20 O F T FI E VITAL AND
tlie derivation of the fpirits into them fhould be intercepted. If it
be faid, that the auricles are ready to contrail when the fyfiolc of the
ventricles begins, but that being weaker mufcles, they mull wait
till the fyfiole is finifhed # : I anfwer, that, if this were true, the au¬
ricles fhould become pale and tenfe while the ventricles are con¬
tracted, fince, when the influence of the nerves acts more flrongly
on any mufcle, it becomes equally hard, whether it be allowed to
contract, and its extremities to approach each other, or not. But
further, as an influx of fpirits into the fibres of any mufcle muff
be immediately followed by an endeavour in them to contract, fo,
if this be prevented, as foon as the fpirits are again intercepted, their
influence to^ produce any contraction will ceafe. This is the cafe
with fuch mufcles as are under the power of the will, where any
hidden but not continued effort, if it is not allowed that inflant to
take effect, immediately vanifhes ; nor is it to be doubted, that the
fame thing muft happen to the auricles of the heart. But, be this
as it will, it is evident, that the alternate motions of the auricles
cannot be owing to any compreflion of their nerves ; fince it is ac¬
knowledged, by the heft anatomifts, that the courfe of thefe nerves
is fuch as cannot fubject them to any alternate preffure ; which is
alfo true of all the cardiac nerves in thofe animals whofe hearts
have only one ventricle*
7. In dying animals, the right ventricle continues to contract after
✓
the left ventricle has ceafed, and the right auricle performs its mo¬
tions for fome time after its ventricle f. But thefe alternate mo¬
tions of the right ventricle and auricle, cannot arife from any com¬
preflion of their nerves ; fince, in the firft cafe, neither the aorta nor
the left auricle are dilated with blood at the end of the fyßole of the
right ventricle ; and in the latter, the pulmonary artery alfo remains
empty. In frogs and fifhes, whofe heart has but one ventricle and
one artery going out from it, the alternate motions of this organ
are as regular as in men and quadrupèdes: and it is well known,
that
* Beîiin. de motu cord. prop. 2. Keii’s anatomy cap. 3. fe&. iv.
f Harvey .de mot. cord, et fang. cap. 4.