Читать книгу Essentials of Veterinary Ophthalmology - Kirk N. Gelatt - Страница 133

Cones are inactive in scotopic conditions, and in such an environment our fovea becomes a relative blind spot. Instead, scotopic vision is possible because of the molecular and anatomical characteristics of both rods and the rod pathway. The unique features make an individual rod more sensitive than an individual cone. Another important feature that enables sensitive scotopic vision is the converging nature of the rod pathways. In cats, it has been estimated that in the peripheral retina, the output of approximately 75 000 rod photoreceptors converges on about 5000 rod bipolar cells, which output to 250 amacrine cells, that converge on one ganglion cell.

In many animal species, increased number and density of rods enhance scotopic vision. It can be appreciated that dogs have a higher maximal rod concentration than cats, even though most people associate the latter with greater scotopic sensitivity. This discrepancy may be explained by the structure of the tapetum, which is less reflective in dogs than in cats.