THE muscles of Compartment I and of the upper part of Com- partment II as far as the junction of the upper and middle thirds of the oesophagus are chiefly striated muscles and are innervated mainly by somatic nerves; the glandular struc- tures of these compartments are, however, associated with
the autonomic system.
The part of the alimentary tube from the lower two-thirds of the oesophagus to the anal canal is activated almost entirely by involuntary muscles and is under the control of the auto- nomic system. It has, therefore, a double innervation, namely, sympathetic and parasympathetic. In the wall of the tube both sets are intimately associated with two nerve- plexuses : one, the submucous (Meissner’s) plexus is found in the submucous coat; the other, the myenteric (Auerbach’s) plexus lies between the longitudinal and circular layers of the muscular tunic. In this connexion it may be pointed out that the somatic motor nerves and the autonomic motor (so-called) nerves are not strictly comparable anatomically. A somatic motor nerve is the final common path for a series of impulses directed fo a specific muscle. The great sympathetic and parasympathetic nerves, on the other hand, are connector neurons, and the final common pathways are the small neurons of the plexuses in and adjacent to the wall of the bowel. The implications of this are discussed in the later sections.
Sympathetic Nerves (Fig. 9).—The greater and lesser splanchnic nerves derived from the 5th-12th thoracic seg- ments of the spinal cord give twigs to the lower end of the oesophagus, and then, on either side, pierce the diaphragm to end in the cceliac (semilunar) ganglion. The nerves emerging from this ganglion form the large cœliac (solar) plexus, one part of which accompanies the cœliac artery and its branches, while another part, the superior mesenteric plexus, follows the branches of the superior mesenteric artery.
Compartments III and IV therefore receive their sympathetic nerve-supply from the cceliac plexus. The cceliac plexus is continued down over the aorta as the aortic plexus which is joined by twigs derived from the ist, 2nd, and 3rd lumbar spinal segments. Part of the aortic plexus accompanies the inferior mesenteric artery and its branches. Compartment V is supplied by this inferior mesen- teric plexus.
It will be seen, therefore, that Compartments III, IV, and V of the alimentary tube receive their sympathetic supply from the 5th-12th thoracic and ist-ßrd lumbar spinal seg- ments. Note specially that these are the spinal segments from which are derived the somatic nerves of the muscles of the abdominal wall and of the costal part of the diaphragm.
Parasympathetic Nerves (Fig. 10). — The para- sympathetic supply of Compartments I-IV is through the vagus nerves, which are the outgoing pathways from the spinal medulla. In the neck, twigs from the vagus nerves are given off to the glands in Compartments I and II and to the wall of the upper oesophagus. The vagus nerves then descend in the thorax, one on either side of the oesophagus, and supply this tube. After passing behind the root of the lung, each nerve divides into branches that combine with those of the opposite side to form simple plexuses on the front and back of the oesophagus. Right and left vagal trunks emerging from the plexuses accompany the oesophagus through the diaphragm. From ‘these trunks that are at first disposed mainly along the lesser curvature of the stomach, twigs are distributed to the anterior (left vagus trunk) and posterior (right vagus trunk) surfaces of the viscus as far as the pyloric antrum (Fig. n ) . Separate branches of the vagal trunks carry the supply to the prepyloric region, pylorus, duodenum, and the more distal parts of Compartment IV. Twigs which join the cceliac plexus are distributed along the branches of the cceliac and superior mesenteric arteries. Although it has not been demonstrated anatomically, it is supposed that the vagus supply extends along the whole of the small intestine and part of the great intestine as far as the middle of the transverse colon.
Compartments V and VI receive their parasympathetic supply through the pelvic splanchnic nerves. These nerves repre- sent the parasympathetic outflow from the 2nd~4th sacral segments of the spinal cord. The fibres for Compartment V run to the inferior mesenteric ganglion, and are distributed with the sympathetic fibres that accompany the branches of the inferior mesenteric artery.
It is important to note that, as regards the parasympathetic supply of the abdominal part of the alimentary tube, Com- partments / / / a n d IV are under the control of the vagi (cerebral outflow), while Compartment V is controlled by the pelvic splanchnics (sacral outflow).
Sensory Nerves.—It is only in the regions of entry and exit of the alimentary tube that receptors for ordinary sensa- tions are present. These regions, therefore, are supplied by the somatic sensory nerves.
In the rest of the alimentary tube there are no receptors for ordinary sensations in the wall, and the bowel is insensitive to ordinary sensory stimuli. Discomfort and pain in the bowel have no specialized sense organs. These two sensa- tions and numerous reflexes, however, require ingoing pathways, and such pathways must be present in the sym- pathetic and parasympathetic nerves of the appropriate segments, although it is not easy to demonstrate them histologically.