; these arteries, like all other branches of the aorta, are
largest at their origin. Where these vessels spring from G, the
descending thoracic aorta, they present considerable caliber; but at
this inaccessible situation, they seldom or never call for surgical
interference. As the intercostal arteries pass outwards, traversing the
intercostal spaces with their accompanying nerves, they diminish in
size. Each vessel divides at a distance of about two inches, more or
less, from the spine; and the upper larger branch lies under cover of
the inferior border of the adjacent rib. When it is required to perform
the operation of paracentesis thoracis, this distribution of the vessel
should be borne in mind; and also, that the farther from the spine this
operation is performed, the less in size will the vessels be found. The
intercostal artery is sometimes wounded by the fractured end of the rib,
in which case, if the pleura be lacerated, an effusion of blood takes
place within the thorax, compresses the lung, and obstructs respiration.

The thoracic aorta descends along the left side of the spine, as far as
the last dorsal vertebra, at which situation the pillars of the
diaphragm overarch the vessel. From this place the aorta passes
obliquely in front of the five lumbar vertebrae, and on arriving
opposite the fourth, it divides into the two common iliac branches. The
aorta, for an extent included between these latter boundaries, is named
the abdominal aorta, and from its fore part arise those branches, which
supply the viscera of the abdomen.

The branches which spring from the abdominal aorta to supply the viscera
of this region, are considerable, both as to their number and size. They
are, however, of comparatively little interest in practice. To the
anatomist they present many peculiarities of distribution and form
worthy of notice, as, for example, their frequent anastomosis, their
looping arrangement, and their large size and number compared with the
actual bulk of the organs which they supply. As to this latter
peculiarity, we interpret it according to the fact that here the vessels
serve other purposes in the economy besides that of the support and
repair of structure. The vessels are large in proportion to the great
quantity of fluid matter secreted from the whole extent of the inner
surface of this glandular apparatus--the gastro-intestinal canal, the
liver, pancreas, and kidneys.

As anatomists, we are enabled, from