The arrangement of these communicating branches is somewhat variable;
they may arise in various combinations, as at first united, or as more
or less completely separated nerves.
The sympathetic trunk (_c_) continues caudad from the caudal end of the
inferior cervical ganglion, lying at first on the lateral surface of the
longus colli, then on the lateral surface of the centra of the vertebræ.
For each vertebra there is a slight enlargement, forming one of the
=thoracic ganglia= of the sympathetic, and from each ganglion one or
two =communicating branches= are given off to the corresponding spinal
nerve. Just caudad of the last rib (but while still in the thoracic
cavity) the sympathetic gives off the large =N. splanchnicus major= or
great splanchnic nerve (_n_), which passes ventrocaudad and pierces the
diaphragm. The sympathetic itself then passes through the diaphragm
laterad of the crus.
=Abdominal Portion of the Sympathetic= (Fig. 164).--The great splanchnic
nerve (_b_) passes through the diaphragm (2) laterad of the crus,
extends two or three centimeters caudoventrad, and joins a large
ganglion lying just caudad of the cœliac artery (4), almost on the
lateral surface of the superior mesenteric (5). This is the =cœliac= (or
semilunar) =ganglion= (_d_). From it nerves branch in all directions,
forming the =cœliac plexus= (_c_), a part of the large =solar plexus=.
Less than a centimeter ventrocaudad of the cœliac ganglion, on the
caudal surface of the superior mesenteric artery (5), is the somewhat
smaller =superior mesenteric ganglion= (_f_); the two are connected by
two strong nerve-trunks.
[Illustration: FIG. 164.--SYMPATHETIC AND VAGUS IN THE ABDOMEN
(SOMEWHAT SCHEMATIC).
1, stomach; 2, crus and cut edge of diaphragm; 3, aorta; 4, cœliac
artery; 5, superior mesenteric artery; 6, kidney; 7, inferior
mesenteric artery; 8, large intestine. _a_, main trunk of sympathetic;
_b_, great splanchnic nerve; _c_, lesser splanchnic nerves; _d_,
cœliac (or semilunar) ganglion; _e_, cœliac plexus; _f_, superior
mesenteric ganglion; _g_, superior mesenteric plexus, following the
artery; _h_, aortic plexus; _i_, inferior mesenteric ganglion and
plexus; _j_, suprarenal plexus on suprarenal body; _k_, renal plexus
following renal artery; _l_, ventral œsophageal branch of vagus,
forming anterior gastric plexus; _m_, dorsal œsophageal branch of
vagus, forming posterior gastric plexus.]
The cœliac ganglion (_d_) receives from the sympathetic (_a_), in
addition to the great splanchnic nerve, two (or three) =lesser
splanchnic= nerves (_c_), which arise from the main trunk of the
sympathetic, the first just as it reaches the abdominal cavity, the
second one or two centimeters caudad of the diaphragm.
From the cœliac and superior mesenteric ganglia a network of nerves
passes in all directions to the abdominal viscera. These nerves follow
especially the arteries, forming plexuses over their surfaces, and
passing with them to the organs which they supply. The cœliac plexus
anastomoses with the =posterior gastric= plexus of the vagus. Parts of
the cœliac plexus following the branches of the cœliac artery may be
distinguished as =hepatic= plexus, =splenic= plexus, etc. A dense plexus
is formed on the surface of the suprarenal body (=suprarenal= plexus,
_j_), and a =renal= plexus (_k_) follows the renal artery to the kidneys
(6). The =superior mesenteric plexus= (_g_) forms a dense network over
the superior mesenteric artery (5) and follows it to the intestine. A
plexus passes caudad on the aorta (=aortic plexus=, _h_). This is a
continuation of the cœliac and superior mesenteric plexuses; it receives
a number of branches from the main sympathetic trunk, lying dorsad of
the aorta. The aortic plexus contains a number of small ganglia.
From the aortic plexus the smaller =inferior mesenteric= plexus (_i_)
follows the inferior mesenteric artery (7) to the large intestine; in
this lies a ganglion of considerable size, the =inferior mesenteric
ganglion=.
In the pelvic region small plexuses are formed in a similar manner, from
branches given off by the main sympathetic trunk, together with branches
from the plexuses of the abdominal cavity. These plexuses supply the
various organs in the pelvis.
The main sympathetic trunk (_a_) passes into the abdomen at the side of
the crus of the diaphragm, lying dorsad of the great splanchnic nerve
(_b_). In the abdomen it lies nearer the middle line than in the thorax,
so that the two trunks of right and left side are separated by but a few
millimeters as they lie on the ventral surface of the centra of the
lumbar vertebræ. There is a ganglion for each vertebra (Fig. 162, _n_),
and from each ganglion are given off one or more communicating branches
to the spinal nerves, and usually a branch which passes ventrad to join
one of the plexuses among the viscera. The first three branches of this
kind (Fig. 164, _c_) are larger and pass to the cœliac and superior
mesenteric ganglia. The sympathetic may be traced into the pelvis, where
it becomes smaller, and is lost as the caudal region is reached.