Microcryoscopic Determination of the Osmolality of Interstitial Fluid in the Living Rabbit Cornea

¹ Howe Laboratory of Ophthalmology, Harvard University Medical School, and the Massachusetts Eye and Ear Infirmary Boston, Mass.; Alfred P. Sloan Student Fellow.
² Howe Laboratory of Ophthalmology, Harvard University Medical School, and the Massachusetts Eye and Ear Infirmary Boston, Mass

A micropuncture method for isolating samples of corneal interstitial fluid is described; valmss obtained were on the order of 10^-7 c.c. and were sufficiently large for freezing point determinations. The effect of loss of carbon dioxide from samples of aqueous humor is discussed. It was concluded that loss of carbon dioxide into mineral oil used to protect the sample from evaporation has no appreciable effect on the freezing point. Corneal interstitial fluid taken from the center of the cornea was found to be isotonic with aqueous humor and 10 milliosmoles hypertonic to arterial serum. Corneal interstitial fluid taken about 2 to 3 mm. from the limbus toas found to he 2 milliosmoles hypotonic to that taken from the center, suggesting an osmotic gradient across the cornea; however, this difference ivas not sufficiently large to be statistically significant. Corneal interstitial fiuid taken 2 to 3 mm. from the limbus teas found to be 8 milliosmoles hypertonic to arterial serum, indicating a relatively large osmotic gradient between this area of the cornea and the limbal capillary bed. These experiments suggest the following generalization: When the evaporation of tears has been temporarily arrested in the rabbit cornea in vivo, the immediate limbal area is the site of a significant osmotic gradient, while the greater part of the cornea is very close to being in osmotic equilibrium with the fluid on its posterior surface.