Chapter 45: Eye Anatomy

The external structure is protected by the fibrous outer coat, comprising the opaque sclera and the transparent cornea. The cornea, which accounts for approximately seven percent of the exterior surface and is the major refractive structure, consists of five traditional layers, including the highly regular stromal layer, which ensures transparency, and the innervated epithelium, which serves as a protective barrier. Internal to the outer coat is the vascular uveal tract, consisting of the posterior choroid, the ciliary body, and the anterior iris. The ciliary body contains smooth muscles and is crucial for regulating the shape of the elastic lens, which is suspended by zonular fibres, thereby adjusting the eye’s focal power during accommodation. The anterior segment is divided by the iris and lens into the anterior and posterior chambers, which, along with the larger vitreous chamber posteriorly, are filled with humours. Aqueous humour, produced by the ciliary body epithelium, supports the avascular cornea and lens metabolically and drains primarily through the trabecular meshwork into the canal of Schlemm (scleral venous sinus). Disruption of this drainage mechanism results in increased intraocular pressure and can lead to glaucoma. The iris acts as an adjustable diaphragm, regulating the size of the pupillary aperture via the parasympathetically innervated sphincter pupillae (constriction) and the sympathetically innervated dilator pupillae (dilation). The internal surface of the eye is lined by the photosensory retina, where rods and cones transduce light into neural activity. The inner retina receives a direct blood supply from the central retinal artery, while the outer retina receives nutrients indirectly from the underlying choriocapillaris of the choroid. The central retina, or macula, contains the foveola, a specialized rod-free, avascular region where inner retinal layers are displaced, ensuring maximum visual resolution due to the dense packing of cones. Specialized retinal glial cells, known as Müller cells, span the neural retina and contribute to the blood-retinal barrier. The output of the retina is transmitted by the axons of ganglion cells, which converge at the optic disc to form the optic nerve. These axons follow a strict retinotopic map, with nasal fibres crossing at the optic chiasma, and they project through the optic tract to the lateral geniculate nucleus and onward via the optic radiation to the primary visual cortex, enabling visual perception.