832 NERVOUS SYSTEM OF THE HORSE 



inclusive. It extends along the bodies of the vertebrse medio-ventral to the thoracic 

 trunk, then crosses the latter ventrally, passes back between the cms of the dia- 

 phragm and the lateral border of the psoas minor, and joins the coeliaco-mesenteric 

 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 

 vertebrae. 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 coeliac 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 coeliaco-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 

 trimk 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 coccygeal 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 coeliac 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 coeliaco-mesenteric 

 ganglia. From the coeliac plexus and its ganglia subsidiary plexuses are continued 

 upon the branches of the aorta. 



' Also known as the solar plexus. 



