12 Dr. Young’s Lecture on the Functions 
law, especially where the distension becomes considerable : 
thus there may be substances which exhibit a force of tension 
proportional to the excess of the square, or the cube of their 
length, beyond a certain given quantity. It is safest therefore 
to reason upon the elasticity of any substance, from experi- 
ments made without any great deviation from the circumstances 
to which the ca] dilation is to be applied. 
For this purpose, we may again employ some of the many 
excellent experiments contained in Hales’s haemastatics. It 
appears, that when any small alteration was made in the quan- 
tity 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 es- 
timating the quantity, which was probably contained in the 
larger vessels of the animal. Hence it follows, that the velo- 
city of the pulse must be nearly the same as that of an im- 
pulse transmitted through an elastic fluid, 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 progressive motion of the blood itself 
adds about eight inches ; and with this velocity, of at least six- 
teen 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 intervention of any very perceptible interval of time. 
The velocity of the transmission of the pulse being known, 
it is easy to determine the degree in which the arteries are 
dilated during its passage through them. The mean velocity 
of the blood in the aorta being eight inches and a half in a 
