A handbook of local therapeutics / edited by Harrison Allen.

Date:
1893
Catalogue details

Licence: Public Domain Mark

Credit: A handbook of local therapeutics / edited by Harrison Allen. Source: Wellcome Collection.

    140/554 (page 128)
    modation. Its exact mode of action is not yet definitely determined. It is known to be local. It must be absorbed and reach the iris directly before it can act. The aqueous humor of an atropinized eye contains sufficient atropine to dilate the pupil of another eye. It causes my- driasis when applied to the eye immediately after death, or, in experi- ments upon animals, after removal of the brain and spinal cord. Its action is upon unstriated muscle, and it does not affect the pupil of birds whose iris muscular fibres are of the striated kind.' It is erroneously stated that atropine dilates the pupil by paralyzing the sphincter fibres and stimulating the radial; the former being under the control of the oculo-motor nerve, and the latter of the sympathetic. The existence of dilator muscular fibres in the iris, however, may now be considered extremely doubtful, and is positively denied by some of the best authorities. According to these authorities the dilatation of the pupil is the result of elastic contraction, and the only structure of the iris that takes an active part in it is the posterior limiting membrane, which is the only one not thrown into folds when the pupil dilates. (Fuchs, V. Graefe Arch., xxxi, p. 89, and Oph. Rev., Jan., 1886.) It is not easy, however, on this view to account for the following facts, viz.; that mydriasis from pathological or experimental paralysis of the third nerve is increased by the application of atropine; that the pupil dilated by atropine im- mediately contracts when the sympathetic nerve is severed and dilates again when the nerve is excited by faradization ; or that cocaine, which is supposed to produce mydriasis by its excitant effect on the sym- pathetic, increases the dilatation of an atropinized eye. Gaskell {Jour- nal of Physiol., Jan., 1886) thinks that in the sphincter muscle of the iris we have an example of a muscular structure supplied by two nerves of opposite character, the one motor and the other inhibitory, and that dilators of the pupil act upon the latter. According to this view atropine increases paralytic mydriasis by inhibition of the tonic contraction of the sphincter muscle. Some observers have thought that the vaso-motor action of atropine, in contracting the iris blood-vessels, is an important factor in producing . mydriasis. Belladonna is extensively used in ophthalmic surgery. As a mydriatic the full effect of atropine may be rapidly obtained by a solution of four grains to the ounce. When one drop of this is instilled in the conjunctival sac of a healthy eye, the dilatation of the pupil com- mences in less than fifteen minutes, and attains its maximum, with complete immobility, in twenty or twenty-five. When it is desired to dilate the pupil merely to facilitate ophthalmoscopic examination, a much weaker solution should be used, as mydriasis produced by the strong application does not completely disappear for nearly, or quite,