Belladonna is employed locally for its sedative action, to relieve pain and allay irritation and inflammation in the conjunctiva, cornea, choroid, or iris.

Mydriatics and myotics are used not only for their action upon the pupil but for their action upon accommodation and intra-ocular pressure.

Mydriatics are employed to dilate the pupil for the purpose of facilitating ophthalmoscopic examination, assisting the detection of cataract commencing in the periphery of the lens, or allowing the patient to see past the edge of a cataract or corneal opacity when this is central in position, and obstructs the vision with a pupil of normal size. They are used to prevent prolapse of the iris, or to restore it to its normal position when already prolapsed in cases of perforating ulcer or mechanical lesion of the cornea. They are employed in iritis to afford rest to the inflamed tissues of the eye, and to keep the iris as far as possible off the surface of the lens and prevent adhesions of its posterior surface to the anterior surface of the lens.

Mydriatics are employed to paralyse the ciliary muscle, and thus destroy the power of accommodation in order to test the condition of the refractive media of the eye in cases of astigmatism, or in cases where the patients either suffer from spasm of the ciliary muscle or are unable voluntarily to relax the accommodation.

Myotics are used to counteract the effect of mydriatics which have been previously employed, or in mydriasis following a blow or paralysis of the third nerve. They are used also to counteract deficiency in tone of the ciliary muscle, as in paralysis of accommodation consequent on diphtheria, asthenopia, a blow on the eye, etc.

Myotics are useful in cases of threatening and commencing glaucoma and often even in more advanced cases of glaucoma, from their power to lessen intra-ocular tension. As a temporary expedient they are often of the greatest service in cases of acute glaucoma. So, also, if perchance the instillation of atropine have induced glaucoma, myotics will not only counteract the mydriasis, but often rapidly restore the intra-ocular tension to the normal standard.1

Mydriatics and myotics may be employed alternately in order to ascertain the presence of any adhesions of the iris, and to break them down if present.

In glaucoma the intra-ocular tension within the anterior chamber is greatly increased, and the increase, according to Tweedy, is due to the natural channel of escape for the aqueous humour through the spaces of Fontana and the canal of Schlemm being obstructed by the iris lying against the cornea. This condition is relieved by myotics, which, by causing contraction of the pupil, draw the iris away from the cornea, and thus allow the fluid to escape through the spaces of Fontana. When the anterior chamber of the eye is shallow and the iris is lying close to the cornea, so as nearly, though not quite, to obstruct the spaces of Fontana, atropine may induce an attack of glaucoma by dilating the pupil and thus packing the tissue of the iris into the angle between it and the cornea, so as to render the obstruction to the spaces of Fontana complete.