OPERATION OF MEDICINES BY NERVOUS AGENCY.

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volition, from which they are quite distinct. They are effected by ner-vous fibres and a nervous centre. The fibres are of two kinds, onetermed incident ewcitor, the other reflex motor. The centre is the greymatter of the true medulla oblongata and medulla spinalis, from whichthe nervous fibres arise.— (See Grainger's Observations on the Structureand Functions of the Spinal Cord , 1837.) The mode of action of theseparts is this: when a physical agent is applied to any of the external orinternal surfaces of the body, an impression is made on, and carried by,the incident excitor nerve to the grey matter, constituting the nervouscentre of the system; and this part, by its peculiar power, excites con-traction through the medium of its reflex motor nerves. Electricity hasbeen suspected to be the secret agent effecting these communications.

When the nostrils are stimulated, the fauces irritated, or cold waterdashed upon the face, filaments of the fifth pair of nerves are the incidentexcitors ; when carbonic acid, or a drop of water, comes in contact withthe larynx, and when the dust of ipecacuanha is inhaled into thebronchia, with the effect of inducing asthma, filaments of the pneumo-gastric nerve are the incident excitors. In all these instances filamentsof the pneumogastric are the rellex motors, by means of which the actionsof sneezing, vomiting, sobbing, closure of the larynx, and asthma, areproduced. “ It is singular,” observes Dr. Hall, ( Lectures on the Nervous System , p. 156, note,) “ that ipecacuanha, taken into the stomach, shouldexcite vomiting, and, inhaled into the bronchia, should excite spasmodicasthma, equally, as it would appear, through the pneumogastric nerve.”Belladonna applied to the eyebrow causes dilatation of the pupil: theincident excitors concerned in this process are the fibres of the portiomajor of the liftli,—while the reflex motors are derived from the third oroculo-motor nerve. In cases of poisoning by this substance, difficulty orimpossibility of deglutition has been observed,—another effect of itsaction on the excito-motory system.

Mr. Grainger is of opinion, that the ganglions of the sympathetic forma part, though to a certain degree an isolated one, of the excito-motorysystem ; and that their action is excited like that of the spinal cord.—{Op. cit. p. 136, el seq.) lie has also suggested (op. cit. pp. 131-2, 155-6-7,) that the motions displayed by plants and the lower animals areexcited, and not voluntary; and that even in plants itmay be effected by a structure analogous in its office, thoughdiffering in its physical characters, to the true spinal sys-tem of animals. Hitherto, however, no one has been ableto demonstrate a nervous system in vegetables. Dutrochet (.Recherches anatomiques et physiologiques sur la structureintime des Animaux et des Vegetaux, 1824,) indeed asserts,that the small points, or spots, observed on the cells andvessels of plants (figs. 17 and 18,) are analogous to the ner-Section of the vous globules of animals ; he calls them nervous corpuscles,medulla of the and regards them as the scattered elements of a diffusedMimosa pudica, nervous system. That globules are found in vegetables inhular bodies'ad- the situation described by Dutrochet no one can deny; buther in g to the the grounds on which he asserts them to be nervous aresides of the cells, very slender. The researches of Leeuwenhoek , Prochaska,Fontana, Sir Everard Home , Bauer, the brothers Wenzel, and Dr. Milne

Fig. 17.