The oblate annular membrane in the middle of the eyeball wall.It is located between the cornea and lens and can be seen through the cornea, commonly known as "black eye".There is a small round hole in the center, called pupil, from which light enters the eye.The iris is mainly composed ofconnective tissueIt is composed of pigment, blood vessel and smooth muscle.The color of the iris varies with the amount and distribution of pigments, generally including black, blue, gray and brown.
There are smooth muscles arranged around the pupil in the iris, called pupillary constrictor (pupillary sphincter), and smooth muscles arranged radially around the pupil, called mydriasis (pupillary dilater).The former is innervated by parasympathetic nerve, while the latter is innervated by sympathetic nerve.When strong light shines on the eyes, the pupillary sphincter contracts to limit the light entering;When the light is weak, the pupil dilate muscle enlarges the pupil to increase the light entering.The size of normal pupil is also related to many factors, such as age and refractive status.Pupils under 1 year old are the smallest, gradually increasing later, and gradually becoming smaller after puberty.The pupil of myopia is larger than that of hyperopia.When people are in pain and panic, their pupils dilate, while when they sleep, their pupils shrink.
Anatomical interpretation
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In the most anterior part of the middle membrane of the eye, located between the cornea and the lens, its cavity is divided into the anterior chamber and the posterior chamber. The iris is disc shaped, and there is a circular hole in the center, called pupil.There are two kinds of smooth muscle fibers in the iris tissue around the pupil: one is radially arranged around the iris, called the dila tor pupillae, which is dominated by sympathetic nerve fibers. When it contracts, it makes the pupil open to absorb more weak light;The other kind surrounds the pupil, called the sphincter pupillae, which is innervated by the parasympathetic nerve fibers in the oculomotor nerve and shrinks the pupil during contraction to reduce strong light stimulation.The epithelial cells in the inner layer of the iris contain melanocytes. Due to the different number and distribution of melanocytes, the human iris presents various colors such as black, blue or gray.
Iris is also called rainbow.It is the uveal tissue located in the most anterior part of the eye, which is developed from the neuroectodermal tissue in the anterior part of the optic cup and the mesodermal tissue in the anterior part.Its starting point is connected with the front edge of the ciliary body, and extends to the free edge in front of the lens, becoming an important diaphragm separating the anterior and posterior chambers of the eye.There is a round hole (or nearly round) in the center or slightly off the nose, which is called the pupil. It has the function of adapting to the intensity of external light to adjust the brightness entering the eye.The iris has two sides: front and back. The front part is divided into iris pupil and iris ciliary part.The root of the iris is attached to the center in front of the ciliary body, forming a part of the posterior wall of the anterior chamber angle. Therefore, the forward and backward movement of the iris root position will directly affect the width of the chamber angle.The thickness of the iris varies. The root of the iris is thin (it is often broken due to eye contusion). It gradually thickens inwards until the iris ring is the thickest part of the iris, about 0.5 mm (due to the existence of the pupillary sphincter). When it reaches the edge of the pupil inwards, it becomes thin again.The color of the iris mainly depends on the amount of pigment in the iris parenchyma, and is related to race. Occidental people lack pigment cells in the iris parenchyma, while pigment epithelial cells behind the iris have normal pigment, and their iris is blue.The iris parenchyma and pigment epithelium of albino patients are lack of pigment cells and pink, which is the result of fundus reflection.The iris of Chinese people is rich in pigment and brown.Histologically, the iris can be divided into six layers from front to back: endothelial cell layer, anterior limiting membrane, parenchyma layer, muscle layer, pigment epithelium layer and inner limiting membrane layer.From the perspective of embryology, the anterior layer of the iris is the uveal part, which originates from the mesodermal tissue, including the endothelial cell layer, the anterior boundary layer and the vascular layer (anterior and posterior lobes);The posterior layer of the iris is the omentum, which originates from the neuroectodermal tissue, including the muscular layer (sphincter and expander), the pigment epithelium and the inner limiting membrane (some people think that this layer does not exist).
