Glaucoma / by Priestley Smith.

  • Smith, Priestley.
Date:
[1891]
Catalogue details

Licence: Public Domain Mark

Credit: Glaucoma / by Priestley Smith. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    indirectly of the intra-ocular pressure. Unfortunately for all methods of ophthalmo- tonometry which have been hitherto devised, and, in fact, for all which appear to be mechanically practicable, the ratio which the depth of the tonometer-impression bears to the internal pressure of the eye is not constant; it is affected by several factors, the influence of which cannot be accurately ascertained, e.g., the size of the eye, the shape of the eye, and the degree of elasticity possessed by its tunics. From a theoretical point of view, it is worth while to note that if it were possible to measure the diameter of the impression instead of its depths we could calculate the internal pressure therefrom. If to the surface of a sphere in which an impression is produced by external pressure we imagine a plane surface to be applied, the line of contact is a circle ; let the term impression-plane denote the imaginary plane bounded by this circle; then the following rule holds good for all cases, irrespective of the size of the sphere, the amount of elasticity possessed by its walls, and the degree of pressure, internal and external. It assumes, however, a perfect pliability in the walls:— Rule.—The external pressure equals the internal pressure per unit of area multiplied by the number of such units in the impression-plane. For example, if a weight of twenty pounds applied externally produce an impression- plane equal to twenty square inches, the internal pressure equals one pound per square inch. Surmising the existence of such an equation, and being unable to test it mathe- matically, I did so in the first place by experiment. My friend, Mr. Mason Worthington, M.A., F.R.A.S., of Clifton College, has since furnished me with a mathematical demonstration of its truth. Even this method, however, would fall short of the desired end, for it would only tell us the amount of the internal pressure during the formation of the impression in the eye, whereas it is the pressure as it exists unmodified by the use of the instrument which we desire to know ; moreover, it leaves out of account the amount of resistance due to rigidity in the tunics themselves. It is certain that, however faultless the construction of the tonometer, and however skilful its application, it cannot be made to yield exact indications of the intra-ocular pressure. So long as eyes differ in size, shape, elasticity, and rigidity, exactitude will be unattainable. But since these same sources of error beset the digital method, also, in addition to one or two others peculiar to itself, especially the varying thickness and firmness of the eyelid, it by no means follows that the mechanical method is useless. My own experience is that the tonometer indicates differences of tension smaller than those which can certainly be detected by the finger, its superior delicacy being chiefly conspicuous in consecutive examinations of the same eye under varying conditions. In certain experiments and investigations it is indispensable to employ some method which, whether more accurate or not, shall, as far as possible, eliminate the effect of the investigator's own wish or prejudice; and here the tonometer gives invaluable aid ; in practice it is little likely to become a substitute for palpation by the fingers. The other symptoms of glaucoma are closely related to the increased tension; they vary in character and in intensity according to the rapidity with which the internal pressure rises, the time of its duration, and the height to which it reaches. In accordance with these variations we recognise three forms of primary glaucoma—the chronic, the sub-acute, and the acute. Each possesses distinctive clinical features, but is not divided from the next by any hard and fast line. Chronic Glaucoma (Syn. Glaucoma Simplex) begins almost imper- ceptibly, progresses slowly, with Httle tendency to exacerbation or re- mission, and leads inevitably, unless arrested by treatment, to blindness. The patient, when first seen by the surgeon, is usually at least fifty years