Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 114

Accommodation is a rapid change in the refractive power of the eye, which is intended to bring the images of objects at different distances into focus on the retina. The stimulus for the accommodative response is a blurred, or defocused, retinal image. In vertebrates, eyes accommodate by one or more of the following mechanisms: (i) changing the curvature or position of the lens; (ii) changing the corneal curvature; (iii) changing the distance between the cornea and retina; or (iv) having two or more separate optical pathways of different refractive powers. Accommodation is most commonly measured using IR photoretinoscopy, which uses reflection of IR light from the fundus to measure dynamic changes in the refractive error. Since mammalian accommodation is mediated by contraction of the smooth ciliary muscle, it can be stimulated by pilocarpine. Humans and other primates accommodate by changing the curvature of the lens (Figure 2.12). To view distant objects, sympathetic innervation induces relaxation of the ciliary body muscle, which in turn leads to stretching of the lens zonule. The increased tension of the zonules results in a greater pull on the lens capsule, thus causing the lens to become more discoid and decreasing its overall axial thickness and refractive power in a process of disaccommodation. To accommodate for near objects, the reverse process takes place. Parasympathetic input induces contraction of the ciliary body muscles, leading to relaxation of the zonular fibers and reduced tension on the lens capsule. In turn, this liberates the inherent elasticity of the lens, resulting in a more spherical lens possessing greater axial thickness and refractive power. Consequently, anterior chamber depth decreases and increases during accommodation and disaccommodation, respectively.

Unlike mammals, chickens (and possibly lizards) also accommodate by changing the corneal curvature. Corneal accommodation is mediated by the anterior ciliary muscle. Contraction of Crampton's muscle, which originates in the sclera and inserts in the cornea, flattens the peripheral cornea and increases the curvature of the central cornea. Corneal accommodation is reported to play an important role in chicken accommodation, contributing 8–9 D. Thus, the reported combined (corneal and lenticular) accommodative power of the eye in young chicks is 25 D, compared with a maximal power of 15 D in children.

Figure 2.11 The effect of vitreous elongation on ocular refraction. (a) A focused emmetropic eye. (b) The refractive power of the eye has not changed, and the light is focused on the same spot as in panel (a). However, due to vitreous elongation, the retina has moved posteriorly, and therefore the light is now focused in front of the retina. As a result, the eye is now nearsighted or myopic.