In low light conditions, this part of the eye is dilated, so more light can reach the retina to improve night vision.
The amount of light that enters the eye is controlled by the iris and the pupil. The opening of the pupil is controlled by the muscles within the iris.
One muscle contracts the opening of the pupil and makes it smaller and another muscle of the iris dilates the pupil making it larger, and controlling the amount of light that enters through the pupil.
The central nervous system is influenced by a variety of factors such as light, vision distance, alertness and cognitive load, these messages are sent and the dilation or constriction of the pupil occurs.
The illumination of the retina is then the product of these incident factors.
Therefore, this dilation or constriction of the pupil allows a rudimentary adaptation to different environmental settings.
The constriction of the circular pupils is achieved by ring-shaped muscles, while the closure of the slit pupils involves two additional muscles that compress the opening laterally, allowing a much greater change in area.
Depth of field for horizontal contours is determined by the vertical extent of the pupil.
The pupils appear generally equal in size and black in color, this color is due to the fact that the light that filters through the pupil is absorbed by the retina and is not reflected in the back of the eye.
Types of pupils
The pupils are classified according to their shape in:
The vertically elongated pupil is a smart adaptation that facilitates stereopsis and to estimate the distances of objects located at ground level while allowing the depth from the blur to the estimated distances along the same.
The vertically elongated pupils can be adequately described as slits, they are vertically elongated pupils to favor nocturnal activity and ambush.
Eyes with vertical slit pupils have an astigmatic depth of field – larger, meaning less blur due to blur.
The predators of nocturnal and polyphasic ambushes generally have vertical slit pupils, some examples are: the domestic cat, the snake, among others.
Also the vertically elongated or subcircular pupil of the lynx is located in this classification.
These types of pupils offer great flexibility in terms of the intensities of light they can receive.
Pupils that have this shape can reach very varied aperture diameters.
When the intensity of light in the environment is very high, the pupils remain practically closed, but if the intensity decreases, they open completely and then acquire an almost spherical shape.
These types of pupils are very suitable for those animals that are active, both during the day and at night.
Circular pupils are presented in those animals that are normally active in bright light and need good vision in these circumstances.
Most diurnal predators with daytime activity and active feeding have circular pupils.
Examples of these circular pupils are man, rabbits, and almost all birds.
The constriction of the circular pupils is achieved by ring-shaped muscles.
The horizontally elongated pupils are roughly rectangular and their appearance changes with dilation and constriction.
Retinal images of horizontal contours are more blurry than images of vertical ones.
Some examples of horizontal pupils are domestic sheep, goats, horses, deer, elk, among others.
Herbivorous animals or land prey are very prone to horizontal pupils.
The horizontal pupil shape allows more illumination from the sides and less from above and below.
These types of pupils help reduce the intensity of sunlight.
The horizontal pupil is present in herbivores and rotates automatically when the animal’s head changes position, and thus manages to remain stable and parallel to the ground.
This mechanism that herbivores with this type of pupil display, allows them to continue grazing and to be able to raise their heads without losing their points of attention in this movement, protecting them from an attack by their natural predators while they feed.