parts of the vascular system. It is in the more
hidden of nature’s operations, that we frequently
discover the most splendid instances of design
in its Author.
The number and distribution of the minute and
capillary vessels, is accurately proportioned and
adapted to the object of the circulation. When
the structure of the part is simple, and the object
of the circulation is its nutrition merely, the
vessels are few in number ; when the part is
more complicated, or other objects besides its
nutrition are to be fulfilled, the number, character,
and mode of distribution of the vessels,
are appropriately modified. 1
1. Of the minute and capillary vessels in the Fin
and Tail of the S tickle-back;— the Gasterosteus
aculeatus of L in n a e u s . *
The fin and tail of the fish are merely natatory
organs. Their structure, like their office, is
simple, and the number and distribution of their
minute and capillary vessels, are strictly in unison
with their character. The rays have but one
centre of movement, that at their origin ; and the
texture which unites them, is merely, a reflex
membrane. This part affords, therefore, an example
of the simplest kind of the minute and
capillary vessels and circulation.
The arteries run along each border of each
ray, and pursue their course to its extremity. At
the extremity of the ray, the artery is reflected,
becomes venous, and pursues a similar but
retrograde course along the outer side of the
artery. The artery, during its whole course, and
at its extremity, gives origin to capillary vessels,
which are few in number, inclose large spaces,
and at length join the veins.
Nearly at the point at which the ray divides
into two smaller rays, the artery gives off a
branch which crosses the ray, subdivides into
two minuter branches, which run along the
internal borders of these smaller rays respectively,
at the termination of which they are reflected
into veins, which, in their turn, unite, cross the
ray, and join the principal vein. In this course,
the artery gives off, and the vein receives a few
capillary vessels.
Sometimes a minute arterial branch, or venous
Toot, crosses the fine membrane between the rays,
especially near the root of the tail ; but in