The eye is a complex and delicate organ. It has many functional parts that all work together to make sight possible. Though many of the parts are the same in different species, animals have developed certain adaptations that best suit their needs.the eye has three main layers: the outer fibrous tunic, middle vascular tunic, and inner nervous tunic.The fibrous tunic is the outermost layer of the eye. In the posterior (back) three-fourth portion it is opaque called sclera and anterior (front) one-fourth portion is transparent/ clear, called “cornea”.
The middle vascular tunic, as the name implies, is a network of blood vessels that supply oxygen and nutrients to the tissues of the eye. The actual area where this network is located is beneath the portion covered by the sclera and is called the "choroid." The choroid of animals highly developed in nocturnal animal presents a reflecting structure, tapetum which gives a greenish glare to the eye at night. Anterior to the choroid is a circular structure called the "ciliary body." The ciliary body has muscles that act on suspensory ligaments called "zonules," which suspend the lens in the correct position. The ligaments are either taut or relaxed based on the action of the ciliary muscles. The iris is the colored portion of the eye that gives different normal coloration to the eye in human as well as in animals. (Brown, bluish, black, white color eye). The opening in the middle of the iris is called the "pupil," which appears as the dark center of the eye. The iris either dilates or constricts the pupil to regulate the amount of light entering the eye. In bright light the pupil will be small, but in dim light the pupil will be very large to let in as much light as possible.
The nervous tunic is a layer of photoreceptor cells called the "retina." These cells are able to change light into electrochemical signals, which are transmitted to the nervous system. There are two types of photoreceptor cells which perform different functions and are named for the shape of the cell. These are the rods and cones. The rods are very light sensitive, so they are most abundant in nocturnal species. The cones need bright light, and they are for sharp image formation and perception of color. Domestic mammals have mostly rods, and are unable to distinguish colors well. Some reptiles and most birds can see color, since they have many cones.
How tappet lucidum works?
Normally in human being whatever amount of light passed through retina where (layer of rod and cones i.e. photoreceptor) change into electrochemical signal and rest light absorbed by choroid and sclera. Here in animals tapetum bounce light back towards the source, ultimately preventing absorption and enhance the effect of dim light that is why nocturnal animal use minimal light to maximum extent and are able to visualize the things in dark.
In animals the tapetum lucidum come in various shapes, sizes and colors. It appear as white structures in some eyes, and as yellow or blue objects in others. The color of tapetum depends largely on the chemical and physical composition of eyes, including the amount of pigment in them and presence of certain substances such as zinc and riboflavin.
Diagram of the three types of the tapetum lucidum present among vertebrates. 1. Incoming light ray; 2. First stimulation on the rods and cones; 3. Reflection on the tapetum lucidum; 4. Second
stimulation on the rods and cones.
Absence of tapetum
Animals without a tapetum lucidum (e.g. primates, squirrels, birds, red kangaroo and pig) generally have diurnal habits, and a red or orange to pale gray fundus reflection. The red to orange background results from reflection of light from choroidal blood vessels. Birds are apparently the only large group of animals in which the tapetum is consistently absent.
By: Shashi Bharti, Dr. Rakhi Vaish, Dr. Nidhi Gupta, Dr. Yogita Pandey and Dr. R.P.Singh