David Hartley

“ For the sensations must be supposed to bear such an analogy to each other, and so to depend in common upon the brain, that all evidences for the continuance of sensations in any one sense, will extend themselves to the rest. ”

“ When old persons are sleepy, it is a morbid affection, and may arise either from an hydropical disposition, whereby the turgescence of the neighbouring parts compresses the medullary substance; or from a defect of nutrition in this substance, which renders it soft and compressible in a preternatural degree. If the venal sinuses, and other blood-vessels, of the brain, be by any accident preternaturally distended, and continue so for a considerable time, they will scarce ever recover their pristine tone and dimensions; and this so much the more, as the person approaches to old age. ”

“ These vibrations are motions backwards and forwards of the small particles; of the same kind with the oscillations of pendulums, and the tremblings of the particles of sounding bodies. They must be conceived to be exceedingly short and small, so as not to have the least efficacy to disturb or move the whole bodies of the nerves or brain. For that the nerves themselves should vibrate like musical strings, is highly absurd; nor was it ever asserted by Sir Isaac Newton, or any of those who have embraced his notion of the performance of sensation and motion, by means of vibrations. ”

“ And as the false and imperfect keys, which turn up to the decypherer in his researches, prepare the way for the discovery of the true and complete one, so any hypothesis that has so much plausibility as to explain a considerable number of facts, helps us to digest these facts in proper order, to bring new ones to light, and to make experimenta crucis for the sake of future inquirers. ”

“ Fourthly, an increase in the force of respiration must also increase the force with which the heart moves, because it propels the blood in greater quantities upon the heart. Fifthly, an increase of force in the heart must increase both itself and the action of respiration, because the blood-vessels of the heart and organs of respiration are particularly near to the heart, and therefore must be particularly under its influence. ”

“ The following instance may illustrate this:—The quantity of matter in bodies is always found to be proportional to their gravity: we may therefore either make the quantity of matter the exponent of the gravity, or the gravity the exponent of it, according as either can be best ascertained; notwithstanding that we are entirely at a loss to determine in what mechanical way each atom contributes to the gravity of the whole mass ”

“ A person may even feel, that the stomach is the seat of the pleasurable impressions made by opiates. We are to suppose therefore, that vivid vibrations, which, however, lie within the limits of pleasure, ascend perpetually from the stomach and bowels, along the par vagum, and intercostal nerves, up to the brain and spinal marrow, diffuse themselves over these, and from thence descend along the nerves into all the parts of the body. ”

“ In the sense of feeling, the continuance of heat, after the heating body is removed, and that of the smart of a wound, after the instant of infliction, seem to be of the same kind with the appearances taken notice of in the eye and ear. ”

“ When the nerves of any part are cut, tied, or compressed in any considerable degree, the functions of that part are either entirely destroyed, or much impaired. When the spinal marrow is compressed by a dislocation of the vertebræ of the back, all the parts, whose nerves arise below the place of dislocation, become paralytic. When any considerable injury is done to the medullary substance of the brain, sensation, voluntary motion, memory, and intellect, are either entirely lost, or much impaired ”

“ In young children, there seems to be a constant moderate pressure of the skull upon the brain. For the brain is of a great relative magnitude in them; and by its endeavour to expand itself, it keeps the sutures from uniting too firmly, till such time as it is arrived at its full growth. It must therefore be compressed in return, by the re-action of the skull. And this may be considered as a circumstance, which concurs to render young children more apt to sleep than adults. ”

“ And as the body is so formed, that great and visible solutions of continuity may be healed again, and the parts restored, in great measure, to their primitive integrity and perfection, by the power of nature, unless where there is a loss of substance, (and yet even here the same end is obtained in part;) so we may suppose, that the power of nature restores all minute solutions of continuity in the constituent infinitesimal particles almost instantaneously, and so that the body receives no perceptible detriment from single instances, though it probably does from frequent repetitions ”

“ Fourthly, It is observed, that vigilance continued, fatigue, and pain, all dispose strongly to sleep. For all vigorous or long-continued vibrations must both generate heat, whereby the blood and juices will be rarefied, so as to compress the medullary substance, and also exhaust this substance of its fluid and active particles, so as to render it more easily compressible, and less susceptible and retentive of vibrations. Great degrees of heat seem to produce an extraordinary propensity to sleep, in nearly the same way. ”

