B R A 



If bcliirve that tlu-y arc removed from tlieir bodif', in c<ini»- 

 «jucncc of tlie painj and i'cnfattons which they (eem to feel 

 in thfm. In cither of iIrI'c cafes a motion is produced in 

 the middle of a nerve, which being tranfmitled to the brain, 

 in attributed by tlie percipient to a difoidercd ftate of the 

 parti, from which it liad been accutlomed to receive im- 

 pielHon";. Again, if a certain dejjrec of ptcflure be made 

 en the brain, all fenfation and voluntary motion arc iiittr- 

 rupted. As thcfe citcumllaiices hi-.ve been fo v.cll explained 

 by Haller, we (hall not enlarge on this fubjecl, but proceed 

 to coiilidcr the means, by which thic; trant'mifllon of motion 

 from one end of the nerve to the other, is probably ef- 

 ferted. 



From the earliell times in which m^n cDutempIatcd the 

 animal economy, icfledinf^ pcrfons imagined, that they 

 perceived fome fubtilainviliblc agent, which operated in the 

 performance of tlic animal funclions. Hippocrates attri- 

 buted aflions, which he could not othenvife account for, 

 to fomcthinff which he called nature ; ParaCvllUo, to the 

 operation of an archxus ; Stahl, to different aiiiina: ; and 

 Harvey fecms to have had an idea of a vital principle. But 

 if any merit is due to fuch opinions, the credit of firfl; de- 

 cidedly leaching that a fabtile invifible matter was didulcd 

 throughout living bodies, and was the caufe of many phe- 

 nomena occuning in them, belongs to Mr. Hunter. This 

 opinion he has endeavoured to prove by experiments, which 

 ■will be noticed under the article I^iie. The mod conclu- 

 five arguments in favour of tiiis opinion will probably be 

 givfn under the phvfolo^v of Muscles, in difcufling the 

 caufe of mufcular motion, and to thefe the reader is re- 

 ferred. 



It has been ciiilomary to confiJer matter as paffive, and 

 to denominnte the active power which puts it in motion, by 

 the name of fpiiit ; but wc relinquilli the purfuitof the pri- 

 mary caufe of the motions of matter, becaiifc w-e kiiov/ no- 

 thing of the fubftances wiiich we ufually denominate matter 

 or fpirit. Thefe appellations have been given to them from 

 their properties, wluch we do know. ^Ve know nothing of 

 matter, but iome of the properties which difieient ipecies of 

 it poffeis. Of that matter," which, for the iiioft part, pre- 

 fciits ilfelf to our notice, when collected together in fuch 

 q;!antity as to be cognizable to the eye and touch, we know 

 that it pofTcflTes a property, called by Sir Ilaac Newton in- 

 ertnefs, an indifpolition to move, tinlefs impelled to it, and 

 a propenfity to continue in motion, unlefs rftaided. But 

 there are other fubftances in the nniverfe which are ufually 

 confidercd as material, fuch, for iollancc, as cleiilricity and 

 magnetilm the motions of which do not item to be regu- 

 lated by the fame law;;, and which are cognizable neither to 

 the eye nor touch, but of the e>-.illence of which rcafon fnr- 

 nillies us with the mod indilputable teftiinony. Of thefe 

 kinds of fubtile and mobile matter, we know that they fre- 

 c(uently aiS upon, and put in motion that which is move 

 ^rofs and inert. We fee magnctifm move even ponderous 

 pieces of iron, and eleftricity difplace and diffcver the molt 

 Itabile fubltances which oppofe its pafTage. The reafons 

 for entertaining the opinions of Mr. Hunter, that there i; a 

 fubtiie, mobile, and invifible fpecies of matter connected with 

 the evident llruclure of living bodies, as magnetifm may be 

 fuperadded to iron or cledricity to various fubllances, will 

 be detailed under different articles in this book. At prefenc, 

 ■we claim pcrmifrion to employ this explanation of the phe- 

 nomena of life, becaufe there appears nothing improbable in 

 it, or incoufiilent to piefent philofophical noliojis ; and it is 

 a fuppolitioH, which appears, not only verifiable, but ade- 

 <|uate to account for the phenomena which occur in living 

 bodies. By employing tlii-j mode of accounting for the 



BRA 



aftions of life, we avoid all inquiry into the primary caufe 

 of the motions of matter ; we contemplate only a fecondary 

 cauie, fuch as takes place in eleilrical and magnetic experi- 

 ments, in which a fubtle fpecies of matter puts in m'otion 

 that which is more grcfs and inert. 



