which they are found. In some they are more accurately
fitted, in others more remissly or carelessly contrived,
and always with a view to their being closed under
a greater or a slighter force of the ventricle.
In the left ventricle, therefore, in order that the
occlusion may be the more perfect against the greater
impulse, there are only two valves, like a mitre,
and produced into an elongated cone, so that they
come together and touch to their middle; a circumstance
which perhaps led Aristotle into the error of supposing
this ventricle to be double, the division taking place
transversely. For the same reason, and that the
blood may not regurgitate upon the pulmonary veins,
and thus the force of the ventricle in propelling
the blood through the system at large come to be neutralized,
it is that these mitral valves excel those of the
right ventricle in size and strength and exactness
of closing. Hence it is essential that there
can be no heart without a ventricle, since this must
be the source and store-house of the blood. The
same law does not hold good in reference to the brain.
For almost no genus of birds has a ventricle in the
brain, as is obvious in the goose and swan, the brains
of which nearly equal that of a rabbit in size; now
rabbits have ventricles in the brain, whilst the goose
has none. In like manner, wherever the heart
has a single ventricle, there is an auricle appended,
flaccid, membranous, hollow, filled with blood; and
where there are two ventricles, there are likewise
two auricles. On the other hand, some animals
have an auricle without any ventricle; or, at all
events, they have a sac analogous to an auricle; or
the vein itself, dilated at a particular part, performs
pulsations, as is seen in hornets, bees, and other
insects, which certain experiments of my own enable
me to demonstrate, have not only a pulse, but a respiration
in that part which is called the tail, whence it is
that this part is elongated and contracted now more
rarely, now more frequently, as the creature appears
to be blown and to require a large quantity of air.
But of these things, more in our “Treatise On
Respiration.”
It is in like manner evident that the auricles pulsate,
contract, as I have said before, and throw the blood
into the ventricles; so that wherever there is a ventricle,
an auricle is necessary, not merely that it may serve,
according to the general belief, as a source and magazine
for the blood: for what were the use of its pulsations
had it only to contain?
The auricles are prime movers of the blood, especially
the right auricle, which, as already said, is “the
first to live, the last to die”; whence they
are subservient to sending the blood into the ventricles,
which, contracting continuously, more readily and
forcibly expel the blood already in motion; just as
the ball-player can strike the ball more forcibly
and further if he takes it on the rebound than if
he simply threw it. Moreover, and contrary to