Lenticular

Lens is a transparent, refracting light body, which has the shape of a biconvex lens, located in the eye between the iris and the vitreous body. After the cornea, the lens is the second refractive medium of the optic system of the eye.

The front surface of the lens (facies anterior lentis) and the most prominent point of it - the front pole (polus anterior) are facing toward the posterior chamber of the eyeball. The more convex posterior surface (facies posterior) and the posterior pole of the lens (polus posterior lentis) adhere to the anterior surface of the vitreous. A conditional line connecting the anterior and posterior poles of the lens, having an average length of 4 mm, is called the axis of the lens (axis lentis). This axis coincides with the optical axis of the eyeball. The rounded peripheral edge of the lens, where the anterior and posterior surfaces converge, is called the equator. The substance of the lens (substantia lentis) is colorless, transparent, dense, contains no vessels and nerves. The inner part - the nucleus of the lens (nucleus lentis) is much denser than the peripheral part - the crust of the lens (cortex lentis).

Outside, the lens is covered with a thin transparent elastic capsule (capsula lentis), which is attached to the ciliary body with the help of a ciliary band (zinn ligament) coming from the capsule of the lens. The capsule of the lens is an unstructured, vitreous, elastic membrane. The capsule of the lens has a selective permeability, as a result of which the chemical composition of the transparent lens is stable.

With the contraction of the ciliary muscle, the choroid itself shifts forward, the ciliated body approaches the equator of the lens, the ciliary band weakens and the lens is sorted out. At the same time, the anteroposterior size of the lens increases, it becomes more convex, its refractive power increases - the lens is placed on a close vision. In the case of relaxation of the ciliary muscle, the ciliary body is removed from the equator of the lens, the ciliary band is stretched, the lens is flattened, its refractive power decreases and the lens is set to a distant vision. The ability of the lens to see at different distances is called accommodation. Therefore, the lens together with the ciliary muscle (ciliary body) and the fibers that connect them is called the accommodation device of the eye.

At a young age, the lens fibers are soft, elastic. When the ciliary muscle is contracted, when the zinn ligament relaxes, the lens takes on a more spherical shape, thereby increasing the refractive power. With the growth of the lens, the centrally located older lens fibers lose water, they become stronger, become thinner, and a dense core of the lens is formed. This process, preventing excessive lens enlargement (which causes the lens to grow throughout life without increasing in size), begins very early, and by 40-45 years there is already a well-formed dense nucleus. The lens fibers surrounding the nucleus form the cortical layer of the lens. With age due to the increase in the nucleus and the reduction of the cortical layer, the lens becomes less elastic, its accommodative capacity decreases. The processes of metabolism in the lens are extremely slow. Exchange takes place with the participation of epithelial cells of the anterior capsule of the lens. They get all the necessary substances from inside the eye fluid that surrounds the lens on all sides.

The lens looks like lentils. The curvature of the anterior surface is 10 mm, the posterior surface is 6 mm, i.e. The posterior surface is more convex, the thickness of the lens (diameter) is 9-10 mm. The lens weighs 0.2 g. At the child the lens has a spherical shape. Identification zones:

1. front and back pole - centers of the front and back surfaces;
2. axis - a line connecting the poles;
3. equator - the front-to-back transition line.

Histological structure of the lens (capsule, epithelium, fibers, nucleus):

1. capsule - collagen-like membrane, part of it (zocular plate) can be separated from the front surface. The front of the capsule is thicker;
2. epithelium are hexagonal cells under the anterior capsule, which are drawn in the region of the equator;
3. the lens fibers are hexagonal prisms. A total of about 2.5 thousand fibers moving toward the center, they grow to the poles, but do not reach the poles. In the junctions of the anterior and posterior fibers with the capsule, seams are formed;
4. nucleus - embryonic and adult human. There are seams in the embryonic core. The adult nucleus, which is formed by compacting the crystal fibers, is formed by the age of 25 years. The lens contains the following substances: water, proteins, mineral salts, lipids, ascorbic acid. In the lens 60% water, 18% soluble protein (alpha, beta and gamma proteins). The main protein - cysteine - provides transparency of the lens. 17% are insoluble proteins (albuminoids) contained in fiber membranes; 2% - mineral salts, a small amount of fats.

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