Vertebrate Eye - Adult Eye

There are no main differences in the structure or composition of the eyes among different vertebrates. This should be clear to you from the diagram. The reptilian eyes are also similar. Let us review in brief the structure of a mammalian eye. The eye of mammalian is a ball-like organ composed of, three concentric layers. The outer most is the selerotic coat or selera made of hard connective tissue in mammals and birds, but it is cartilaginous in lower vertebrates. In front it is continuous along with the transparent cornea. The middle coat is the vascular and pigmented choroid that is continuous with the iris. The perforation in the centre of iris is the pupil in front of the lens. The inner coat is the retina containing the outer pigmented retina and the inner neural retina. The latter consists of the light sensitive cells (cones and rods), ganglion cells and other neurons.

Figure: Diagram of Vertebrate Eye

The nerve fibers from ganglion cells from the optic nerve that connects the eye with the brain. Cones are concentrated at fovea in the neural retina and there are no light receptive cells in the region of macula in which the optic nerve passes on way to the brain. The anterior part of retina is non-sensory and is continuous along with the ciliary body which contains smooth muscles. The lens is located behind the iris held in place by the ligament. Contraction and relaxation of these muscle fibers change the distance among the lens and cornea and also their convexities. The space between the cornea and iris is the anterior chamber, and that among the iris and ligament is called the posterior chamber; these spaces are continuous with each other and filled with a fluid(aqueous humor). The large cavity of the eye ball among the lens and neural retina is the vitreous body. The accessory structures of the eye involve eye lids, nictitating membrane, conjunctiva, lacrimal glands and extrinsic intraocular muscles of the eye orbit. As you would notice eye is an very complex organ. The cells that make up the various parts of the eye are contributed by ectoderm and mesoderm. The retina and lens are ectodermal; the cornea, iris and ciliary body are made from cellular contributions from both ectoderm and mesoderm. The extrinsic intraocular muscles are purely mesodermal and thus are the choroid and sclera. The eyelids and lacrimal glands are ectodermal derived from the epidermis.