Sodium has been shown to enter the aqueous predominantly or wholly by secretion. Therefore, if the aqueous were a mixture of a secretion and any considerable amount of ultrafiltrate, the osmotic pressures of these two components would have to be markedly different. A blood pressure reduction which changes the amount of ultrafiltrate in the aqueous would consequently change the osmotic pressure of the aqueous.
The osmotic pressure difference between the aqueous humours of both eyes was measured in rabbits after the blood pressure of one eye had been kept at 30 mm Hg below the normal level for 1.5—2 hours. No significant difference in osmotic pressure was detected. Therefore, the amount of ultrafiltrate in the aqueous cannot be considerable.
This investigation has been made possible by a grant from the Lennander Foundation of the University of Uppsala. Miss G. Thelin AND Mr. A. Rüütli have given most valuable assistance