VAN 121: Veterinary Gross Anatomy-II (2+2)

IRIS

• It is a muscular diaphragm placed in front of the lens and is visible through the cornea.
• It is pierced centrally by an elliptical opening, the pupil.
• The ciliary border is continuous with the ciliary body and is connected with the ligamentum pectinatum iridis.
• The pupillary border surrounds the pupil. The upper margin bears small masses of pigmentary tissue called corpora nigra or granula iridis.
• The anterior surface is dark brown in colour and is lined by the reflection of the corneal endothelium. It forms the posterior boundary of the anterior chamber.
• The posterior surface is dark. Its central part is in contact with the lens but peripherally it is separated from it by an annular space called the posterior chamber.
• It is lined by endothelium. The anterior and posterior chambers communicate through the pupil and contain aqueous humor.

Structure

•  This iris chiefly composed of a framework of connective tissue called the stroma iridis containing numerous pigment cells.
• The muscular tissue is unstriped and consists of a sphincter pupillae around the pupil and a dilator pupillae with fibres radiating from the sphincter to the ciliary border.
• The anterior surface of the iris is covered by a continuation of the endothelium of the cornea. Beneath this is a condensation of stroma in which the cells are close together and are filled with pigmented granules.
• The colour of the eye is determined by the pigmentation of the iridic stroma. If it contains little pigment, the pigmented epithelium on the posterior surface shows through and gives a blue coloration.
• In the cat, there is diffuse yellow pigmentation of the stromal cells and so the eye has a golden sheen. In albinos, the pigment is absent here, as elsewhere, and the iris is pink is colour.

Iris angle

• It is the angle of the anterior chamber and is so called because in meridional section, it has an angular shape.
• It is occupied by a loose spongy tissue, the meshwork of iris angle, whose outer border forms the inner wall of canal of Schlemm.
• The spaces of the meshwork are called spaces of Fontana and are in communication with the anterior chamber.
• The canal of Schlemm appears in meriodional sections as one or more endothelial lined oval spaces.
• It communicates with the anterior ciliary veins in the neighboring scleral tissue.
•  Together with the spaces of Fontana, the canal forms a means of exit for the intraocular fluid. The aqueous humor secreted by the epithelium over the ciliary processes, passes from the posterior chamber, through the pupil into the anterior chamber, on reaching the angle of iris, passes through the spaces of Fontana to the canal of Schlemm.

Blood supply

• Ciliary arteries.

Nerve supply

• Long and short ciliary nerves. Parasympathetic fibres from oculomotor nerve to sphincter pupillae whereas sympathetic fibres innervate dilator pupillae.