Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 52

The crystalline lens is a transparent, avascular structure that focuses light onto the retina. It is suspended within the eye by zonules arising from the ciliary body epithelium (i.e., pars plicata) and attaching circumferentially to the lens capsule at the lens equator. The lens is also held in place posteriorly within a shallow depression in the anterior vitreous (i.e., the patella fossa), and the iris rests against it anteriorly. In many mammals, birds, and reptiles, the lens is biconvex; the degree of convexity (i.e., shape) changes during accommodation due to the elasticity of the capsule and the pliability of the lens fibers. In young mammals, the lens is quite soft, with only a small, central, denser nucleus. The lens grows throughout life, with newly formed fibers added continuously to the outermost cortex, causing compression of the central, older zone of lens fibers. This results in a hardening of the central nucleus (i.e., nuclear sclerosis), which reduces accommodation ability as the lens ages.

The refractive power of the lens is less than the cornea because the change of refractive index is much greater at the air–cornea interface than at the aqueous–lens and lens–vitreous interfaces. Contraction of the ciliary body muscle reduces tension on the lenticular zonules, changing the shape of the lens and resulting in an alteration of the dioptric power. Of the roughly 60 diopters of total refractive power of the eye, the lens contributes approximately 13–16 diopters in humans. In dogs, the dioptric power of the lens contributes approximately 40 diopters. The remaining refraction is provided by the cornea.

The lens is proportionately larger in domestic animals than in humans. The dog lens has a volume of approximately 0.5 ml and averages 7 mm in thickness at the anteroposterior axis, with 10 mm equatorial diameter. The ratio of lens volume to entire globe volume ranges from 1:8 to 1:10. The equine lens, on the other hand, has a volume of approximately 3 ml, 12–15 mm average anteroposterior axis thickness, approximately 21 mm equatorial diameter, and a lens–globe ratio of 1:20. Lens volumes of sheep, cattle, and pigs fall between these volumes, thicknesses, and diameters. The lens consists of an enveloping basement membrane called the lens capsule, an anterior epithelium, and lens fibers occupying two main zones: the nucleus and the cortex (Figure 1.47).