Hyoscine and the Mydriatic Alkaloids

The physiological phenomenon of mydriasis may be produced by the action of a number of alkaloidal bodies, and is known to bear a certain relationship to their chemical structure. A dilatation of the pupil may theoretically be produced by a paralysis of the circular muscle fibres of the iris, or of the endings of the oculo-motor nerve, or by the stimulation of the radial muscle fibres, or of the sympathetic nerve endings. The mydriatic alkaloids (of which atropine is the best known) act in the first-mentioned manner?that is, they paralyse the sphincter fibres and the oculomotor nerve endings. Whether atropine also stimulates the sympathetic nerve endings is uncertain ; it seems probable that it does not. Cocaine, on the other hand, owes its mydriatic effect mainly to its stimulation of the dilator fibres, and moreover it is without effect on the ciliary muscle and the light reflex. For this reason it is not usually classed among the