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these muscles are relaxed; and consequently, by the reaction of the cartilages of the ribs, and the stretched abdominal muscles, &c. the cavity of the thorax is lessened, i. e. expiration is performed; which, on account of the disagreeable sensation which begins to be felt in the lungs, is soon succeded by a new inspiration. Although, in ordinary breathing, we are but little sensible of this uneafiness, arising from the difficult passage of the blood thro’ the lungs after expiration is finished; yet if one attends to it, and restrains inspiration for some time, it becomes very perceptible: and, as in asthmatic patients, the laborious contractions of the inspiratory muscles are beyond all quesțion owing to an anxiety and sense of suffocation in the breast; so it is highly reasonable to think, that in healthful people, the gentler stimulus of the warm blood accumulated in the pulmonary vessels, is the ordinary cause of inspiration.
. Further, a variety of phænomena concur to persuade us that the blood acting as a stimulus on the vessels of the lungs, after expiration, is the cause of the fucceeding contraction of the inspiratory muscles. Thus we observe, that as the blood flows in greater or less quantity through the lungs, inspiration and expiration more quickly or slowly succeed each other: hence the quick breathing observed in a smart fever, or upon violent exercise.— Though the quantity of blood flowing through the lungs remains the same, yet, if its heat and bulk be increased, respiration becomes more frequent: hence in bagnios, and in the warın summer's air, we breathe oftener, than in our common rooms, and in more temperate seasons.- Again, when any
obstruction happens in the pulmonary vessels, which renders the palsage 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 disorders consequent upon the lungs being obstructed.---If a portion of the lungs be rendered useless, or be wholly consumed by an ulcer, the patient is shortbreath'd, and subject to asthmatic fits, upon the least fatigue, or upon any incsease of motion or rarefaction in the blood.
• Since therefore it appears, that the motions of respiration are always proportional to the quantity of blood thrown into the pulmonary vessels, and its easy transit thro' them, this fuid ought undoubtedly to be esteemed the cause which excites, regulates and continues these motions; and
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fince respiration is more frequent and laborious, when a lefs quantity of blood passes with greater difficulty through the lungs, than when a larger stream flows through their vefsels with more ease; these increased motions of the thorax cannot be owing to the inspiratory muscles being more plentifully supplied with blood and spirits, but must proceed from the stimulus or uneasy sensation accompanying the difficult paffage of the blood through the pulmonary vefsels, or its ftagnation in them. And does not this plainly shew, why blood-letting gives. more speedy relief in fits of difficult breathing, than any other remedy?"
Having thus accounted for inspiration as effected by a stimulus, exciting the energy of the fentient principle to remove the uneasy sensation, by a determination of the nervous influence into the intercostal muscles 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 seem necessary to expiration, which na. turally fucceds, when the inspiratory muscles cease to act, by the elastic renitency of the cartilages of the ribs, and of the stretched pericardium and peritonaum, and not from any superadded musculer 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 state of compleat expiration, after all muscular action is ceased.' He still further establishes this position, and, as we apprehend, very strongly, by a close and ingenious application of some observations on the long intervals between inspiration in the case of comatose patients, whether naturally fo, or from too much opium, whereby the attention of the mind to the stimulus, from the blood accumulated in the lungs, is abated; but where nevertheless expiration fucceeds in the usual manner.
He observes next, that respiration differs from most of the involuntary motions, as we can, at pleasure, accelerate, retard, or even stop, for a considerable time, the motions of the intercostal muscles and diaphragm ; but adds, that, notwithitanding this difference from the proper involuntary motions, it does not perfectly agree with the voluntary, as it is regularly and unconiciously performed in feep. Alter an ingenious ratiocination on the efficient cause of this, he concludes,
< But, whatever may be the efficient cause, which thus subjects respiration to the government of the will; the final cause of this difference between it and the other
vital motions is pretty evident: for were it not that the motions of the muscles employed in respiration may be väried at pleasure, we should not only be unable to evacuate the urine and fæces, but must have been deprived of the happiness and advantage of communicating our thoughts to one another in the way of speech.'
