trials was, that the vibratory alternations recur between twenty and
thirty times in a second, but varying in number in proportion to the
degree of force exerted by the muscle.
The utmost frequency which he has observed, he estimated at 35, and the lowest 15. But he considers the visible unsteadiness of an aged or infirm person, to arise from a less frequent repetition of the same motions.
In the second part of this lecture, which treats of sea-sickness, the author described an irregularity that he observed in his respiration, after having suffered some days from that affection, which appeared to be an involuntary effort of the constitution to relieve itself, by counteracting the effects of the motion of the ship. .
In waking from a disturbed sleep, he remarked that each effort of inspiration was suspended for a time, and was then taken with a certain feeling of adaptation to some unknown motion of the sea. In reflecting afterwards upon this observation, it appeared to him that the act of inspiration might afford relief by means of its effect upon the circulation. For since, when the skull is trepanned, the effect of inspiration in withdrawing blood from the brain is manifested by the alternate heaving and subsidence of the brain, in alternate motion with the opposite states of the chest, the act of inspiration must tend to counteract any cause propelling blood to the head. And such a cause of pressure will manifestly occur in the descent of a ship by the subsidence of a wave on which it rests. When a person is stand- ing erect upon deck, the motions of the column of blood contained in his vessels may be compared to those of quicksilver in a barometer. When the deck descends, the fluids no longer press with their whole weight against the force which supported them. The mercury continuing to be pressed with the whole weight of the atmosphere is seen to rise in the tube containing it, and so also the blood continuing to be pressed with the same elasticity of the vessels, which before supported its whole weight, is now driven upwards by the excess of force, and the most distressing sensation of sickness is then felt in consequence of its pressure upon the brain. But if an effort of inspiration be exerted at the same instant, it cannot but lessen this propensity, and have some effect in relieving the consequences.
An opposite effect is also noticed by the author to arise from motion in an opposite direction. For when a person rises very suddenly from an inclined position, and is at the same time, by previous fatigue, more than usually sensible of the consequences, he perceives a temporary sensation of faintness and giddiness, by partial abstraction of blood from the vessels of the brain; and may immediately relieve these symptoms by descending again suddenly to his former posture.
The explanation contained in the third part of the lecture, of the salutary effects of external or passive motion, is founded upon one necessary consequence which, he observes, must take place from mere mechanical agitation. Since the direction in which the circulation of the blood is carried forward inanimal bodies, is given solely by the position of the valves that are to be found in the circulating
