.
There are internal and external accessory structures. One of the external structures are the eyelids. These are skin folds that cover the external part of the eye and close when in order to prevent the eye from being damaged by light when it is very intense. Eyelids also protect eyes from being damaged by blows. Finally, they extend lubricating fluids over the eye surface.
There are a group of long and thick hairs on the eyelids edge called eyelashes. They protect the eye from little particles, that are captured by the hairs.
The eyebrows are a group of hairs that form a sort of arc above the eye. Their function is preventing sweat and water that run down from the upper part of the head from falling into the eye.
There are group of glands associated located in the edge of the eyelids, called Meibomian glands (also known as tarsal glands). They lubricate the eye surface and protect them from infections.
Finally, the Lacrimal Glands are located in the upper lateral part of the orbit, and they are responsible for humidifying the eye surface. They produce tears that cover the eye surface and flow to the canals situated in the internal corner of the eye, that are connected to the lacrimal sac to drain the liquid.
The sensitive part of the eye is enclosed in a structure called ocular globe. Its diameter is between two and three centimetres long. And its surface can be divided into three consecutive layers, called fibrous, vascular and nervous tunics.
- Fibrous tunic: this is the outermost covering of the ocular globe. It has two parts. The anterior one is called the cornea, and the posterior one is called the sclerotic. The cornea is a transparent fibrous tissue that covers the iris. Its shape is curved in order to focus and concentrate the incident light. The sclerotic is a dense layer made up of connective tissue, that covers the posterior or internal part of the ocular globe. It has a hole that is crossed by the optic nerve.
- Vascular tunic: this is the intermediate layer. It has three parts called the choroid, ciliary body and iris.
- Choroid: it is an extremely vascularised layer that covers the posterior part of the ocular globe in order to irrigate the retina.
- Ciliary Body: it is located in the anterior part of the ocular globe, behind the cornea, covering the entrance of the incident light. It has a muscle called the ciliary muscle that surrounds an internal structure called the crystalline. The crystalline is a transparent structure in the shape of a biconvex lens. It is responsible for projecting and focusing the incident light on the retina. The ciliary muscle changes the curvature of the crystalline so it can focus on the retina the image of different objects, depending on the distance they are.
- Iris: this is the outermost part of the vascular tunic. It is a disc with a central hole called pupil. Light crosses the pupil to reach the crystalline. This internal hole of the pupil can change its diameter in order to control the amount of light that crosses the iris to reach the retina. When it is dark, the pupil dilates to perceive more light, when it is bright the pupil contracts to prevent the light from causing damage to the retina.
- Nervous Tunic (Retina): this layer covers the internal surface of the posterior part of the ocular globe. It is responsible for perceiving light, due to the activity of the photoreceptors, called rods and cones.