gg ON THE FUNCTIONS OF THE HEART AND ARTERIES. 



Pressure re- by which the tabe] is burst. Now it has been ascertained 

 quired to burit ^ jy^^ Hales, that the pressure, required for bursting one 

 the carotids ot J . , „ i • ^ 2. 2^. ^ c 1 c 



H 4f>i. of the carotids of a dog, is equal to that ot a column of 



water one hundred and ninety feet high ; nor does he re- 

 mark, that the artery was very materially dilated ; and de- 

 ducting from this height the five feet, which express the 

 actual pressure in the arteries of a dog, the remaining one 

 hundred and eighty five feet will give a velocity of at least 

 fifty four feet in a second, for the propagation of the pulse 

 in the dog. It is not however ascertained, that all the 

 membranes, which may have surrounded the artery in this 

 experiment, are called into action in its ordinary pulsa- 

 tion; much less that the force, developed by their tension, 

 •varies precisely according to the general law of perfectly 

 elastic bodies; but this mode of calculation is still amply 

 sufficient to make it probable, that the velocity of the pul- 

 sations, in the larger arteries, must amount to at least forty 

 feet in a second, although some very considerable deductions 

 must be made, on account of the resistances of various 

 kinds, which cannot be comprehended in the calculation. 

 The artery The artery must not be supposed to subside, immedi- 



uoesnotsub" gtelv after each pulsation, precisely to its original dimen- 

 side to its for- , •' . . '^ . "^ i . r 11 • j . 



aier dimen- sions; smce it must remain somewhat tuller, in order to sup- 



sions immedi- p^y ^^^ capillary arteries, and the veins, in the interval 

 between the two successive pulsations: and in this respect 

 it differs from the motions of a wave through a canal, which 

 is open on both sides: but the difference may be under- 

 stood, by supposing a partial reflection of the pulse to take 

 place at every point where it meets with any resistance, 

 which will leave a general distension of the artery, without 

 any appearance of a retrogade pulsation. 



/To be concluded in our next. J 



