of the microscope. It is however by no means
difficult to observe. And it becomes still more
obvious if the circulation be impeded or languid.
It is, indeed, by rendering the circulation slow, by
means which will be described hereafter, that the
fact of the transition of the minute arteries into true
capillaries, is made most obvious. In the ordinary
state of the circulation, the termination of the extreme
minute artery is apt to he lost to view; and
the change of rapidity in the circulation only adds
to this obscurity by making the capillaries themselves
more obvious.
This is not the only mode of transition from an
arterial to a capillary vessel. Sometimes one of
the branches only, in the case of subdivision,
becomes capillary, whilst the other remains arterial,
pursues its course and again subdivides variously ;
sometimes the artery becomes all at once singularly
contorted, turning variously and giving origin
to various capillary branches and the slower
circulation.
The capillary vessels themselves are situated
intermediately between the minute arteries and
veins. They are in a certain sense cylindrical,
that is, of uniform diameter throughout their
course ; and however frequently they may unite
and divide and form anastomoses and even circles,
they constantly retain the same size. The arteries
and the veins viewed merely in their trunk and extreme
branch, or origin, on the other hand, may be
considered as cones, the former becoming gradually
smaller, the latter, gradually larger, in their
course.
The minute arteries then gradually divide and
subdivide ; the veins unite successively into larger
and larger roots and trunks, and frequently
anastomose ; the capillaries unite, divide, reunite,
and redivide, and anastomose continually, so as to
form a complete net-work of vessels of uniform
character and dimensions.
Plate II. represents the arrangement of the
minute vessels. The comparative modes of distribution
of the arteries and veins, and the frequent
anastomosis of the latter are accurately copied.
Plate III. pourtrays the ultimate divisions of the
minute arteries into their final branches, and of
these into the true capillary vessels. The former
are marked by continuous lines ; the latter, by
small circles arranged in the order of the vessels.
The first arterial branch to the left, is seen to give