Eye (глаз) – an organ of perception of light stimulation in some invertebrates (in particular, cephalopods), all vertebrates and humans. Most invertebrates function of the eye are less complex organs of vision, such as compound eyes. In vertebrates, the eye pair, are located in their sockets of the skull – and orbits consist of your own eyes or eyeballs, connected through the optic nerve to the brain, eyes and paranasal parts: age (see. the lids), lacrimal apparatus and eye-motor muscles. All vertebrates have a common type of eye structure.
The structure of the eye
The eyeball has a spherical shape. The central point of the front surface of the eye called the anterior pole, a point located on the rear surface, in the place of origin of the optic nerve – the posterior pole. The line connecting the poles of the eye is taken for an anatomical axis, it coincides with the geometrical axis. In the front, facing the light of the eye is located diopter (light-refracting) apparatus (refractive media system, which includes the cornea (See. The cornea), a transparent lenticular lens – the lens, aqueous humor and filling the cavity of the eye vitreous, as well as serving to accommodate the ciliary body and iris, or the iris), transmitting the image on a photosensitive retina (See. Retin-A), or retina.
The wall of the back of the eyeball consists of three shells, tightly adjacent to each other. The dense outer shell – the sclera – has a supporting and protective value; it gives the eye shape being like a skeleton. At the open front side of the eye, the sclera becomes thin and transparent cornea. Under the sclera is richly endowed with blood vessels choroid, the front of which is in the form of a thin plate forms the iris, which has a hole in the middle – the pupil. In the tissue of the iris in most vertebrates are special muscles – sphincter and dilator that, expanding and narrowing the pupil to regulate entry into the eyes of the light rays, and the iris, ie. A., Plays the role of the diaphragm.
Of pigmentation of the iris containing pigment cells – chromatophores – and transmission through the transparent cornea, eye color depends. If there are no chromatophores, or outer layer of the retina is devoid of pigment, the iris shine through the blood vessels of the choroid and eyes are red (see. Albinism). Painting of the iris sometimes varies depending on the age, sex and animal habitats (dark color eyes of many young birds and bright in old or milky-white color of the iris in young and orange-yellow in older, for example, in goshawk). Behind the iris ciliary body located – ring roller containing muscle fibers. To him using fiber bundles Zinn suspended bag lens. Reduction of ciliary body causes in most vertebrates change in curvature of the lens, ie. E. The accommodation (See. Accommodation) (adaptation of the eye to the distant or near vision).
The inner shell photosensitive eye – the retina. At the posterior pole of the eye is yellow spot; somewhat closer to the center line of it is land, devoid of sensitivity to light – a blind spot. At this point is collected from the entire retinal nerve fiber of the optic nerve as extending further into the brain. Different classes of vertebrate eye has a number of characteristics.
The fish eye cornea is totally flat and spherical lens:
Eye of fish (pike longitudinal section of the eye): 1 – the retina; 2 – pigment layer; 3 – choroid; 4 – the optic nerve; 5 – the lens; 6 – supporting ligament of the lens; 7 – muscle to move the lens; 8 – the tendon of the lens; 9 – Iris; 10 – cornea; 11 – the sclera; 12 – silver shell.
In the cavity of the eye juts from the choroid special crescent process containing smooth muscle fibers that attach to the bag of the lens. Thus, the accommodation of the eye in fish is achieved by moving the lens. In the back of the choroid often contains a special layer of cells filled with crystals of light pigment so-called silver shell. Some fish have a shiny layer – mirror (tapetum) that reflects light rays back to the retina, resulting in an apparent glow eyes of some fish in almost complete darkness (eg sharks). In some deep-sea fish eyes are reduced, while others are well adapted to capture faint light underwater depths – huge, telescopic eyes.
Four-eyed fish pupils have elongated in the vertical direction; cornea is divided by a horizontal bar in the upper and lower sections. When the fish floating on the surface, the upper part of her eye is able to survey the air environment, the bottom – water.
Amphibians cornea differs big bulge:
Eye amphibian (frog eye longitudinal section): 1 – the lower oblique muscle of the eye; 2 – inferior rectus muscle of the eye; 3 – the lower eyelid; 4 – muscle, attracting the lens; 5 – the optic nerve; 6 – nictitating membrane; 7 – pupillary muscles; 8 – scleral cartilage; 9 – superior oblique muscle; 10 – upper rectus; 11 – the upper eyelid; 12 – Zinn fiber.
In the iris and ciliary body has an underdeveloped musculature; Accommodation of the eye by moving the lens by means of a special muscle, attracting the lens forward to the cornea, as well as by the pressure of ciliated muscle.
In reptiles (Fig. 1), excluding the Gutter and the turtles, and birds (Fig. 2), except for kiwis, vitreous juts from the entrance of the optic nerve outgrowth characteristic, abundantly supplied with blood vessels, – crest. Visual apparatus of birds in many respects superior to the eyes of other animals. The eyeball has a very large size and unique structure, thereby increasing the field of view. Birds that have particularly keen eyesight (vultures, eagles), the eyeball is elongated “telescopic” form. In the retina, there are up to three yellow spots.
Fig. 1. Eye reptile (snake eyes longitudinal section): 1 – the muscles of the iris; 2 – the front wall of the lens; 3 – cornea; 4 – ciliary body with muscles; 5 – suborbital scales; 6 – the sclera; 7 – supraorbital scales; 8 – fused eyelids; 9 – Zinn bunch.
Fig. 2. Eye of a bird of prey (longitudinal section): 1 – the cornea, sclera in passing; 2 – choroid; 3 – ciliary body; 4 – vitreous; 5 – Iris; 6 – the lens; 7 – the optic nerve; 8 – ridge; 9 – retina; 10 – the sclera; 11 – front camera.
The mammalian eye in its form approaches to the eyes of amphibians. In aquatic mammals (eg, whales) it is the convexity of the cornea and a high refractive index resembles the eyes deep-sea fish. In carnivores, pinnipeds and kitoooraznyh the inner surface of the choroid forms, as well as a number of fish, the brilliant pigment layer – mirror.
Reduced eyes are in cave fish and amphibians (eg Proteus); living in underground mammals (eg, moles) they differ or deep bedding under the skin, or the lack of the lens, iris and some layers of the retina.