<P> The final branch which is important for the digestive system is the inferior mesenteric artery, which supplies the regions of the digestive tract derived from the hindgut, which includes the distal 1 / 3 of the transverse colon, descending colon, sigmoid colon, rectum, and the anus above the pectinate line . </P> <P> The enteric nervous system consists of some one hundred million neurons that are embedded in the peritoneum, the lining of the gastrointestinal tract extending from the esophagus to the anus . These neurons are collected into two plexuses - the myenteric (or Auerbach's) plexus that lies between the longitudinal and the smooth muscle layers, and the submucosal (or Meissner's) plexus that lies between the circular smooth muscle layer and the mucosa . </P> <P> Parasympathetic innervation to the ascending colon is supplied by the vagus nerve . Sympathetic innervation is supplied by the splanchnic nerves that join the celiac ganglia . Most of the digestive tract is innervated by the two large celiac ganglia, with the upper part of each ganglion joined by the greater splanchnic nerve and the lower parts joined by the lesser splanchnic nerve . It is from these ganglia that many of the gastric plexuses arise . </P> <P> Early in embryonic development, the embryo has three germ layers and abuts a yolk sac . During the second week of development, the embryo grows and begins to surround and envelop portions of this sac . The enveloped portions form the basis for the adult gastrointestinal tract . Sections of this foregut begin to differentiate into the organs of the gastrointestinal tract, such as the esophagus, stomach, and intestines . </P>

Explain the structure and function of digestive system