The relation between capillary pressure and secretion. II.— The secretion of the aqueous and the intraocular pressure

Abstract
Many observations have been recorded of the- aqueous or intraocular pressure. The method generally used has been to pass a hollow needle through the cornea and connect this with a manometer. J. Herbert Parsons* says: “ The first essential is . . . that no fluid shall enter or leave the eye whilst the needle is being inserted.” It is no less essential that the aqueous shall not escape along the track of the needle or into the manometer. Although expressing the opinion cited above, Parsons gives the figure of a needle employed by him, which not only has an open end, but also two side holes, one nearer to, one farther from, the open end; so contrived, he says, to give free admission to the aqueous fluid. Such a needle seems to us especially liable to allow the escape of the aqueous during insertion. Starling and E. E. Henderson used a needle with an open eyehole, from which fluid was allowed to escape at a pressure conjectured to be greater than the pressure of the aqueous. By this means they believed to prevent the escape of aqueous during insertion. It is not certain that this means is effectual, since pressure on the eyeball raises the intraocular pressure greatly, and considerable pressure may be required at the moment of insertion. The method also permits the entry of fluid into the eye should the pressure of the fluid employed be greater than that of the aqueous. We have measured the aqueous pressure by means of a specially constructed hollow needle, used in conjunction with the method employed by one of us (L. H.) when measuring the intracranial or cerebro-spinal fluid pressure.* The needle (fig. 1) consists of two hollow tubes, one revolving within the other.

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