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these muscles are relaxed; and confequently, by the reaction of the cartilages of the ribs, and the ftretched abdominal muscles, &c. the cavity of the thorax is leffened, i. e. expiration is performed; which, on account of the difagreeable fenfation which begins to be felt in the lungs, is foon fucceded by a new inspiration. Although, in ordinary breathing, we are but little fenfible of this uneafinefs, arifing from the difficult paffage of the blood thro' the lungs after expiration is finished; yet if one attends to it, and restrains infpiration for fome time, it becomes very perceptible: and, as in asthmatic patients, the laborious contractions of the infpiratory muscles are beyond all queftion owing to an anxiety and fenfe of fuffocation in the breaft; fo it is highly reafonable to think, that in healthful people, the gentler ftimulus of the warm blood accumulated in the pulmonary veffels, is the ordinary cause of inspiration.

Further, a variety of phenomena concur to persuade us that the blood acting as a stimulus on the veffels of the lungs, after expiration, is the caufe of the fucceeding contraction of the infpiratory muscles. Thus we obferve, that as the blood flows in greater or lefs quantity through the lungs, inspiration and expiration more quickly or flowly fucceed each other: hence the quick breathing obferved in a smart fever, or upon violent exercife.- Though the quantity of blood flowing through the lungs remains the fame, yet, if its heat and bulk be increased, refpiration becomes more frequent: hence in bagnios, and in the warm fummer's air, we breathe oftener, than in our common rooms, and in more temperate seasons. Again, when any obftruction happens in the pulmonary veffels, which renders the paffage of the blood through them more difficult than in health, respiration is more laborious, and more frequently repeated: hence the quick breathing in peripneumonies, and other diforders confequent upon the lungs being obftructed.---If a portion of the lungs be rendered ufelefs, or be wholly confumed by an ulcer, the patient is shortbreath'd, and fubject to asthmatic fits, upon the least fatigue, or upon any increase of motion or rarefaction in the blood.

'Since therefore it appears, that the motions of refpiration are always proportional to the quantity of blood thrown into the pulmonary veffels, and its eafy tranfit thro' them, this fluid ought undoubtedly to be efteemed the cause which excites, regulates and continues thefe motions; and Gg3

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fince refpiration is more frequent and laborious, when a lefs quantity of blood paffes with greater difficulty through the lungs, than when a larger ftream flows through their veffels with more eafe; these increafed motions of the thorax cannot be owing to the infpiratory mufcles being more plentifully fupplied with blood and fpirits, but muft proceed from the fimulus or uneafy fenfation accompanying the difficult paffage of the blood through the pulmonary veffels, or its ftagnation in them. And does not this plainly fhew, why blood-letting gives more fpeedy relief in fits of difficult breathing, than any other remedy?'

Having thus accounted for infpiration as effected by a ftimulus, exciting the energy of the fentient principle to remove the uneafy fenfation, by a determination of the nervous influence into the intercoftal mufcles and diaphragm ; and having answered the most obvious objections to it with much force and judgment, he observes, that an effort of the mind does not feem neceffary to expiration, which naturally fucceds, when the infpiratory mufcles cease to act, by the elastic renitency of the cartilages of the ribs, and of the stretched pericardium and peritoneum, and not from any fuperadded mufculer contraction of them at this time, and very little even of the abdominal muscles. This he thinks evident from the thorax of dead animals being in a ftate of compleat expiration, after all mufcular action is ceafed.' He still further establishes this pofition, and, as we apprehend, very ftrongly, by a clofe and ingenious application of fome obfervations on the long intervals between infpiration in the cafe of comatofe patients, whether naturally fo, or from too much opium, whereby the attention of the mind to the ftimulus, from the blood accumulated in the lungs, is abated; but where nevertheless expiration fucceeds in the ufual manner.

He obferves next, that refpiration differs from most of the involuntary motions, as we can, at pleasure, accelerate, retard, or even ftop, for a confiderable time, the motions of the intercoftal mufcles and diaphragm; but adds, that, notwithstanding this difference from the proper involuntary motions, it does not perfectly agree with the voluntary, as it is regularly and unconciously performed in fleep. After an ingenious ratiocination on the efficient caufe of this, he concludes,

But, whatever may be the efficient caufe, which thus fubjects refpiration to the government of the will; the final caufe of this difference between it and the other

vital motions is pretty evident: for were it not that the motions of the mufcles employed in refpiration may be väried at pleasure, we fhould not only be unable to evacuate the urine and fæces, but muft have been deprived of the happiness and advantage of communicating our thoughts to one another in the way of speech.'

