Studies on the retina. The structure of the retina of phrynosoma cornutum

Abstract

Estudios sobre la retina.La estructura de la retina de Phrynosoma cornutum.La estructiira general de la retina de Phrynosoma es muy semejante a la de la retina de Chameleon. Existe un Brea central muy desarrollada en forma de una convexidad circular situada encima del punto de entrada del nervio 6ptico, conuna fovea que alcanza el mliximum de desarrollo en su'centro. Un pecten c6nico muy vascularizado se proyecta dorso-anteriormente hasta cerca de un milimetro en la camara posterior del ojo. La retina posee solamente conos, asemajhndose bajo este punto de vista a la retina de otros saurios diurnos. Los conos exhiben una variaci6n considerable en tamafio, forma y estructura. Los conos de la fovea estBn muy atenuados y son cilindricos, diferencihndose por estos caracteres de 10s de forma chnica, situados en la regi6n extra-foveal. La presencia de conos dobles no ha podido demostrarse. Bajo condiciones ordinarias de iluminacih el pigmento se extiende sobre las cklulas visuales, llegando casi hasta las paraboloides, except0 en la regi6n de la fovea, en la cual solo 10s segmentos externos estBn recubiertos por el.

The Visual Cells and Visual Pigments of the Vertebrate Eye

“…A fovea is present and this is often of the deep, convexiclivate type. Cones are numerous or exclusively present (ROCHON-DuVIGNEAUD, 1917;DETWILER and LAURENS, 1920) and in these, yellow oil-droplets are common. The visual cells of Anolis lineatopus (Fig.…”

Morphology, Characterization and Distribution of Retinal Photoreceptors in the South American (Lepidosiren paradoxa) and Spotted African (Protopterus dolloi) Lungfishes

“…Without a reference image of P. dolloi double cones from Pfeiffer (1968), this is difficult to confirm. Double cones are present in many teleost fishes (Stell and Hárosi, 1976;Collin, 1997;Collin and Shand, 2003;Pignatelli et al, 2010), and most terrestrial vertebrates such as birds (Hart, 2001), amphibians (Mariani, 1986), and diurnal reptiles (Detwiler and Laurens, 1920;Sillman et al, 1997), and some marsupials and monotremes (Young and Pettigrew, 1991;Ahnelt and Kolb, 2000;Ebrey and Koutalos, 2001). The function of double cones is still under consideration, but there is evidence that they aid in the detection of motion and discrimination in fine spatial detail rather than chromatic visual tasks in birds (for review see Hart and Hunt, 2007) but are involved in color discrimination in reef teleosts such as Rhinecanthus aculeatus (Pignatelli et al, 2010).…”

Prey snapping and visual distance estimation in Texas horned lizards, Phrynosoma cornutum

“…Precise accommodation requires high visual acuity. Based on the structure of the eye (M.O., personal observation) and the retina (Detwiler and Laurens, 1921) it is apparent that horned lizards have high visual acuity, comparable with that of chameleons (Ott and Schaeffel, 1995). Therefore, the eye of Phrynosoma should be capable of detecting small deviations in image focus and, hence, be capable of precise accommodation control.…”

“…For the eye of Phrynosoma, this equation yields a value of 0.31·D for the depth of field [pupil diameter=1.5·mm, as measured in the living eye, and maximal resolved spatial frequency calculated by the equation SF=PND/ͱ3 ϫ photoreceptor-distance ϫ 57.3 (Reymond, 1985), assuming a receptor spacing of 2·µm (after Detwiler and Laurens, 1921) and a posterior nodal distance (PND) of 0.6 ϫ axial length (5·mm) of the eye]. With this conservative calculation (it is very likely that the actual resolving power of the eye is higher than estimated), the lizard would have a depth of field of 0.022·mm at a focal distance of 2·cm, 0.06·mm at 3·cm, 0.1·mm at 4·cm and 0.159·mm at 5·cm.…”