SECT. XLIII. OF REST. Sleep and wakefulness bear a great resemblance to exertion and reft; as wakefulness is the natural state of action, in which the animal machine is fatigued and wafted, and fleep the state of cafe, in which it is refrefl ed and repaired. Thus we may look upon the time of being awake and active as the time of wearing out the animal frame; and the time of fleep and reft, as that in which it is repaired and recruited; for, in action, the irritable principle is continually taken from the muscular fibres, which cannot otherwise be replaced than by rest. 1. Of Voluntary Action. " Not only the will, by which the electric fluid is fent into the muscles, but the muscles themselves feem, as it were, to get fatigued by exertion, and require a certain. time to recruit their powers: for in every contraction of a fibre, there is an expenditure of the irritable principle; and where the exertion of the voluntary powers has been for fome time increased, and the muscles or organs of motion have in confequence acted with greater energy, their propenfity to activity is proportionally leffened; which can be afcribed to nothing else but the exhauftion or diminution of the IRRITABLE PRINCIPLE. Indeed every one must have experienced the refreshment arifing from repose, and it is an established fact, that for a horse to perform a long journey, he should be previously kept at reft for feveral days in the stable. Upon waking after profound fleep we ftretch our limbs, which arifes from the accumulation of the irritable principle in the fibres. From the fame caufe, when the muscles of our face have been long in a state of inaction, we yawn and children and young animals, who have abundant irritability, are impatient of confinement, and seem never easy but in a change of pofition. 2. Of Involuntary Action. After animal fibres have for fome time been exerted into contraction, a relaxation fucceeds, even though the exciting caufe continues to act. In refpect to the irritative motions this is exemplified in the peristaltic contractions of the bowels, and the beatings of the heart; which ceafe and are renewed alternately, though the ftimulus of the aliment and blood continue to be uniformly applied in fenfitive motions, as in fits of the stone and gravel, and in parturition, though the ftimulus is perpetual. In our muscular exertions it is experienced, as no one can hang long by the hands, however vehemently he wills so to do; and the changes of our attitude evinces the neceffity of relaxation to those muscles which have been long in action *. The application of a ftimulus, whether that stimulus be material or mental, excites the correfpondent irritable and fentient principles into action, and this caufes the contraction of the fibre and the fenfation of the nerve. On the contraction of the fibre a part of the irritable principle + becomes expended, and the fibre, as was faid before, ceafes to contract, though the ftimulus Vide page 459. The two principles, viz. the irritable and fentient principles, are here mentioned as diftinct. Dr. DARWIN confounds them under one and the fame title, and calls them by the very expreffive term, SENSORIAL POWER. Whether this be the irritable principle itself (whatever this principle may be), or oxygen, or electricity, upon which its powers depend, the reasoning is equally true: for as a philofopher (Dr. FOTHERGILL) well expresses it, CAUSA LATET-VIS EST NOTISSIMA. The caufe may lie concealed; but its power is most evident. VOL. II. 30 continues continues to be applied; till in a certain time the fibre, having received a supply of the irritable principle, is ready to contract upon the application of a ftimulus. If the ftimulus on the contrary be withdrawn, the fame quantity of irritable principle foon becomes refident in the fibre as before its contraction; as appears from the readiness for action of the large locomotive muscles of the body in a fhort time after common exertion, and in those muscular fibres which are fubject to constant ftimulus, as the arteries, glands, and capillary veffels; and on this account those muscular fibres become afterwards excitable into their natural actions by a much weaker ftimulus; or into unnatural violence of action by their accustomed flimulus, as is feen in the hot fits of intermittent fevers, which are in confequence of the previous cold ones. Thus as the minute veffels of the skin are constantly stimulated by the fluid matter of heat; if the quantity of this stimulus of heat be a while diminished, as in covering the hands with fnow, the veffels cease to act, as appears from the paleness of the fkin; if this cold application of fnow be continued but a short time, the irritable principle, which had habitually been fupplied to the fibres, becomes now accumulated in them, them, owing to the want of its being expended by their accustomed contractions; and thence a lefs ftimulus of heat will now excite them into violent contractions. Thus on a frosty day with a bleak wind, the face of a perfon exposed to the wind is at firft pale and fhrunk ; but on turning the face from the wind, it becomes foon of a glow, with warmth and flushing. The glow of the skin in emerging from the bath is owing to the fame cause; for by leffening the quantity of heat for a minute or two by going into the cold bath, a great accumulation of the irritable principle is neceffarily produced. On emerging from the bath the irritable fibre is therefore thrown into greater exertion by the stimulus of the common degree of the warmth of the atmosphere, and a great increase of animal heat enfues*. This experiment of cold bathing prefents us with a fimple FEVER-FIT; for the pulfe is weak, fmall, and quick, during the cold immersion; and becomes strong, full, and quick, during the subsequent glow of heat; till at length all these unnatural exertions fpontaneoufly fubfide with the increased irritability that pro *This has been more fully explained when on the subject on the manner of catching cold, page 432. |