The beginning of refpiration in animals is the fubject of the next fection, and might undoubtedly have followed as fome appendage to it with as much propriety as the motrons of the internal ear were annexed to thofe of the pupil. But the much greater length of this may have been one reafon for making it a diftinct fection. Our author here then fimply afcribes the commencement of refpiration to the fame caufe that continues it, viz. to an uneafy fenfation, though the folution of this problem, he fays, has been vainly attempted by fome great phyfiologifts. He obferves the foetus needs neither food nor air ab extra, the mother's juices received through the veffels of the placenta fupplying the former; and the particular circumftances of the heart of the fætus, and the humours of the mother, which have already undergone the action of the air, rendering any further admiffion of it unneceffary. But the neceffity of both commencing with the birth, as the uneafy fenfations of hunger and thirft faithfully admonifh us of the neceffary periods of nutritition; fo the uneafinefs from want of fresh air, which fuggefts a continual renewal of it, may not improperly be called, as the doctor aptly remarks, the appetite of breathing. And here he judiciously reflects, that, as no one ever thought of accounting for hunger or thirst, merely from the mechanical conftruction of the, ftomach, gullet, and fauces, without recurring to a fentient principle, it must be unreasonable and unphilofophical to attempt explaining the action of refpiration, independent of the principle which commences and continues it. A great part of this fection is employed in refuting the different opinions of Pitcairn, Boerhaave, and Haller, on the cause of the firft refpiration in animals. These are undoubtedly very confiderable names in phyfic to diffent from: but as dr. Whytt has endeavoured, in these refearches, to go to the bottom of his fubject, he has only acted up to the philofophical axiom of nullius in verba, fince he appears to be fuperior to cavilling, and reafons with force and candour.

And now having evinc'd the various vital and involuntary motions to be owing to fome ftimulus, acting imme

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diately on the organ moved, or on fome neighbouring part, with which it seems to have a peculiar fympathy, he proceeds to enquire, in his 10th fection, into the reason of muscular contraction from a ftimulus. Here then, after premifing, that the means by which the will contracts the voluntary muscles, is wholly beyond our investigating, he seems justly to reject that opinion which afcribes involuntary motion to an elaftic power of the animal fibres; reflecting, that an elastic body, of whatever kind, is mere inactive matter, without a power of generating motion; as its recoiling, in proportion to the force that wound it up, is in confequence of its being acted on, and not the effect of its own agency. He finds as fufficient reafons for repelling the hypothefis, that would impute it to a number of little fprings, which fome have supposed the animal fpirits lodged in the mufcular fibres to confift of; and which, being put into vibratory motions by the application of stimuli, dilate the fibres, and fhorten the muscle. He mentions the opinion of muscular action's arifing from an ebullition confequent on the mixture of the nervous and arterial fluids; or from the peculiar energy of fome etherial or electrical matter in the nerves, which may be under the regulation of the will in fome cafes, and may be neceffarily determined to exert their influence in involuntary motion, from the mechanical action of heat, or other ftimuli: but, in confequence of many ftrong objections, he fhews thefe alfo infufficient to account for the alternate contraction of irritated muscles; and adds, that every attempt towards explaining their motions from properties, which their fibres, confidered as mechanical inftruments, ever fo exquifitely framed, or nicely adjusted, can be supposed to be endued with, must be fruitless; very juftly concluding, if such effects of ftimuli cannot be deduced from any properties or powers belonging to animal fibres, as mere MATERIAL organs, they must be referred to an active SENTIENT PRINCIPLE animating them. This, he obferves, will eafily account for their alternate contractions; fince the fentient principle, to difpel the uneasy sensation from irritation, determines the nervous influence more strongly than ufual into the fibres, till the uneafinefs being removed by repeated contractions, the mufcle returns to a ftate of reft: whereas, in the contraction from a ftimulus, if the muscle were to be confidered as a mere mechanical organ, its entire contraction fhould continue during the equal action of the ftimulus. For in the contraction of the f enfitive

fenfitive plant from touching, tho' it has fome resemblance to the cafe of animal fibres, (but which happens indifferently from blunt or pointed bodies, from a drop of brandy or water) there are no alternate contractions or relaxations, no indication of feeling, but all is effected by mere mecha nical appulfe or contact. And as the contractions of irritated muscles do not follow the law of vibration in elastic, bodies, (which fucceed from first to laft with equal velocity) but become remarkably flower as they decrease in ftrength, and before they entirely cease, the doctor thinks it amounts to a clear demonftration, that they cannot be owing to any elaftic vibrations excited in the mufcular fibres, or their contain'd nervous fluid. And this, were it neceffary, feems ftill further evinced from the contraction. of animal fibres, to which the ftimulus is not applied, and which have no nervous communication with the part it is applied to, as in the contraction of the fpincter pupiliæ from the forcible action of light on the retina, which inftigates the fentient principle to a removal or alleviation of the irritating caufe: many other inftances of which conduct in the animal economy this ingenious contemplator of it obferves; further reflecting, that the very remembrance or idea of things formerly applied to different organs will often produce almoft the fame effect as their repeated application. This he fufficiently exemplifies from the fight of very grateful food, of naufeous phyfic, and from the very apprehenfion of being tickled. The many involuntary motions and remarkable alterations produced in the body by the affections of the mind are further adduced, and many specified, as collateral fupports of our author's notion of the cause of mufcular contraction, and do not a little establish it. This fection contains many other curious and pertinent arguments on the fubject; for which we may, after this pretty liberal abridgment, refer to the work; concluding it in our author's own words:

• Upon the whole, as nature never multiplies causes in vain, it seems quite unphilofophical to afcribe the motions of the mufcles of animals from a ftimulus to any hidden property of their fibres, peculiar activity of the nervous fluid, or other unknown caufe; when they are so easily and naturally accounted for, from the power and energy of a known fentient PRINCIPLE.'

The 11th fection, which treats-Of the share the mind has in the production of involuntary motion, is truly curi ous, and replete with various and extenfive erudition, which

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