The ring like opening based in the middle of the iris is called pupil. It regulates the amount of light that enters the eye. The tissues present inside the eye suck most of the light that falls on the pupil. That is the main reason behind why the pupil looks dark or black in color. The image of the pupil when seen from outside is much larger than its actual size. This is because the cornea magnifies it manifold.
Soft muscles surround the pupil. When the light enters the eye, iris controls the amount of light that enters the eye by changing the size of the pupil according to the amount of light. This phenomena can be explained by the fact that when we go out in the sun, our eyes contract and take a few seconds to adjust to the rays of the sun. Like wise, when the eyes see something interesting or unusual it tends to open more, and hence there is a phrase in English ‘wide eyed'. The eyes get smaller in presence of light and in darkness the pupils widen.
The iris comprises of two types of smooth muscles. The roundish group is known as the sphincter pupillae, and another is radial group known as the dilator pupillae. When the sphincter pupillae reduce in size, it makes the iris contract, reducing the size of the pupil. The dilator pupillae is connected with the sympathetic nerves. It makes the iris to dilate when the dilator papillae contract. The pupils and the structure of the eye is in such a way that both work together in perfect harmony. Both the eyelids bat together, see the same distance and allow the same quantity of light to enter the pupil.
The size of the pupils will reduce when pilocarpine is given as eye drops to any person. This is because the parasympathetic activity increases. As opposed to this, atropine will temporarily stall the parasympathetic action and result in dilation of the pupil. Dilation of pupil can occur either as a result of increase of the sympathetic nerve activity in the neck, or when the adrenaline is infused. The maximum size of the pupil is different from person to person. The pupil of a 15 year old may be in the range of 5 mm to 9 mm. After the age of 25 the size of the pupil goes on decreasing.
When exposed to sun light or bright light, the pupils gets narrower to restrict the excess light rays entering the eyes. In the dark, the pupil widens to adjust to the lack of light. In addition to this, the pupil dilates when a person experiences certain emotions like pain or fear or happiness. A few drugs can cause the pupil to constrict or dilate. For example, use of alcohol and opioids reduces the pupil. Whereas drugs like atropine, marijuana, LSD, mescaline, psilocybin mushrooms, cocaine and amphetamines widens the pupil. Decrease in the size of the pupil is also called miosis. Medicines that lead to miosis are called asmiotic.