Distensibility of the eye.

IN clinical ophthalmology the intra-ocular pressure can rarely be determined directly, and reliance must be placed on measurements of the tension of the coats of the eye. The instruments used for the estimation of ocular tension (tonometers) measure the depth of indentation of the, globe produced by a known weight. The depth of indentation depends on several factors, the most important of which are the intra-ocular pressure and the distensibility of the eye. Variations in distensibility can have a large influence on the calibration scales of tonometers and therefore some knowledge of the relationship between changes in volume in the eye and the resultant changes in intra-ocular pressure is necessary. Schulten (1884), Koster (1895, 1901), Greeves (1913), Ridley (1930), and Clark (1932) investigated such volume-pressure relationships in the eyes of various species and, in spite of some differences in detail, the common feature of all the results of these investigators was that the change in pressure induced by a given change in volume in an eye was not constant but depended on the initial intra-ocular pressure. Smaller changes in pressure resulted from the same change in volume at lower intra-ocular pressures than at higher intra-ocular pressures. Friedenwald (1937) attempted to derive an expression relating the pressure and volume changes which could be applied over, a wide range of intra-ocular pressures. In an analysis of the results of Koster, Ridley, and Clark, he claimed that above pressures of 5 mm. Hg a linear relationship existed between the volume increment and the logarithm of the pressure. Accepting this relationship, he formulated an equation: