ANATOMY/PHYSIOLOGY REFRESHER

The cornea is the transparent anterior portion of the fibrous outer layer of the globe. It is made up of several layers – the outer epithelium, the middle stroma and the inner endothelium. The acellular layer of Descemet’s membrane lies between the stroma and endothelium (Figure 20.1). In dogs and cats the corneal thickness is approximately 0.45–0.55 mm. The epithelium itself is multi-layered and non-keratinized and produces its basement membrane. Cells are attached to each other and to the basement membrane via interdigitations of the cell membranes and hemidesmosomes. The stroma is the thickest layer of the cornea, made up of multiple layers of fine collagen-containing cells. Descemet’s membrane is an elastic, acellular layer between the stroma and endothelium. The endothelium itself consists of just one layer of cells and therefore is the thinnest cellular layer. Despite its small size, the endothelium is critical for normal corneal functioning as will be seen later on. The corneal surface is bathed by the pre-ocular tear film. Further details of this can be found in the section on the nasolacrimal system (Chapter 15 and cases in Chapters 16 and 17).

Figure 20.1 Cross-section of the normal cornea.

Sensory innervation to the cornea is provided by the long ciliary nerves which derive from the ophthalmic branch of the trigeminal nerve (cranial nerve V). The nerves branch superficially – this means that more pain-sensitive nerve endings are present in the epithelium and anterior stroma compared to the deeper stroma and is particularly relevant when considering corneal ulcers: shallow ulcers are often more painful than deeper ones, and as such one must be slightly wary when owners return for a follow-up consultation and report that their pet is much more comfortable with its eye. Hopefully this means the ulcer has healed, but it could be a worrying sign that the ulcer has actually got much deeper!

The cornea has several functions. Along with the sclera, the posterior continuation of the outer fibrous coat of the globe, it provides structural support while protecting the intraocular contents. It also has a major role in both the refraction and transmission of light. It actually has more refractive power than the lens since the light rays pass from air to fluid (aqueous) as they traverse the cornea – crossing from aqueous through the lens has less refractive power. Corneal transparency is vital for normal functioning. This is maintained by the absence of blood vessels and pigment, the lattice organi-zation of the cellular structures and the small size of the collagen fibrils. The endothelium has pumps to remove extra fluid from the cornea which renders it in a relatively dehydrated state and this also assists in maintaining transparency. If this pump fails, then dense corneal oedema can result, giving a blue–grey colouration to the cornea and increased thickness.