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Show 16 INTRODUCTION • Of the forces which act in the .l:lnimal Body. . t ly the organ of voluntary rna- The muscular ~bre lS :oth: it is also the most powerful of tion, for we have JUS~ see t to produce those transmutations the agents employed y_na ul.rfe Thus the :fibres of the intes-y to vegetatiVe I e. . so necessar . . . which causes the ahmen-tines produce the peristaltiC motion, h them. the fibres tary matter therein co~tained :;:sgse~~:~gthe cir;ulation and of the heart and arteries are through it of all the secretions, &c. d' f the Volition contracts the :fibre through the me mhm o . I ta fibres such as those we ave men· nerve; an~ thelinvo ?n :~by them it is probable that these tioned, bemg a so anima e '. nerves are the cause of their contractiOn: e of All contraction, and generally speakmg, every f ch~ng . l dimension in nature, is produced by a change. o c emi~a 't' n though it consist merely in the flowmg or ebbmg composi IO ' 1 · h lso are pro· of an im onderable fluid, such as ca oric; t us a ~ duced th~ most violent movements known upon earth, explo sions, &c. (}Se tha+ the There is, consequently, good reason to _supp . .. J nerve acts upon tlle fibre through the mediumh o f ahn' Impfo n derable fluid, and the more so, as it is proved t at t IS ac IOn is not mechanical. · h The medullary matter of the whole ~erv?us syst~.m IS o: d must be able to exercise Its pecuhar func ~ogenehous, an .t ·s ~ound. all its ramifications are abundantly tions w erever I 1 11 ' supplied with blood vessels. All the animal fluids- being drawn f'rom the blo~d by secre~ tion, we can have no- doubt that such is the case Wl~h the ner vous fluid, and that the medullary matter secretes It. On the other hand, it is certain that the medullary matter is the sole conductor of the nervous fluid ; all the othelr to~· ganic elements restrai. n an d arres t 1· t ' as glass arrests e ec ri· cityT.h e external causes which are capab l e 0 f Pr od u cing sen'scaa·l tions or causing contractions of the fibre are all chemt INTRODUCTION. 17 agents, capable of effecting decompositions, such as light, caloric, the salts, odorous vapours, percussion, compression, &c. &c. It would appear then that these causes act on the nervous fluid chemically, and by changing its composition ; this appears the more likely, as their action becomes weakened by continuance, as if the nervous fluid needed the resumption of its primitive composition, to fit it for a fresh alteration. The external organs of the senses may be compared to sieves, which allow nothing to pass through to the nerve, ex· cept that species of agent which should affect it in that particular place, but which often accumulates it so as to increase its effect. The tongue has its spongy papillre which imbibe saline solutions; the ear, a gelatinous pulp which is violently _agitated by sonorous vibrations; the eye, transparent lenses which concentrate the rays of light, &c. &c. It is probable, that what are styled irritants, or the agents which occasion the contractions of the fibre, exert this action by producing on the fibre, by the nerve, a similar effect to that produced on it by the will; that is, by altering the nervous fluid, in the way that is requisite to change the dimensions of the fibre which it influences: but with this process the will has nothing to do, and very often the ME is entirely ignorant of it. The muscles separated from the body preserve their susceptibility of irritation, as long as the portion of the nerve that remains with them preserves the power of acting on them-with this phenomenon the wiH has evidently no connexion. The nervous fluid is altered by muscular irritation, as well as by sensibility and voluntary motion, and the same necessity exists for the re-establishment of its primitive composition. The transmutations necessary to vegetable life are occasioned by irritants; the aliment irritates the intestine, the blood irritates the heart, &c. These movements are all independent of the will, and generally (while in health) take place without the knowledge of the M~; in several parts, th~ nerves that produce them are e.ven differently arranged from thos.e that are appropriated to sensation or dependent on the will, and VoL. 1.-C I f |
