ON THE FUNCTIONS OF THE HEART AND ARTERIES. Q^ 



nature, which differ materially from this general law, espe- The lavvr of 

 ciall}- where the distension becomes considerable : thus there vlTyr'*^ '"^^ 

 may be substances, which exhibit a force of tension pro- 

 portional to the excess of the square, or the cube of their 

 length, beyond a certain given quantitj-. It is safest 

 therefore to reason upon the elasticity of any substance, 

 from experiments made without any great deviation from 

 the circumstances to which the calculation is to be applied. 



For this purpose, we may again employ some of the many Velocity of the 

 excellent experiments contained in Hales's hoemastatics. It transmission of 



II 1. 1 • 1-1 the pulse. 



appears, that, when any small alteration was made in the 



quantity of blood contained in the arteries of an animal, 

 the height of the column, which measured the pressure, 

 was altered nearly in the same proportion, as far as we are 

 capable of estimating the quantity, which was probably 

 contained in the larger vessels of the animal. Hence it fol- 

 lows, that the velocity of the pulse must be nearly the same 

 as that of an imijulse transmitted through an elastic fluidj 

 under the pressure of a column of the same height, as that 

 which measures the actual arterial pressure: that is, equal 

 to that which is acquired by a. heavy body falling freely 

 through half this height. In man, this velocity becomes 

 about fifteen feet and a half in a second ; to which the pro- 

 gressive motion of the blood itself adds about eight inches; 

 and with this velocity, of at least sixteen feet in a second, 

 it may easily happen, that the pulse may appear to arrive 

 at the most distant parts of the body without the interven- 

 tion of any very perceptible interval of time. 



The velocity of the transmission of the pulse being Degree of dila.-"^ 



known, it is easy to determine the degree in which the ar- *^''°p *^* ^^® 



' •' ° artenes. 



teries are dilated during its pass-age through them. The 



mean velocity of the blood in the aorta being eight inches 

 and a half in a second, its greatest velocity must be about 

 three times as much, since the contraction of the heart is 

 supposed to occupy only about one third part of the inter- 

 val between two successive pulsations ; and if the velocity . 

 of the pulse is sixteen feet in a second, that of the blood 

 itself must be about one eighth part as great; so that the 

 column of blood occupying eight inches may occupy only 

 seven ; hence the diameter must increase in the ratio of 

 VoL.XXVII— Sept. 1810. F about 



