Sence organs: Eye II

Histology 2016-2017
Department of Anatomy &Histology: Dr.Rajaa Ali
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Sence organs: Eye II
Sclerocorneal Junction/Limbus (fig.)
o The sclerocorneal junction is marked by a shallow depression externally and it is the place where the transparent cornea becomes continuous with the opaque sclera.
o It contains an important circular canal lined by endothelium called canal of Schlemm or scleral venous sinus concerned with drainage of aqueous humour as this canal is connected externally to the anterior ciliary veins and internally to the anterior chamber through spaces of Fontana at iridocorneal angle. In section, the canal appears as an oval cleft whose inner wall is
o formed by trabecular meshwork enclosing spaces of Fontana.
o Aqueous humour is dialysate of blood. It is a clear watery fl uid (similar to CSF in composition) produced by the ciliary epithelium, covering the ciliary processes into the posterior chamber. From there it passes into the anterior chamber via pupil and is continuously reabsorbed (drained) through the canal of Schlemn into the bloodstream. Aqueous humour serves as a medium for exchange of metabolites to and from the avascular lens and cornea. It is also responsible for the intraocular pressure (normal pressure is 20 mmHg).
o Normally, the canal of Schlemn contains no blood, as the communication between the canal and the ciliary veins are oblique and fl attened preventing the refl ux of blood into the sinus. However, under venous congestion blood may pass into the canal.
o Obstruction to the drainage of aqueous humour leads to sustained increase in the intraocular pressure/tension, causing glaucoma. If untreated, this may cause damage to retina resulting in blindness.

Middle Vascular Pigmented Coat/Uveal Tract
o This is a heavily pigmented vascular coat that absorbs light that has passed through the retina, creating a dark chamber for the eyeball.
o It consists of three parts, namely, choroid, ciliary body and iris.
o The arteries of this layer are derived from branches of ophthalmic artery and are responsible for nourishment of 1st–4th layer of retina.