Agents Affecting the Organs of Special Sense.

Mydriatics—are agents which cause dilatation of the pupil of the eye (mydriasis). Some act only when applied locally, others only when taken internally,—but the principal ones, Atropine and its congeners, act both locally and internally, and at the same time, produce temporary loss of accommodation, the eye remaining focussed for distant objects, and the intra-ocular tension being increased. The loss of accommodation is due to the drug's causing paralysis of the ciliary muscle,—and the dilatation of the pupil to its producing stimulation of the end organs of the sympathetic nerve, and paralysis of the end-organs of the 3d cranial (motor oculi) nerve; the former increasing the power of the radiating muscular fibres of the iris, and the latter lessening the power of its circular fibres. The smallest quantity of Atropine which will affect the pupil is stated by Donders as the 1/700000th of a grain. Cocaine acts by stimulation of the ends of the sympathetic alone, and does not paralyze the 3d nerve. As a mydriatic it has peculiar qualities, which should make it preferred by eye-surgeons. These are referred to in detail in the body of the book under the title Coca. The chief mydriatics are:—

Atropine. Cocaine. Anaesthetics, (at last).
Hyoscyamine. Duboisine. Gelsemine, (locally only).
Daturine. Homatropine. Belladonnine, (internally only).

Myotics, (muv, to close) ,—are agents which produce contraction of the pupil (myosis). The principal local myotic, and the only one used in ophthalmic practice, is Physostigmine, (Eserine), which acts by stimulating the circular muscular fibres of the iris—at the same time contracting the ciliary muscle so that the eye is accommodated for near objects only, and diminishing intra- ocular tension,—in all of which respects it exactly antagonizes the action of Atropine, (see above). The other myotics are as follows, viz.:—

Pilocarpine. Ergot. Nicotine (locally).
Muscarine. Gelsemine (internally). Anaesthetics (at first myotics, but mydriatics at last).
Morphine. Lobeline (internally).

Ocular Sensibility is increased by Strychnine, the vision being thereby rendered more acute, and the field of visions being enlarged. This drug also increases the field of vision for blue colors, while Eserine diminishes it for red and green, and Santonin causes objects to appear of a violet color at first, and of a greenish-yellow color afterwards.

Amblyopia, or impairment of vision due to nerve changes, is often produced temporarily by Quinine, and may be permanently induced by Alcohol and Tobacco, also by Lead and Urea poisoning.

Visions are caused by several drugs, the action of which is probably exerted on the sight centres in the cerebrum (angular gyrus and occipital lobes),—rather than on the eye itself. The delirium and hallucinations produced by Alcohol are familiar examples,—the objects raised thereby being usually animals, as snakes, toads, dogs, etc. Cannabis Indica often causes pleasant visions, while Sodium Salicylate, in some persons, produces very disagreeable ones. The Bromides, if taken in continued large doses, may, in the typhoid condition which follows, cause visions of such intense character that they are often impressed permanently on a brain which, at the time, was utterly unconscious of all its real surroundings. Digitalis may produce subjective sensations of the continued presence of light,—and Cannabis Indica, among the many curious effects produced by its ingestion in large doses, frequently gives origin to similar disturbances of the visual function, of indefinite and varied character.

The Ear is affected by several drugs. Strychnine and Morphine increase the excitability of either the auditory nerve, or the auditory centre in the superior temporo-sphenoidal convolution,—making that faculty much more acute than nominal. Quinine, Antipyrin, and Salicin produce hyperaemia of the auditory apparatus, and cause subjective noises, such as humming, buzzing or ringing, which are very unpleasant to their subject.

The Smell is rendered more acute by Strychnine, and is depressed by all the cerebral depressants, and by such agents as cause changes in the nasal mucous membrane, as Potassium Iodide.

The Taste is not heightened or lowered as a faculty by any drugs that we know of. Each substance makes its own peculiar impression on the nerve-organs of taste, and that of one agent may be so potent as to overcome the impressions left by another agent. The so called "after-taste" of drugs is often entirely different to the original taste, and substances which are excreted by the salivary glands, (the Iodides, etc.) leave a very persistent after-taste behind them.

A Compend of Materia Medica, Therapeutics, and Prescription Writing, 1902, by Sam'l O. L. Potter, M.D., M.R.C.P.L.