This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
Belladonna is a feeble local irritant, has upon the circulation either a moderately excitant effect, or no direct effect whatever, and powerfully stimulates the brain. It undoubtedly operates on the system through absorption. The proofs of this fact are that it produces the same effect to whatever surface it may be applied, even when introduced into the blood, and that its active principle has been detected in the urine. It is asserted that the urine of a rabbit which had been fed upon it. caused dilatation in the pupil of a cat to which it was applied. According to M. Runge, of Berlin, belladonna, stramonium, and henbane, are the only substances which have this effect on cats. (Orfila, Toxicologic.) The circumstance that this plant, as well as others of the same natural family of Solanaceae, is eaten by some animals with impunity, while it kills others, proves that the medicine acts on the susceptibility of the tissues, and not by a chemical reagency upon their organization. From the succession of phenomena produced, it may be inferred that belladonna acts with a powerful irritant influence, primarily on the centres of conscious perception in the brain and annular protuberance, and subsequently on the spinal marrow; as sensibility to pain is lost, while yet reflex action continues, showing a persistent sensitiveness of the gray matter of the cord. The congestion of the cerebral membranes and those of the upper part of the spinal cord, observed after death, confirms this view of the operation of the poison. (Lemattre).
* The generally admitted fact, that plants poisonous to man are taken with impunity by certain of the lower animals, does not, according to M. Lemattre, imply that the active principles are not poisonous to these animals, if absorbed into the circulation. M. Lemattre found that all animals, upon which the trial was made, were susceptible of the poisonous action of atropia, and the other solanaceous alkaloids. The impunity, therefore, with which rabbits, goats, etc., may eat the leaves of the belladonna and stramonium plants must be attributed to the non-absorption of the alkaloids from the stomach, or so slow an absorption that sufficient does not enter the circulation at one time to produce deleterious effects. This may be explained in part by the fulness of the stomach with various fresh vegetable matters, which interfere with the rapid digestion of the poison. (Archives Gen., Juillet, 1865, p. 89; also Lancet, Sept. 2, 1865, p. 269.) - Note to the third edition.
One of the most curious effects of this medicine, and of other Solanaceae, is that which they exert on vision and the pupil. The dilatation of the pupil is probably owing to au irritant influence upon the centres of the sympathetic nerve, a branch from which governs the action of the dilating fibres of the iris. Its influence on vision may be ascribed, when complete amaurosis exists, to a constriction of the blood-vessels of the retina, depriving it for a time of blood, and, of course, of sensibility. but a less degree of visual disturbance, such as presbyopia, or an inability to see near objects, while the distant are visible, is probably referable to the action of the sympathetic on the ciliary muscles, thereby disturbing the accommodating powers of the eye. When belladonna is locally applied to the eye, the effect is much more prompt than from its internal use, and quite as great if not more so. But, under these circumstances, the vision is not affected, and the nervous centres of the retina, therefore, not acted on. Generally only the eye to which the application is made is affected; but it is asserted that the other eye sometimes participates; in which case it is possible that the medicine may have been absorbed, and reached the cerebral centres. The dilatation of the pupil and dimness of vision appear to be direct, without any preceding excitation of the organ.