832 NERVOUS SYSTEM OF THE HORSE 
inclusive. It extends along the bodies of the vertebra medio-ventral to the thoracic 
trunk, then crosses the latter ventrally, passes back between the crus of the dia- 
phragm and the lateral border of the psoas minor, and joins the cceliaco-mesenteri¢ 
ganglion. It is small at its origin, but becomes considerably larger than the sym- 
pathetic trunk. Near its termination it may present a small splanchnic ganglion, 
from which and from the nerve filaments go to the aorta, the oesophagus, and the 
vertebrie. The fibers of the nerve are derived chiefly from the spinal cord; hence 
its white appearance as compared with the sympathetic trunk. 
The small splanchnic nerve (N. splanchnicus minor) is formed by roots derived 
from the last two or three thoracic ganglia. It runs back with the great splanchnic 
nerve, but ends in the cceliac or in the renal and adrenal plexuses. 
The splanchnic nerves are quite variable. The greater splanchnic is often blended more or 
less with the sympathetic trunk and may be separate only in the posterior part of the thorax. 
The lesser splanchnic may be included in the greater, and thus appear to be absent. Its roots 
communicate by filaments with the great splanchnic. 
4. The abdominal part of the sympathetic trunk (Pars abdominalis systematis 
sympathici) lies along the medial border of the psoas minor, above the aorta on the 
left side and the posterior vena cava on the right (Fig. 656). The trunk is smaller 
than the thoracic part, and presents usually six small fusiform lumbar ganglia (G. 
lumbalia), which are connected by rami communicantes with the ventral divisions 
of the lumbar nerves. Visceral branches go to the aortic and pelvic plexuses 
and to the cceliaco-mesenteric ganglion. 
5. The pelvic and caudal part of the sympathetic trunk (Pars pelvina et cau- 
dalis systematis sympathici) begins at the last lumbar ganglion and extends along 
the pelvic surface of the sacrum medial to the emergence of the ventral branches 
of the sacral nerves (Fig. 576). At the third or fourth segment of the sacrum the 
trunk divides into medial and lateral branches. The medial branch inclines to the 
median plane and unites with the opposite branch. At the junction there is often 
found the small coccygeal ganglion (G. impar), which lies on the coccygeal artery 
at the joint between the first and second coccygeal vertebrae. A filament from the 
ganglion accompanies the artery. The lateral branch communicates with the last 
two sacral nerves and joins the ventral cocecygeal nerves. Sacral ganglia (G. 
sacralia) occur near each of the first three sacral foramina, and are connected by 
gray rami communicantes with the ventral branches of the sacral nerves. The 
visceral branches are distributed through the pelvic plexus. They supply motor 
fibers to the longitudinal, and inhibitory fibers to the circular, muscular coat of the 
rectum; motor fibers to the bladder and uterus; and the vaso-dilator fibers (Nn. 
erigentes) to the penis. 
According to von Schumacher, minute segmental coccygeal ganglia occur along the caudal 
branches of the sympathetic trunk. The lateral branch is regarded by van der Broek as an 
aggregate of rami communicantes. 
THE ABDOMINAL AND PELVIC PLEXUSES 
The chief plexuses which distribute nerves to the viscera and vessels of the 
abdominal and pelvic cavities are two in number, the coeliac and the pelvic. From 
them fibers proceed to form numerous subsidiary plexuses which are named accord- 
ing to the organs which they supply or the vessels which they enlace. 
The ceeliac plexus (Fig. 575)! is situated on the dorsal wall of the abdominal 
cavity, in relation to the aorta and the origin of its chief visceral branches. It is 
formed by the splanchnic nerves, branches of the vagi, and filaments from the 
anterior lumbar ganglia of the sympathetic. It contains the cceliaco-mesenteri¢ 
ganglia. From the eceliac plexus and its ganglia subsidiary plexuses are continued 
upon the branches of the aorta. 
1 Also known as the solar plexus. 
