Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
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a The refractive power of the cornea and tears is not additive. Rather, that of the former arises from the latter, and from its interface with air. The net power of the tears and the anterior and posterior cornea is 43 D.
Lens
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.
Vitreous
The next refractive tissue is the vitreous. Though there is little refraction as light passes from the lens into the vitreous (due to their similar refractive indices), the vitreous plays an important role in refractive development of the eye. Vitreous elongation increases the axial length of the eye, thereby increasing the refractive path of light and inducing myopia, or nearsightedness (Figure 2.11). In certain fish, this mechanism serves to increase ocular refraction and compensate for loss of corneal refractive power. In different goldfish strains, for example, the vitreous body can contribute anywhere from 37% to 70% of the total axial length of the eye.
Table 2.14 Eye size (ascending order) and corneal power (descending order) in selected animal species.
| Species | Axial length (mm) | Corneal power (D) | References |
|---|---|---|---|
| Goldfish | 4.2 | 129 (in air) | Hughes (1977) |
| Rat | 6.3 | 112.7 | Hughes (1977) |
| Chicken | 8.9 | 108 | Cohen et al. (2008) |
| Guinea pig | 8.9 | 83.9 | Howlett & McFadden (2007) |
| Sea otter | 14.0 | 59.2 | Murphy et al. (1990) |
| Rhesus monkey (4 months) | 16.3 | 56 | Qiao‐Grider et al. (2010) |
| Rabbit | 18.0 | 44.6 | Hughes (1977); Wang et al. (2014) |
| Cat | 21.3 | 43.0 | Habib et al. (1995) |
| Dog | 19.5–21.9 | 37.8–43.2a | Gaiddon et al. (1991); Nelms et al. (1994); Rosolen et al. (1995) |
| Ostrich | 38/0 | 25.3 | Martin et al. (2001) |
| Elephant | 38.8 | 21.3 | Murphy et al. (1992a) |
| Horse | 39.2 | 16.5 | McMullen & Gilger (2006) |
| 43.7 | 15.7–19.5 | Farrall & Handscombe (1990); Miller & Murphy (2017) |
a The range of values in the dog probably reflects a breed difference, because larger breeds have flatter corneas.
Accommodation
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