Middle layer of the eye consists of three parts (diagrams 167, 168): uvea or choroid (vascular layer), iris, ciliary body. Uvea or choroid (vascular

Middle layer of the eye (uvea, iris, ciliary body, pupil)

Middle layer of the eye consists of three parts (diagrams 167, 168): uvea or choroid (vascular layer), iris, ciliary body.

Uvea or choroid (vascular layer)

The uvea contains many arteries, veins and capillaries; its function relates to feeding of other eye layers and formations. In front choroid becomes the ciliary body, which has a smooth and glossy ciliary muscle and bundles that are attached to the outer edge of the crystalline lens.


The iris — is the front part of the choroid, which is placed in the pupil. The color of the iris is predetermined by ratio of melanin pigment groups (black, brown, yellow). If there’s a small amount of pigments, a human’s eyes are gray or blue. Newborn’s iris has little pigment, so their eyes are always dull gray (dairy). Later they darken and take on color, which is determined by the content of a certain pigment. Taken from http://worldofschool.org

Diagram 167. Structure of the eye: 1 — retina; 2 — vitreous; 3 — the visual axis; 4 — the optical axis; 5 — the central fossa; 6 — yellow spot; 7 — the optic nerve; 8 — the sclera; 9 — choroid; 10 — ciliary body; 11 — rear camera; 12 — front camera; 13 — lens; 14 — cornea; 15 — iris; 16 — ciliary muscle; 17 — conjunctiva
Diagram 168. Eye (front view): 1 — the upper eyelid; 2 — nasolacrimal channels holes; 3 — nasolacrimal duct; 4 — lacrimal caruncle; 5 — iris; 6 — lower eyelid; 7 — the pupil; 8 — the sclera; 9 — eyelashes; 10 — lacrimal gland


Pupil (figure 168) is the hole in the iris through which the light penetrates into the middle of the eye. Its diameter varies depending on the light level: the more light around — the more narrow he is, the less light — so it is wider. In the dark, the pupil dilates. Changing its lumen occurs due to a reflex, which protects the eyes from bright light. Pupil size change as a result of contraction of the iris smooth muscle: one of them are managed by sympathetic, and others — by the parasympathetic nervous system. During the excitation of the parasympathetic part of the autonomic nervous system pupil narrows and sympathetic — it expands. For example, if there is fear — the pupils dilate, and therefore people say: “fear has big eyes”. The reactions of both pupils to the light are united. For example, when effecting one eye pupil with the light, the other eye also narrows.

  • Explain the interrelation between the iris structure and functions.

  • Show the interaction of the sympathetic and parasympathetic nervous system on the example of the pupillary reflex.