Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 112

As noted, the refraction that occurs as light passes from the cornea into the AH and during its passage through the aqueous has little overall significance. Therefore, the next significant refractive structure through which light passes after the cornea is the lens. As in the case of the cornea, the refractive power of the lens is determined by both its refractive index and its curvature. In humans and in many nonaquatic species, the refractive index of the lens nucleus is about 1.41; it decreases gradually toward the cortex, forming a bell‐shaped refractive index curve known as the gradient index. In humans, the calculated refractive power of the lens is approximately 22 D.

The second factor determining lenticular refractivity, the lens curvature, also differs between aquatic and nonaquatic species. Generally, it can be said that the lens is spherical in fish and aquatic mammals, while it is more discoid (i.e., less spherical) in terrestrial species. Therefore, the lens will have a higher refractive power in the former compared to the latter (Table 2.14). The reason for the increased refractive index and lens curvature in aquatic species is the loss of corneal refractive power underwater. Of course, the curvature (and, hence, the refractive power) of the lens can also be changed actively through a process termed accommodation.