349 



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 cer- 

 tain 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 barome- 

 ter. 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 continu- 

 ing to be pressed with the same elasticity of the vessels, which be- 

 fore 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 in- 

 spiration 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 mo- 

 tion 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 tem- 

 porary 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 circu- 

 lation of the blood is carried forward in animal bodies, is given solely 

 by the position of the valves that are to be found in the circulating 



