84 ANALYTIC SECT. IX. 
cxcrv. The heart being enveloped in the peri- 
cardium, and moistened by the liquid contained 
in this membrane, cannot be said to move in a 
vacuum; but it should be recollected, that the 
thorax being an air-tight cavity, and the left lung 
not pressing equally on the whole of the peri- 
cardium, the periphery of the heart is not subject 
to the whole pressure of the atmosphere. This 
arrangement of parts in which the heart is placed, 
must considerably aid the natural expansibility 
of its fibres, and augment its power of suction. 
cxcv. The influence of the heart’s expansion 
on the fluids, within the sphere of its action, 
appears extremely plain, if we reason simply 
from cause to effect, or vice versd. Supposing 
the right auricle to have contracted, the moment 
that it again expands, a temporary vacuum is 
formed, which ‘is instantly filled with blood from 
the vene cave, upon the plain laws of hydro- 
statics. This suction of the ‘auricle causes an 
undulatory motion in the blood of the vena: cave, 
which Haller calls contraction of these vessels. 
It is obvious that the quantity of blood drawn 
from the vene: cavae must depend on the degree 
of expansion in the fibres of the auricle. 
cxcvi, The filling of the auricle speedily alters 
the state of its fibres, the blood not only neutral- 
izes the expansive force, but communicates a 
