of the Heart and Arteries. 2 1 
auricle, while it must retard, for a moment, the column which 
is approaching, at a time that it could not be received. 
There is no difficulty in imagining what services the mus- 
cular coats of the arteries may be capable of performing, 
without attributing to them any immediate concern in support- 
ing the circulation. For since the quantity of blood in the 
system is on many accounts perpetually varying, there must 
be some means of accommodating the blood vessels to their 
contents. This circumstance was very evident in some of 
Hales's experiments, when after a certain quantity of blood 
had been taken away, the height of the column, which mea- 
sured the tension of the vessels, frequently varied in an 
irregular manner, before it became stationary at a height 
proportional to the remaining permanent tension. Haller 
also relates, that he has frequently seen the arteries com- 
pletely empty, although in some of his observations there was 
probably only a want of red globules in the blood which was 
flowing through them. Such alterations in the capacity of 
the different parts of the body are almost always to be attri- 
buted to the exertion of a muscular power. A partial con- 
traction of the coats of the smaller arteries may also have an 
immediate effect on the quantity of blood contained in any 
part, although very little variation could be produced in this 
manner by a change of the capacity of the larger vessels. 
According to this statement of the powers which are con- 
cerned in the circulation, it must be obvious that the nature 
of the pulse, as perceptible to the touch, must depend almost 
entirely on the action of the heart, since the state of the arte- 
ries can produce very little alteration in its qualities. The 
greater or less tension of the arterial system may indeed render 
