nerves of the heart, and organs of respiration, and the regions of the brain and spinal marrow corresponding thereto, be exempted, they may be agitated even with more vigorous vibrations, on account of the abatement in the other parts, because the vibrations raised in these regions during sleep, by such of their causes as take place then, will be hindered from diffusing themselves freely, and abating their own force thereby, as soon as the other regions are collapsed and compressed. These causes are, first, the heat of the blood, and pulsation of the arteries of the medullary substance; both which, when increased on any account, must farther increase themselves by a reflected influence, since both increase the force of the heart. Secondly, the fulness and distention of the lungs. These arise from the rarefaction of the blood, and accumulation of it in the veins just before sleep (at which time respiration is languid), and must at last stimulate the organs of respiration to a vigorous exertion of themselves; i.e. raise vigorous vibrations in the region of the brain corresponding thereto, just as in the cases of sighing, and recovering from the night-mare. Thirdly, we may suppose, that the heart, and muscles of respiration, do not exert themselves during vigilance, with a degree of force at all approaching to their utmost powers, as the limbs do; and therefore, that they, and the corresponding regions of the brain, may be qualified for a vigorous exertion during sleep. 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.
Cor. I. By laying together what has been delivered concerning sleep, in this proposition, the difference between the states of sleep and vigilance may be thus set before the reader, in one view. In sleep, the nerves of the five external senses are indisposed to receive vibrations, and the objects themselves are either absent, or impressed feebly. The nerves of the stomach and bowels sympathize with these at first, but recover themselves at the instant of sleep, the impressions of the aliment, &c. being then made with unusual vigour; and this continues during the time of sleep. In like manner the muscular system becomes inactive in general; the heart, however, and muscles and respiration, are excepted, and even exert themselves with an extraordinary degree of force. The blood is rarefied so as to take up more space upon the whole; and as there is more in the veins, and particularly in those of the brain, and spinal marrow, than in a state of vigilance, the medullary substance is hereby exposed to a constant uniform compression; whereas, in vigilance, the action of the muscles squeezes the blood out of the veins, and cools it, unless this action be violent, or long continued.
