Corpus: Iris

In veterinary medicine, the term iris' refers to a flat ring of tissue in the eye that is part of the centre of the eye and surrounds the central visual opening (pupil). It regulates the width of the pupil and thus the intensity of light entering the eye.

The iris is located between the cornea (cornea) and the lens (lens oculi) and thus divides the space of the eye into the anterior and posterior eye chamber. The peripheral edge of the iris (margo ciliaryis) is connected to the ciliary body. There is also a connection to the peripheral section of the cornea through the pectinate ligament. The central edge that borders the pupil is known as the pupillary margin (margo pupillaris).

The iris is divided into different layers from rostral to caudal. The anterior surface (facies anterior) is patchily lined with endothelium from the anterior chamber of the eye. In the peripheral ciliary zone (annulus iridis major), conspicuous folds are formed. In contrast, the central and thinner zone (annulus iridis minor) is characterised by lower folds.

The iris stroma itself consists of connective tissue fibres, pigment cells, nerve fibres and blood vessels. The sphincter pupillae muscle is located close to the pupillary margin and surrounding the pupil. It is parasympathetically innervated and is responsible for constricting the pupil (miosis).

The posterior surface (posterior facies) is covered by two epithelial layers. These layers represent the anterior section of the pars caeca retinae and are also known as the pars iridica retinae. The posterior layer is a pigmented layer, while the anterior layer is formed by the radial dilator pupillae muscle. This muscle is sympathetically innervated and is responsible for pupil dilation (mydriasis).

The size of the pupil and the amount of light reaching the retina is regulated by the sphincter and dilator pupillae muscles of the iris (pupillary mechanism). A contraction of the sphincter pupillae muscle causes the pupil to constrict, while contractions of the dilator pupillae muscle cause it to dilate.

In domestic mammals, the pupil is round at maximum dilation, as in humans. When constricted, this circular shape is only maintained in dogs, large wild cats and almost in pigs. The pupil of the domestic cat is a high vertical slit, in ruminants and horses it is an elongated transverse oval.

In hoofed animals there are strongly pigmented grape granules (granula iridica) on the upper and lower pupillary margin of the iris. As a rule, the upper granules are larger and more numerous than the lower ones. The function of the granules is not yet fully understood (2021), although additional secretion of aqueous humour or glare protection against bright light is suspected.

• Salomon FV, Geyer H, Uwe G. 2008. anatomy for veterinary medicine. 2nd, updated and expanded edition. Stuttgart: Enke Verlag in MVS Medizinverlage Stuttgart GmbH & Co. KG. ISBN: 978-3-8304-1075-1