Extensibility of strips of sclera and cornea.

Log p =KU V, ogO Where Po Initial pressure P=Pressure after a change in intra-ocular volume A V K=a coefficient of ocular rigidity. Friedenwald stated that the value of K remained constant for individual eyes above a pressure of 5 mm. Hg. In a recent study (Perkins and Gloster, 1957), however, it has been found that for the rabbit eye this fundamental assumption is invalid, for K increased as the pressure at which it was determined increased (Fig. 1, overleaf). Indeed, we were unable to formulate a simple expression for the relationship between changes in volume and changes in pressure in the eye of this animal, and suggested that a study of the extensibility of the sclera and cornea might help to explain our observations. Little work appears in the literature on this problem. Schelske (1864) gave figures for a coefficient of elasticity for the cornea and sclera of human and rabbit eyes. In human eyes he found that there was a small difference (approximately 7 per cent.) between the coefficient for meridional and equatorial strips of sclera, but that the cornea was more than twice as extensible as the sclera. In the rabbit the cornea was found to be only some 20 per cent. more extensible than equatorial strips of sclera. Weber (1877) found a large variation in the extensibility of strips of sclera from different human eyes, but gave no figures for the cornea. Ischreyt (1899), using cattle sclera, measured the weight required to break strips of sclera, the extension of the strips at the moment of breaking, and the extension produced by increasing weights. The weights used, however, produced tensions in the sclera corresponding to intra-ocular pressures far above physiological levels.