Histology of the rat and cat retinas. Histological section of a normal adult Sprague-Dawley rat and cat retina stained with toluidine blue. Three vertical bars on the left show the assignments of the three MRI-derived layers. CH, choroidal vascular layer. In the cat, there is an additional, vascularized layer known as the tapetum. The rat and cat histological slides are drawn approximately to scale. Adapted from Fig. 5 of Cheng et al, Proc Natl Acad Sci U S A,6 and Fig. 1 of Shen et al., J Magn Reson Imaging.48