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In pursuance of our purpose to avoid uveal tissue, we split the cornea, and place the trephine as far forward as such splitting will allow, and we bear on the trephine in such a way that it cuts through on the corneal edge of the wound first.
To summarize: The aqueous has direct access to the anterior uveal venous system; the physiological thickening of the strands of the meshwork of the iris angle supplies a mechanical obstruction between the anterior chamber and the venous sinus of Schlemm; intra-ocular pressure stands at the same level as the intra-venous, consequently, the hydrostatic pressure is the same on both sides of the iris angle meshwork, because the canal of Schlemm is a secondary venous system; lastly, the outflow of aqueous into the venous sinus is by diffusion, not by filtration, because the pressure is the same on both sides of the meshwork.
Second: The object of trephining is to tap and permanently drain the aqueous fluid from the anterior chamber of the eye into the sub-conjunctival space; in doing so it is essential to avoid as far as possible all interference with the uveal tissue. The purpose of an iridectomy is to avoid the danger of the iris in the neighborhood of the wound being drawn and impacted in the trephined hole.
In acute glaucoma there is congestion of the entire uveal tract, the congestion partaking more of a venous stasis than of an active or arterial congestion. The vessels of the ciliary process, which are larger and more tortuous in adults of advanced years than in the young, become enormously distended, causing almost complete obliteration of the perilental space.
The third group are cases associated with glaucoma only as causes. In eyes with low nutritive power, or subject to exacerbations of increased intra-ocular pressure, uveal inflammations may prove disastrous.
The spongy nature of this meshwork affords free access of aqueous to the venous sinus of Schlemm, thence by tributaries into the supra-choroidal space and anterior uveal venous system. Fuchs's iris cripts afford direct access of aqueous to the veins of the iris. Furthermore, two simple principles are taught by physics: Fluids are incompressible and they seek the lowest hydrostatic level.
The increase of intra-ocular pressure noted in cases of uveal inflammation, to be presently referred to, may be due to some such tendency. But it is rational to ascribe to obstruction of the filtration angle of the anterior chamber, the important part it has been supposed to play in the pathology of glaucoma.
In some cases the iris becomes adherent to the head of the ciliary processes and, when atrophy of the ciliary body occurs, is drawn backward at the base of the iris by the receding tissues. If the hypertension persists or is repeated at varying periods, a slow atrophy of the uveal tract sets in.
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