CHAPTER VI.
A BRIEF ACCOUNT OF THE SINGULAR PHENOMENON
OF A CAUDAL HEART IN THE EEL.
It is well known to the comparative anatomist,
that the fish tribe possess only a simple pulmonic
heart. This organ gives origin to a large artery,
which, soon after it quits the heart, divides into
four branches, of which one pair goes to each pair
of branchise. From the branchiae, the blood is returned
through vessels which unite and forms
the aorta. The aorta, in its turn, again subdivides
and sends branches to the different parts
of the body. From each of these the vessels
again reunite, form the vena cava, and convey
the blood once more to the heart.
Through the whole of this course, it has been
supposed that the pulmonic heart alone, with the
aid of some subsidiary powers of the circulation,
propelled the blood.
I have discovered a structure in one species
of fish, which will lead us to view this opinion
with distrust, and which will point out to us
the fact of an unsuspected addition to the power
and action of the heart, in some species of
animals.
This structure is seen, even with the naked eye,
in the tail of the eel. Its form, action, and
connexions are, from the degree of transparency
of the part, still better traced by the assistance
of the microscope. Placed under this instrument,
a particular spot near the extremity of the tail
in the eel, easily discovered, has the appearance
represented in Plate X.
The drawing represents the ventricle of this
caudal heart. The different vessels unite and
form a connexion with this ventricle near its
highest point.
The course pursued by the blood in these
vessels is marked by the arrows, and uniformly
tends towards the highest point of the ventricle ;
from this point it seems to be slowly propelled or
drawn into the ventricle; by a sudden contrac