Distensibility of the human eye.

TiE relationship between changes in pressure and changes in volume within the eye is of practical importance in the calibration of tonometers and in tonographic calculations, and the coefficient of ocular rigidity, K, proposed by Friedenwald (1937) has been employed generally for these purposes. Contrary to Friedenwald's assertion, it has been found that this coefficient is not a constant, but varies with intra-ocular pressure in the eyes of rabbits (Perkins and Gloster, 1957a, b) and of cats (Macri, Wanko, Grimes, and von Sallmann, 1957). In both species, the values of K increased markedly as the intra-ocular pressure increased. In view of these findings, it is of considerable importance to determine whether the coefficient of rigidity of human eyes is similarly dependent upon the intra-ocular pressure. Little attention has been given to this matter, although MacDonald (1955) produced some tonometric evidence which suggested that scleral rigidity was inversely related to intra-ocular pressure in living human eyes, and McBain (1957) reported that the rigidity of enucleated human eyes decreased as the intraocular pressure increased. Our observations, which have already been described briefly (Gloster, 1957), are in general agreement with the above findings and this paper gives a fuller account of our results.