Migration of chorioidal tapetal pigment in the spur dog Squaluls acanthias

Abstract

This research has been concerned with the movement of pigment in the occlusible tapetum lucidum of the spur dog Squalus acanthias. The pigment expands and covers the reflecting plates of the tapetum in light, and retreats and uncovers the plates in darkness. Movement of pigment in one eye occurs independently of that in the other. Normal movement of pigment is not affected by cutting the optic nerves and oculomotor nerves, by sectioning the cord immediately behind the medulla, nor by excising the pituitary and pineal glands. It is concluded that the process is not under nervous or hormonal control. It occurs normally in excised opened eyes that are kept in oxygenated Ringer solution, but not under partial or fully anoxic conditions. The pigment cells of the chorioid seem to be behaving as independent effectors, sensitive to light. The pigment expands in tapeta freed of retina, in light or darkness. No explanation is yet available for the latter anomalous result. Measurements were made of the reflectivity of isolated tapeta, and curves are presented showing the progressive decrease of reflectivity accompanying expansion of pigment, and changes in the colour composition of the reflected light.