Thermal gradients in the chick eye: a contributing factor in experimental myopia

Domestic chicks were reared for 4 weeks with a plastic dome or a plastic ring glued to the skin surrounding the right eye. The domes, which degraded retinal images by reducing high spatial frequencies and contrast, have been reported to produce enlargement of the ocular globe and large myopic refractive errors. The rings, which do not produce refractive errors, did not affect vision and served as a control for the mechanical effects of having a device glued to the circumorbital skin. At the end of the rearing period, the chicks were anesthetized and a thermoprobe (a thermocouple in a 29 gauge needle) was inserted into the eye along the optic axis. Temperature readings were made at 1 mm intervals to a depth of 12 mm. Temperature readings also were taken of the circumorbital skin and the air inside the dome. The results indicated that the temperature in the dome eyes was elevated from 2.8 to 5.2 degrees C at the cornea and 0.7 to 2.0 degrees C at the axial sclera. Smaller elevations were found in the ring eyes. Two dome chicks that had lost their devices 24-48 hr prior to temperature measurement had thermal gradients that were nearly identical to those from untreated control eyes. Measurement of the air temperature inside the dome revealed a temperature elevation of nearly 4.0 degrees C above that recorded at an equal distance from control eyes. The circumorbital skin of treated eyes was 0.96 degrees C warmer than the comparable tissue of untreated eyes.(ABSTRACT TRUNCATED AT 250 WORDS)