. A treatise on the diseases of the eye. from the annexed edge of the fixed index (m) is held in a line with the inner edge of thepupil, and then the movable one (/) is gradually screwed up till its edge MYDRIASIS. 291 corresponds exactly with the outer edge of the pupil. The interval betweenthe two indices represents the diameter of the pupil.^—H.] Causes.—Before entering upon the different causes which may producemydriasis, it will be well briefly to consider the action of certain substancesupon the condition of the pupil, either in increasing or in diminishing itssize. Certain su
. A treatise on the diseases of the eye. from the annexed edge of the fixed index (m) is held in a line with the inner edge of thepupil, and then the movable one (/) is gradually screwed up till its edge MYDRIASIS. 291 corresponds exactly with the outer edge of the pupil. The interval betweenthe two indices represents the diameter of the pupil.^—H.] Causes.—Before entering upon the different causes which may producemydriasis, it will be well briefly to consider the action of certain substancesupon the condition of the pupil, either in increasing or in diminishing itssize. Certain substances, more especially belladonna, hyoscyamus, and stra-monium, have the power of producing a marked dilatation of the pupil, andare hence termed mydriatics. We shall here, however, confine our attentionto the action of atropine upon the pupil and the accommodation. Innumerous experiments made by Donders,^ it was found that if a solution offour grains of sulphate of atropine to a fluidounce of water was applied to the [Fig. 108.]. eye, the pupil began to dilate within fifteen minutes, arriving at the max-imum degree of dilatation in from twenty to thirty-five minutes, and finallycomplete immobility ensued. The younger the individual and the thinnerthe cornea, the more rapid was the action. The diminution in the power ofaccommodation commences somewhat later than the dilatation of the pupil,but gradually returns, together Avith the mobility of the pupil, after somedays. After the lapse of forty-two hours there is generally a slight diminu-tion in the size of the pupil, accompanied by some accommodation, whichincreases with tolerable rapidity up to the fourth day, but does not becomeperfect till about the eleventh day. The weaker the solution of atropine,the longer will it take to act, and the less and more transitory will be itseflTect. By employing an extremely weak solution (one grain to eight or tenfluidounces of water), we may dilate the pupil without affecting the ac
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