“ Hence, in the proposition, I term the medullary particles, which vibrate, infinitesimal.
Now that external objects impress vibratory motions upon the medullary substance of the nerves and brain (which is the immediate instrument of sensation, according to the first proposition) appears from the continuance of the sensations mentioned in the third; since no motion, besides a vibratory one, can reside in any part for the least moment of time. External objects, being corporeal, can act upon the nerves and brain, which are also corporeal, by nothing but impressing motion on them. ”

“ Thus magnetism mixes itself with the gravitation both of bodies upon the surface of the earth, and with that of the moon to the earth: a polar virtue of the same kind seems to have a principal share in the formation of natural bodies, especially those whose parts cohere in regular figures: electricity may also extend, without being excited by friction or otherwise, to small distances, and join with the just-mentioned polar virtue in making the parts of bodies cohere, and in some cases in regular figures. ”

“ Light is so nearly related to heat, that we must suppose the argument of analogy to be particularly strong in respect of it: but besides this, we have an independent argument for the existence of vibrations here, also for their communication by a subtle fluid, if we admit Sir Isaac Newton’s hypothesis concerning the cause of the alternate fits of easy reflection and transmission, as I have above remarked. ”

“ The glands are filled during sleep, and consequently, by drawing off from the fulness of the blood-vessels, prepare the body for vigilance, and are themselves fitted for the functions to be then performed; i.e. to excrete their proper fluids from muscular compression, or vibrations running up their excretory ducts, in the manner to be hereafter explained. The medullary substance is, in like manner, fitted and prepared for vigilance, whether it be of a glandular nature, or not. ”

“ If we suppose the circulation to cease entirely at the surface of the body, from the cold, then will warm blood circulate through the internal parts alone; and these parts will continue to be defended from the cold by the external ones, for a time. And thus the body will approach to the common state of a person going to sleep. ”

“ The same hypothesis is well suited to the rarefaction, fluidity, dissolution, and other changes of texture which heat produces in bodies, according to their various natures. And if we farther consider that all bodies, contiguous to each other, come, after a short time, to the same degree of heat, viz. that of the circumambient air; those which are hotter losing something, and those which are colder gaining; and yet that the air is not necessary for the conveyance of heat, as appears from Sir Isaac Newton’s experiment of the two thermometers above-mentioned ”

“ Wines, and grateful aliments, dispose to sleep, partly by their immediate effects on the stomach, partly by their effects after they are absorbed. But the degree not being so great as in opiates, it may more easily be overcome by a variety of common or vigorous impressions; in which case the vivid vibrations, excited by the wine or aliment, will illuminate all the impressions, and add strength to all the motions. ”

“ For the sounds which we hear are reflected by the neighbouring bodies, and therefore consist of a variety of sounds, succeeding each other at different distances of time, according to the distances of the several reflecting bodies; which yet causes no confusion or apparent complexity of sound, unless the distance of the reflecting bodies be very considerable, as in spacious buildings. Much less are we able to distinguish the successive pulses of the air, even in the gravest sounds. ”

“ And when persons exposed to extreme cold are overcome by a pleasing but fatal sleep, it seems as if the internal parts were affected with a preternatural warmth, from the vigorous sensations and concomitant vibrations impressed on the external parts by the cold, and thence ascending to the brain. ”

“ But, besides this, we may conceive, that the opiate particles excite vibrations of the same kind in all the parts of the body, after they are taken into the blood, and circulate with it, till such time as, by a perfect assimilation, they lose all their peculiar qualities. ”

“ If the vibrations go beyond the common limit of pleasure and pain in one part of the brain, at the same time that they fall short of it in others, the result will be a pleasure or pain, according as this or that sort of vibrations prevails ”

“ In like manner we are to suppose the particles which vibrate, to be of the inferior orders, and not those biggest particles, on which the operations in chemistry, and the colours of natural bodies, depend, according to the opinion of Sir Isaac Newton. ”