Veterinary Gross Anatomy-II

NERVOUS TUNIC 

• The retina forms the innermost tunic of the eyeball and extends from the entrance of the optic nerve to the iris. It consists of three parts; a posterior part the essential region containing the special neuroepithelium, the pars optica retinae. This terminates around the ciliary body where it ends in a regular circular line, the ora ciliaris retinae (ora serrata in man). Hereafter, the retina loses its nervous elements. The part of the retina over the ciliary body is the pars ciliaris retinae and that over the iris is the pars iridica retinae. Pars iridica retinae are a layer of pigmented cells that covers the posterior surface of the iris. The area centralis retinae is a rounded spot situated above the optic papilla and corresponds to the macula lutea of man. The entrance of optic nerve forms a definite oval area -the optic papilla or blind spot. The optic nerve fibres converge from all parts of the pars optica to the papilla, where they collect into bundles that traverse the lamina cribrosa of the chorioid and sclera, and constitute the optic nerve.
• The pars optica retinae is that part of the eye which transforms the stimulus of light into nerve impulses, resulting in the sensation of vision. Embryologically, the retina arises from the two layers of ectoderm that form the optic cup. The outer layer gives rise to the pigment epithelium; the inner layer forms the remainder of the retina. In pathological detachment of the retina, or as generally occurs in the fixed and preserved specimens, the primary embryonic cavity between the two layers become reformed and the two parts of the retina gets separated, the pigment epithelium remaining adherent to the chorioid. The strictly nervous portion is firmly attached to the underlying structures at only two regions, at the ora ciliaris retinae and at the optic disc. The retina detached from the pigment epithelium is a thin, delicate membrane, which in life is transparent.