2. Plexus of the Thoracic Aorta. This con sists of delicate filaments which are derived from the thoracic portion of the gangliated chain of the sympathetic; several filaments also pass between it and the cesophageal plexus. Above, it is continued into the cardiac plexus, from which it derives some branches, and below it accornpanies the vessel through the aortic opening in the diaphragm, to terminate in the cceliac plexus.
C. Abdominal Plexuses of the Sympathetic — The abdominal plexuses of the sympathetic are larger and more numerous than those oc curring in any of the other cavities of the body. They correspond in number with the branches of the abdominal aorta, and accom pany them in their course to the different viscera. From the plexuses occurrina on the larger arteries, off-sets pass, which °form a number of subsidiary plexuses upon the smaller vessels. The chief abdominal plexuses are the eceliac, superior mesenteric, renal, infrrior mesen teric, and superior and inferior hypogastric plexuses.
1. The eceliac, solar, or epigastric plexus is the largest of the plexuses of the sympa thetic. It is situated in the upper part of the cavity of the abdomen, on both sides of the aortic opening in the diaphragm, extending across the anterior part of the aorta, and is covered in front by the stomach. It sur rounds the cceliac axis, and extends down wards as far as the origin of the superior mesenteric artery. It usually contains two ganglia; these present a somewhat crescentic form, and have on this account been termed the semilunar ganglia. They are situated one on each side of the plexus towards its upper part, and are commonly surrounded by a num ber of smaller ganglia. The solar plexus receives the splanchnic nerves, also some branches from the posterior gastric plexus of the pneumogastric ; it likewise receives fila ments from the plexus which has been de scribed as surrounding the thoracic aorta, as well as others from the three or four upper lumbar ganglia. The offsets from the plexus present the same plexiform arrangement as the plexus itself, and are named according to the arteries which they accompany ; they are the phrenic, or diaphragmatic, superior coro nary, hepatic, splenic, and renal plexuses.
(a) The diaphragmatic plexuses are tvto in number, a right and left, and consist of several delicate filaments derived from the upper part of the semilunar ganglia. They often present several small ganglionic en largements, and accompany the diaphragmatic arteries, sinking with them into the muscular substance of the diaphragm, where they com municate with branches of the phrenic nerve.
(b) The superior coronary plexus accom panies the left coronary artery of the stomach, along its upper border, and is distributed to the anterior and posterior walls of the organ, its filaments uniting with the branches sup plied by the pneumogastric nerves, chiefly with those which are distributed to the posterior wall of the stomach. It extends to
the pjloric orifice, where it joins branches of the hepatic plexus.
(c) The hepatic plexus, of considerable size, ascends along with the hepatic artery ; it receives some filaments from the pneumo gastric nerve, and also communicates, as has been already mentioned, with the superior coronary plexus of the stomach. Branches leave it for the duodenum and head of the pancreas ; and others pass with the right gastro epiploic artery, along the greater curvature of the stomach, forming the inferior coronary plexus of the stomach. On entering the trans verse fissure of the liver, the hepatic plexus divides into a right and a left portion, which accompany the divisions of the hepatic artery and vena port, ramifying upon theru—an off set from the hepatic plexus passes to the gall-bladder, along with the cystic artery.
(d) The splenic plexus surrounds the artery of the same name, passing with it and its branches to the spleen. Offsets pass from the splenic plexus to the pancreas, and to the stomach, which form the pancreatic and left gastro-epiploic plexuses.
2. The superior mesenteric Plexus appears as a prolongation downwards of the cceliac plexus, and is the largest of the offsets fur nished by it : it also receives some filaments from the right pneumogastric nerve. It sur rounds the superior mesenteric artery, form ing for it a close plexiform sheath, and sends offsets along its branches, which accompany them as they pass between the layers of the mesentery to the duodenum, small intestine, ccecum, and ascending and transverse colon. The highest of these unite with the nerves which pass along the pancreatico-duodenal ar tery, while those which are distributed to the transverse colon communicate with the nerves derived from the inferior mesenteric plexus. The nerves which accompany the arteries to the intestines present at first a plexiform arrangement but in their course through the mesentery, several of them are seen to run alongside the vessels, sometimes separated a short distance from them. Communicating branches pass between them in the same way as between the arteries. On reaching the intestine they enter it at the part where the mesentery is attached, and dividing into finer twigs, soon disappear in the substance of its coats. Many appear to become lost in the muscular coats, while some may be traced through these, ending apparently in the mu cous coat, or in the sub-mucous cellular tissue.