 
                            OUTFLOW  VI

                        CHOROIDAL CIRCULATION 

            Let us make a model of drainage from the choroidal
       circulation.

            In Issue No. 11 of the Glaucoma Letter we discussed the
       venous drainage from the retinal circulation.  Inferences
       that can readily me made from retinoscopic observation alone
       made it possible for us to give explanations for the
       phenomena of A-V nicking, of increased V/A ratio in
       arteriosclerotic and hypertensive vascular disease.  Such
       observations also suggested a possible mechanism for the
       selective distribution of glaucomatous optic nerve damage,
       and by extension suggested that it might be impairment of
       the venous circulation that was responsible for the optic
       atrophy and excavation in glaucoma rather than impairment of
       arterial circulation as has hitherto been assumed.  In
       addition, and perhaps not least in importance, reflections
       on the mechanisms of venous drainage from the retinal
       circulation forced on us as they were by the simple process
       of observation, by the need to explain and the desire to
       understand what is so readily observable, provide us with
       tools to think about venous drainage in general and give us
       courage to consider other mechanisms of venous drainage in
       the eye that are more or less inaccessible to inspection.
       We have long been so concerned about the mechanisms by which
       aqueous humor drains from the eye, that we have taken the
       drainage mechanisms of the retinal circulation for granted,
       and the choroidal circulation we have ignored as if it did
       not even exist.  The mountaineer who aspired to climb Mount
       Everest but spurned to practice ascent of lesser summits
       would have little chance of success.

            Nor is it necessary to assume that there should be some
       a priori similarity between the drainage of blood and that
       of aqueous.  To be sure, the issue of similarity and
       difference when we undertake to analyse and to describe
       phenomena, biologic and otherwise, is a touchy one.  The
       reason we systematically deny judgment from similarity is
       that it betrays the veracity of objective determinations.
       When we point out that one thing is similar to another, we
       reveal the nature of our intellectual apprehension, but
       unless we are so bold as to postulate an apriori generic
       identity, we say nothing at all about the thing that we
       purport to explain.  Rather we demonstrate our inability to
       grasp that which we seek to understand.  To say that Jones
       is like Smith, where Jones and Smith are by nomenclature
       different, is to admit that we are unable to distinguish one
       from the other, is to admit that we comprehend neither of
       them, and to demonstrate the particular level at which our
       understanding is arrested.

            There is, however, another aspect to this issue which
       becomes apparent once we forgo our insistence that we should
       comprehend and control the object and accept the fact that,
       where what we know is limited, the way in which we know it
       may be the only facet of our cognitive experience
       susceptible  to correction and improvement.


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Copyright 2006, Ernst Jochen Meyer