If it be admitted, that fenfation exills in the brain, and 

 volition proceeds from that part, it ticceflarily follows, that a 

 motion muft be tranfmitted along the nervous cords, when- 

 ever thefe effiCls take place. It was formerly fnppofed that 

 thefe cords were pafljve, and ir.ight be made mechanically 

 to vibrate ; but their want of clailicity and tenfion, and thtir 

 pulpy origins and terminations, arc circumftances which ren- 

 dered this fiippofjtion inadmiffible. Phyllologifts were, 

 therefore, induced to fuppofe, that the nervous fibrils were 

 tubular, and that they contained a fubtile fluid, by means 

 of which fuch motions were tranfmitted. Haller's oi)inion 

 of the ncrvnus fluid appears fenlible and accordant to the 

 philofophical opinions of his time. He fays, " Si vero co- 

 gitata nollra de ipfa natura fpirituum proferre jubcremur, ac- 

 tivuin ad niotum a voluntate et a feuiu concipiendum aptif- 

 fimum cekrimum omne fenfuum acie fubtiiius, tamen hafte- 

 nus igne et jethere et eledio, ct magnetica materie craffius 

 facerc elementum, ut et contineri vafis et vinculis coerceri 

 aptum fit, et dtnique manifeftura e.K cibis nalci et reparari 

 queat." But, at prefent, no one will doubt the poflibility 

 of a fubtile and mobile fluid inhering in cords, and moving 

 along tlieai without being contained and confined in tubes. 

 Does nor elcftricity move along a wire ? and is it not pro- 

 bable that a fubtile fluid may inhere in a fubftance, in confe- 

 qucnce of an attrafkion v.hich it poffelTes to it, and more 

 particularly, if the fubftance which attracts it is fnrrounded 

 by others, which may be regarded as non-conduflors. The 

 celerity with which motions are tranfmitted from the 

 tangible extremities of thofe nerves which are moft dillant 

 from the brain, and the celerity with which volition 

 is tranfmitted to mufcles in confequence of fenfations thus 

 occafioned, are fufficient to convince us that tiiefe effeCts 

 muil be produced by the medium of a very mobile matter. 

 It is not neceflary to fuppofe, when motions of a fubtile 

 matter are tranfmitted along the nervous cords, that an 

 evident motion of the viiible matter of thofe cords fhould 

 take place. This opinion, that fenfation is caufed, and voli- 

 tion directed through the medium of a fubtile, invifible mat- 

 ter, inhering in the nervous cords, is fo like the opinion of 

 Hartley, of the phenomena of the nervous fyflem being 

 produced by the vibrations of an aether, that it is not necef- 

 lary, in this place, further to profecute the fubjeft. We' 

 rather refer the reader to his work, for a more ample illuf- 

 tration of this theory. 



Fornierly it was fuppofed, that the motions of the nerves, 

 which caufe fenfation, were the mere effeft of an impulfe 

 made on their tangible extremities by the bodies which we 

 feel. It fcems to be an improvement in modern phyfiology, 

 to impute the nervous motions to an aftion begun in them, 

 in confequence of the ftimulation which they fnffer from 

 fuch impulfes. This opinion is contended for by Doflor 

 Darwin, in his paper on " Ocular Speftra", infert'td in the 

 Philofophical Tranfaftions ; and Mr. Home has further 

 fliewn, in the Croonian Ledure for the year jSoi, inferted 

 in the Philofophical Tranfaftions, that nerves have an irri- 

 tability refembling that, of mufcles, which produce a con- 

 traftion in them when they are divided. This opinion 

 alTifts us in uiiderftanding how our fenfations may be very 

 vivid from the fiightefl impulfes ; fnch as, for inftance, 

 take place fiom the application of the particles of odour to 

 the oKaftory nerves, for it is not the impulfe, but the con- 

 fequcnt aftion, that is tranfmitted to the fenforium; and why 



we . 



