OTHER INVOLUNTARY MOTIONS. 39 
were in a violent Rate, will of themfelves endeavour to return to 
their natural one *» 
The ventricles of the heart, in confequence of the relaxation 
which happens to their fibres after their fyfiole is finifhed, give no 
refiflance to any caufe that begins to dilate them, but will not, 
without violence, allow their fides to be removed from each other, 
fo much as happens during their diaftole. 
From the experiments of the Reverend Dr Hales, compared with 
thole of Dr Langrifh, it appears that the capacity of the left ven¬ 
tricle of the heart of an ox, in confequence of the natural relaxa¬ 
tion of its fibres, is to its capacity, when fully dilated by the re¬ 
fluent blood, nearly as 1 to f. As therefore the relaxation of 
the heart at the end of every fyfiole, is owing to the contraction 
of its fibres ceafing at that time ; fo its full diafioh is produced by 
the returning venous blood, which enters its cavities with a confi- 
derable force. Without this, it is impoffible that any relaxation of 
the heart could produce its diafiole; fince a hollow mufcle, fuch as 
the heart or the bladder, can never be fully dilated by means of 
its internal mechanifm, or without the -affiftance of a diftending 
caufe introduced into its cavities. But although the diafiole, or full 
dilatation of the ventricles of the heart, muff necefiarily be afcribed 
to the force of the refluent blood ; yet this alone, without a pre¬ 
ceding relaxation of their fibres, would be infufficient to produce 
that effect. ’Tis true indeed, that the contraction of the auricles, 
and the ?momentum of the refluent blood, are, in fome fenfe, antago- 
nifts to the ventricles J ; but both thefe taken together falling much 
fliort 
# Dr Langrifh is of opinion, that when the heart is in fyßole, fome of its fibres are always 
ftretched beyond their natural tone ; fo that by their elaftic reftitutive property, they aCt in a 
certain degree as antagonifts to the contracted fibres,and fo contribute to unbend them at the 
end of every Jyßole ; Cronean lectures, p 55. But whatever may be in this, it does not ap- 
p ar that the heart is, by its make, better fitted to relax itfelf, than the other muicles ; fince 
thefe, or even a few of their fibres, when feparated frem the body, and io deprived of their 
antagonifts, are obferved to be alternately contracted and relaxed like the heart. Vid. SeCt. 
2:iv. below. 
f Vid Hales's Statical effays, vol. 2. p. 25.; and Langrififis Cronean leCtnres, No. 147. 
% It is a mifiake to think, that no blood is puflred into the ventricles of the heart during 
their diaßole, except what was contained in the auricles properly fo called. A certain quan¬ 
tity