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HIINDGUT LARGE INTESTINE o Where water is absorbed from indigestible residues of liquid chyme, converting it to feces, which are stored temporarily and allowed to accumulate until defecation occurs o Consists of Cecum; appendix, ascending, transverse, descending, and sigmoid colon; rectum; and anal canal o Can be distinguished from small intestine by Omental appendices Small fatty omentum-like projections teniae coli 3 longitudinal bands o Mesocolic tenia to which transverse and sigmoid mesocolons attach o Omental tenia, to which omental appendices attach o Free tenia, to which neither mesocolons nor omental appendices are attached These are made of smooth muscle They begin at the base of the appendix as the thick longitudinal layer of the appendix separates into 3 bands and run the length of the large intestine They broaden and merge with one another again at the rectosigmoid junction into a continuous longitudinal layer around the rectum Their tonic contraction shortens the wall with which they are associated, and the wall becomes baggy looking, forming… Haustra Sacculations of the wall of the colon between the teniae It’s bigger around CECUM AND APPENDIX o Cecum is the first part of the large intestine—continuous with ascending colon Is a blind intestinal pouch Lies in the iliac fossa of the right lower quadrant of the abdomen, inferior to the junction of the terminal ileum and cecum May be palpable through the anterolateral abdominal wall if distended with gas or feces Lies close to inguinal ligament Almost entirely enveloped by peritoneum, but has no mesentery Commonly bound to lateral abdominal wall by one or more cecal folds of peritoneum Terminal ileum enters the cecum obliquely and partly invaginates into it Ileal orifice enters cecum between ileocolic lips (superior and inferior) o o o o Folds that meet laterally forming ridges called the frenula of the ileal orifice Is usually closed, appearing as ileal papilla on the cecal side Appendix is a blind intestinal diverticulum that contains masses of lymphoid tissue Is on the posteromedial aspect of cecum, inferior to the ileocecal junction Has short triangular mesentery, the mesoappendix, which derives from the posterior side of the mesentery of the terminal ileum Attaches to the cecum and proximal part of appendix Position of the appendix is variable, but it is usually retrocecal ARTERIAL SUPPLY Ileocolic artery, terminal branch of the SMA Appendicular artery, branch of ileocolic artery, supplies the appendix VENOUS DRAINAGE Tributary of SMV, the ileocolic vein NERVES Superior mesenteric plexus Sympathetic fibers are from lower thoracic part of spinal cord Afferent nerve fibers accompany these to T10 spinal segment Parasympathetics are from vagus nerves COLON o Four parts Ascending Second part of the large intestine Passes superiorly on the right side of abdominal cavity from the cecum to the right lobe of the liver, where it turns to the left at the RIGHT COLIC FLEXURE (hepatic flexure) o Flexure lies deep to ribs 9-10 and is overlapped by inferior part of liver Is narrower than the cecum and is secondarily retroperitoneal along the right side of the posterior abdominal wall (awesome picture of arteries to colon on pg. 250) Is usually covered by peritoneum anteriorly and on its sides, BUT it has a short mesentery in 25% of people Is separated from anterolateral abdominal wall by the greater omentum Deep vertical groove lined with parietal peritoneum, the RIGHT PARACOLIC GUTTER, lies between the lateral aspect and the adjacent abdominal wall ARTERIAL SUPPLY OF ASCENDING COLON AND RIGHT COLIC FLEXURE o Branches of SMAILEOCOLIC AND RIGHT COLIC ARTERIES o These anastomose with each other and with the right branch of the MIDDLE COLIC ARTERY This is the first of many anastomotic arcades, that is continued by LEFT COLIC and SIGMOID ARTERIES to form the MARGINAL ARTERY/JUXTACOLIC ARTERY This artery parallels and extends the length of colon VENOUS DRAINAGE o Ileocolic and right colic veins tributaries of SMV NERVES from superior mesenteric nerve plexus TRANSVERSE Third, longest, and most mobile part of the LI Crosses the abdomen from right colic flexure to left colic flexure, where it turns inferiorly to become descending colon LEFT COLIC FLEXURE/SPLENIC FLEXURE is usually more superior and acute and less mobile that the right colic flexure o Lies anterior to inferior part of left kidney and attaches to the diaphragm through phrenicocolic ligament Transverse colon and its mesentery, the transverse mesocolon, loop down, often inferior to level of iliac crests o Mesentery is fused with posterior wall of omental bursa Root of transverse mesocolon is continuous with parietal peritoneum posteriorly ARTERIAL SUPPLY o MIDDLE COLIC ARTERY, branch of SMA VENOUS DRAINAGE o SMV NERVES from superior mesenteric nerve plexus via right and middle colic arteries—transmit sympathetic, parasympathetic (vagal), and visceral afferents DESCENDING Is secondarily retroperitoneal Continuous with sigmoid colon Has short mesentery in 33% of people Passes anterior to lateral border of left kidney Has LEFT PARACOLIC GUTTER on its lateral aspect SIGMOID Links descending with rectum Rectosigmoid junction is indicated by termination of the teniae coli Usually has a long mesentery—sigmoid mesocolon o Is free to move at its middle part, so can result in volvuli Root of sigmoid colon has inverted V-shaped attachment extending medially and superiorly along the external iliac vessels and then medially and inferiorly from the bifurcation of the common iliac vessels to the anterior aspect of the sacrum o Left ureter and division of left common iliac artery lie retroperitoneally, posterior to apex of the root of the sigmoid mesocolon Omental appendices of sigmoid colon are long and disappear when sigmoid mesentery terminates ARTERIAL SUPPLY o LEFT COLIC AND SIGMOID ARTERIES—branches of IMA o At left colic flexure, second transition occurs in blood supply of abdominal part of the alimentary canal: SMA (midgut) to IMA (hindgut) VENOUS DRAINAGE o IMV, flowing to splenic vein and hepatic portal vein after that NERVES o Sympathetics from lumbar part of sympathetic trunk via lumbar splanchnic nerves, superior mesenteric plexus, and periarterial plexuses following IMA and branches o Parasympathetics from pelvic splanchnic nerves via inferior hypogastric plexus independent of arterial supply to this part of the digestive tract o Orad to the middle of the sigmoid colon—Pain sensations travel with sympathetics while reflex information follows parasympathetics to the vagal sensory ganglia o Towards end of sigmoid colon—all visceral afferents follow parasympathetic fibers to sensory ganglia of S2S4 RECTUM AND ANAL CANAL o Rectum is fixed (primarily retroperitoneal and subperitoneal) part of LI o Is continuous with sigmoid colon at S3 level o Is continuous inferiorly with anal canal Clinical correlates 254-261