Dr. Edward Jackson's Paper on Etiology and Classification of Glaucoma
Discussion,
Francis Lane, M.D.
Chicago.
Not one of the theories thus far propounded to explain the essential cause of increased intra-ocular tension is satisfactory. Our present day knowledge apparently ceases with a more or less incomplete understanding of the mere circumstance under which increase of tension in general depends.
The question of the source of the normal intra-ocular pressure must first be solved before any discussion of a pathological increase can be engaged in. This question primarily hinges on whether the corneo-sclera is to be regarded as an unelastic capsule with a fixed volume, or as a yielding envelope with an ever changing capacity.
This brings us at once to the consideration of that theory which probably has held our attention for the longest period of time, i. e., the volumetric theory. According to it, the normal intra-ocular tension depends on the volume of fluids within the eyeball. Any variation in the quantity of the contents gives rise to a change in the pressure, therefore, the globe has been regarded as "an elastic capsule, whose capacity, form, and internal pressure depend on the balance struck between a constant inflow, or formation of aqueous, and a proportionate outflow or resorption." (Henderson.)
Hill has satisfactorily demonstrated that, under physiological conditions, the hydrostatic pressure within the eye and the skull is identical; it rises and falls simultaneously; it is the same as the cerebral venous pressure; it is constantly varying, depending directly on the general circulation. Upon these findings Henderson based his opinion that the physiological properties of the tunica fibrosa and the skull are identical, realizing at the same time, that the rigidity of the corneo-sclera, because of its fibrous nature, is not as firm as the cranium. In accepting this belief the inference was that the cubic capacity of both coverings is fixed. Applying these conclusions to the eye, it can be said that the pressure of the fixed intra-ocular volume varies with the venous tension within the bulb, which in turn is influenced by the general circulation. Such a conception, while not strictly in accord with recognized physiological teachings, proves that the normal intra-ocular pressure is not a question of volume content, but that it is purely a question of pressure of a fixed volume within an unyielding capsule. Dr. Jackson virtually puts aside the volumetric theory with his statement, that "the balance of intra-ocular pressure is not maintained by the slight distensibility of the sclero-corneal coat." Further discussion on the inadequacy of the volumetric theory need not detain us.
It is well to recall a few anatomical features because of their bearing on the theories herein considered.