The beginning of respiration in animals is the subject of the next section, and might undoubtedly have followed as some appendage to it with as much propriety as the niotrons of the internal ear were annexed to those of the pupil. But the much greater length of this may have been one reason for making it a distinct fection. Our author here then finply ascribes the commencement of respiration to the same cause that continues it, viz. to an uneasy sensation, though the solution of this problem, he says, has been vainly attempted by some great phyfiologifts. He observes the fætus needs neither food nor air ab extra, the mother's juices received through the vestels of the placenta fupplying the former ; and the particular circumstances of the heart of the fætus, and the humours of the mother, which have already undergone the action of the air, rendering any further admission of it unnecessary. But the necessity of both commencing with the birth, as the uneafy fenfations of hunger and thirst faithfully admonith us of the necessary periods of nutritition; fo the uneafiness from want of fresh air, which suggests 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 thirit, merely from the mechanical construction of the ftomach, gullet, and fauces, without recurring to a sentient principle, it must be unreasonable and unphilofophical to attempt explaining the action of respiration, independent of the principle which commences and continues it. A great part of this section is employed in refuting the different opinions of Pitcairn, Boerhaave, and Haller, on the cause of the first respiration in animals. These are undoubtedly very confiderable names in phyfic to diflent from: but as dr. Whytt has endeavoured, in these researches, to go to the bottom of his subject, he has only acted up to the philofophical axiom of wellius in veria, since he appears to be fuperior to cavilling, and reasons with force and candour.
And now having evinc'd the various vital and involuntary motions to be owing to some stimulus, acting imme
diately on the organ moved, or on some neighbouring part, with which it seems to have a peculiar fympathy, he proceeds to enquire, in his roth section, into the reason of muscular contraction from a stimulus. Here then, after premising, that the means by which the will contracts the voluntary muscles, is wholly beyond our investigating, he seems juftly to reject that opinion which ascribes involuntary motion to an elastic 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 consequence of its being acted on, and not the effect of its own agency. He finds as sufficient reasons for repelling the hypothesis, that would impute it to a number of little springs, which fome have supposed the animal spirits lodged in the muscular fibres to consist of; and which, being put into vibratory motions by the application of stimuli, dilate the fibres, and shorten the muscle. He mentions the opinion of muscular action's arising from an ebullition consequent on the mixture of the nervous and arterial fluids; or from the peculiar energy of some etherial or electrical matter in the nerves, which may be under the regulation of the will in some cases, and may be necessarily determined to exert their influence in involuntary motion, from the mechanical action of heat, or other stimuli : but, in consequence of many strong objections, he shews these alfo infuificient 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 instruments, ever so exquisitely framed, or nicely adjusted, can be supposed to be endued with, must be fruitless; very juftly concluding, if such effects of stimuli 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 observes, will easily account for their alternate contractions; since the sentient principle, to dispel the uneasy sensation from irritation, determines the nervous influence more strongly than usual into the fibres, till the uneasiness being removed by repeated contractions, the muscle returns to a State of reft: whereas, in the contraction from a stimulus, if the muscle were to be considered as a mere mechanical organ, its entire contraction should continue during the equal action of the stimulus. For in the contraction of the
sensitive plant from touching, tho' it has some resemblance to the case 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 succeed from first to last with equal velocity) but become remarkably lower as they decrease in strength, and before they entirely cease, the doctor thinks it amounts to a clear demonstration, that they cannot be owing to any elastic vibrations excited in the muscular fibres, or their contain'd nervous fuid. And this, were it necessary, seems still further evinced from the contraction of animal fibres, to which the stimulus is not applied, and which have no nervous communication with the part it is applied to, as in the contraction of the spineler pupiliæ from the forcible action of light on the retina, which inftigates the fentient principle to a removal or alleviation of the irritating cause: many other instances of which conduct in the animal economy this ingenious contemplator of it observes ; further reflecting, that the very remembrance or idea of things formerly applied to different organs will often produce almost the same effect as their repeated application. This he sufficiently exemplifies from the fight of very grateful food, of nauseous physic, and from the very apprehension 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 supports of our author's notion of the cause of muscular contraction, and do not a little establish it. This section contains many other curious and pertinent arguments on the subject; 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 unphilosophical to ascribe the motions of the muscles of animals from a stimulus to any hidden property of their fibres, peculiar activity of the nervous Auid, or other unknown cause; when they are so easily and naturally accounted for, from the power and energy of a known sentient PRINCIPLE.'
The rith section, which treats Of the share the mind has in the production of involuntary motion, is truly curie ous, and replete with various and extensive erudition, which