The intra-ocular pressure depends greatly on the amount of fluid contained in the vitreous, and this in turn is determined by two factors :(1) The amount of fluid secreted by the ciliary body.

(2) The freedom with which fluid escapes at the angle of the anterior chamber.

The aqueous humour and the fluid which nourishes the vitreous and crystalline lens are chiefly secreted by the ciliary processes. It ultimately passes out from the anterior chamber of the eye by a number of small openings (/, Fig. 75) close to the junction of the cornea and iris into the canal of Schlemm (c, s, Fig. 75), thence into the anterior ciliary veins. Some of it also passes into the perichoroidal space, and out through the lymphatics.

The intra-ocular pressure may be increased by (a) more rapid secretion from the ciliary processes, or (b) interference with its outward flow from the eye, or (c) by increased quantity of blood in the vessels of the iris. It may be diminished by the contrary conditions.

More rapid secretion from the ciliary process probably takes place under nervous conditions which are not at present well known. Interference with the flow of the aqueous humour out of the anterior chamber may occur in aquo-capsulitis, in which the openings from the anterior chamber into the spaces of Fontana are occluded by a coating of inflammatory lymph; also in glaucoma where these openings are shut by the iris being pressed forward against the cornea, as in Fig. 75, and in iritis where the iris is much congested and the communication between the posterior and anterior chambers is interrupted by complete adhesion of the pupillary edge of the iris to the anterior capsule of the lens (total posterior synechia). The secretion is probably diminished by the action of atropine. In glaucomatous states where the periphery of the iris lies in contact with the cornea the outward flow through the spaces of Fontana may often be increased by Calabar bean, which, by causing contraction of the circular fibres of the iris, flattens the arch of the iris and, drawing it away from the cornea, reopens the contracted angle between the cornea and iris, and permits the passage of fluid through the spaces of Fontana.1

There are few or no experiments on the tension in the vitreous humour of the eye, though by the term intra-ocular tension is usually intended the pressure in the vitreous humour. The degree of intra-ocular tension is usually ascertained by pressing the finger secundum artem upon the eye and observing whether it is harder or softer than usual, or by pressing upon the sclerotic with an ivory point attached to a registering spring, and noticing the pressure required to produce an indentation. These methods of experiment are valuable clinically, but the tension can be more exactly ascertained in animals by passing a small trocar into the anterior chamber and connecting it with a manometer. The results of experiments even by this method are not entirely in accordance. The most recent ones by Graser 2 appear to show that the tension depends to a great extent upon the height of the blood-pressure generally : contraction of the pupil diminishes, and dilatation increases the intra-ocular tension. Eserine causes temporary increase at first, but after contraction of the pupil comes on, the tension is diminished. Atropine in doses sufficient to dilate the pupil increases the tension. The precise effect of atropine on intra-ocular tension in man is disputed. From clinical observation the truth would seem to be that in a perfectly healthy eye and in ordinary iritis atropine and other mydriatics diminish tension, whereas they increase the tension when the anterior chamber is shallow from narrowing of the iridic angle. In glaucomatous states atropine and other mydriatics almost always rapidly increase tension. This action of atropine and its allies not only makes them dangerous in cases of glaucoma, but where this disease has been impending it has been at once brought on by their use. From its power to diminish tension eserine is useful in glaucoma.

Fig. 75.   This diagram (which I owe to the kindness of Mr. J. Tweedy) represents a section through the corneo scleral region, ciliary body and iris, of a healthy eye (left side), and of a glaucomatous eye (right side) : k, cornea; s, sclerotica; i, iris; /, spaces of Fontana; c s, canal of Schlemm.

Fig. 75. - This diagram (which I owe to the kindness of Mr. J. Tweedy) represents a section through the corneo-scleral region, ciliary body and iris, of a healthy eye (left side), and of a glaucomatous eye (right side) : k, cornea; s, sclerotica; i, iris; /, spaces of Fontana; c s, canal of Schlemm. In the glaucomatous eye the ciliary body is atrophied, and the iris lies against the cornea, preventing the escape of fluids through the spaces of Fontana and canal of Schlemm.

1 J. Tweedy, Practitioner, Nov. 1883, vol. xxxi. p. 321.

2 Graser, Archiv f. exp. Path. u. Pharm., Bd. xvii. Heft 5.