The escape of the outflow of fluid from the eye is ultimately through the veins. The general venous blood pressure is so low (often negative in the great veins of the neck during inspiration) that no obstacle can come from it to the ocular outflow. The venous blood pressure permits the eyeball to become perfectly soft. We have all seen tension of 5 mm., or even less; and general venous pressure does not rise to the normal intra-ocular tension. Increased intra-ocular pressure requires that there must be some obstacle that keeps the intra-ocular fluid from reaching the general venous system. This may be in the lymph drainage system of the eye; but it may also be in the ocular veins themselves. Experimentally the eyeball can be made to burst by tying all the venous outlets from it. I have seen very high intra-ocular tension develop in a few hours after general thrombosis of the orbital veins. The absence of the canal of Schlemm is noted in congenital buphthalmos. The enlargement of the anterior perforating veins is an old symptom of chronic glaucoma. Obstruction to outflow of blood through the vorticose veins, by the increased intra-ocular pressure, has long been a recognized explanation of the malignant tendency of glaucoma--a part of the vicious circle established in this disease. There is reason that we should give careful attention to the views of Heerfordt and Zirm, that obstruction to the venous outflow may be the effective cause of the disease. Zirm believes the venous plexus of the choroid is an essential part of the mechanism for the regulation of intra-ocular tension, the necessary vaso-motor control depending on nerve centers situated in the iris. _Nerve Control_ The accurate control of normal intra-ocular pressure, by mutual adjustment of inflow and outflow of fluid, is scarcely conceivable without some highly specialized, extremely sensitive nerve mechanism to preside over it. This is suggested by analogy with the regulation of secretion in the lacrimal, salivary, or peptic glands, or the maintenance of blood pressure in the heart and arteries. Clinical observations point the same way. Many patients connect their attacks (especially their earlier ones of ocular discomfort, impaired vision, haloes around the light, and dilated pupil) with social excitement, anxiety, worry, anger or fatigue. A patient of mine gave up her card parties, because an exciting game generally ended in blurred vision, a rainbow around the light, and a dilated pupil, and sometimes an aching eye. Another woman watching beside her dying husband and exposed to extreme cold, had her first attack of glaucoma, so severe as to destroy the sight of one eye. The other eye, also affected at the time, recovered good vision, and has remained several years without a second attack and without treatment.