Chapter 27: The esophagus, stomach and intestines
The alimentary canal comprises the mouth, pharynx, esophagus, stomach, and intestine. Details of the development of the canal from the embryonic gut, and also the complicated question of rotation of the gut, should be sought in books on embryology. Briefly, the stomach and upper half of the duodenum are derived from the foregut and supplied by the celiac trunk. The distal half of the duodenum, the jejunum and ileum, cecum and appendix, ascending colon, and two thirds of the transverse colon are derived from the midgut and supplied by the superior mesenteric artery. The distal third of the transverse colon, the descending colon, sigmoid colon, rectum, and the upper part of the anal canal are derived from the hindgut and supplied by the inferior mesenteric artery. Hence the developmental and vascular junctions are at (1) the middle of the duodenum, where the bile duct ends, and (2) the left part of the transverse colon.
The esophagus is a conducting tube for food, whereas the stomach and intestine, together with their associated glands, are concerned with the digestion of food and the excretion of undigested material. The products of digestion traverse the gastro-intestinal mucosa to capillaries that ultimately form the portal vein. The portal vein gives rise to a second set of capillaries (sinusoids) in the liver, and these ultimately form hepatic veins.
The alimentary canal presents sphincteric mechanisms at junctional areas: pharyngoesophageal, gastro-esophageal, pyloric, ileocolic, and anal. The sphincters are under neural and hormonal control, and they help to prevent regurgitation of contents.
After its cervical and thoracic course, the esophagus traverses the diaphragm and joins the stomach at the gastro-esophageal junction. The junction is a barrier to the reflux of contents from the stomach back into the esophagus. A complicated sphincteric segment extends several centimeters above the gastro-esophageal junction and is partly in the thorax, partly in the diaphragm, and partly in the abdomen. The lower esophageal sphincter is a physiological sphincter, since there is no obvious thickening of muscle. The muscle in this region normally remains contracted except when swallowing (alcohol relaxes it). Also, the lower ~1 inch of the esophagus is in the abdomen, where intra-abdominal pressure can help keep it closed. The esophageal hiatus of the diaphragm can also press on the esophageal wall. Finally, the angle of entry to the stomach (cardiac incisure; angle of His) can result in a flap-like closure of the lower esophageal entry when the stomach is full. All of these mechanisms contribute to prevention of reflux. Hiatal herniae (where the gastroesophageal junction is above the diaphragm) defeat several of these mechanisms.
An important portal-systemic anastomosis takes place between the esophageal and left gastric veins. Stimulation of the lower end of the esophagus (e.g., by reflux of acid gastric contents) may cause pain ("heartburn") deep to the sternum or in the epigastrium. Diaphragmatic herniae occur most commonly through the esophageal opening and are termed hiatal herniae.
The stomach (Gk, gaster, "belly"; adjective gastric from Latin) is comprised of a cardia, fundus, body, and pyloric part (based on histological differences) (fig. 27-1A); greater and lesser curvatures; and gastro-esophageal and pyloric openings.
The esophagus enters the stomach at the gastro-esophageal opening, and the immediately adjacent portion of the stomach is termed its cardiac part. The fundus is the part of the stomach above the level of the cardiac opening. It usually contains swallowed air and hence is visible radiographically. The body of the stomach lies between the fundus and the pyloric part. The pyloric part comprises the pyloric antrum followed by the pyloric canal. The pyloric opening, or pylorus, between the stomach and duodenum, is surrounded by the pyloric sphincter. Congenital thickening of the sphincter is known as congenital hypertrophic pyloric stenosis, which produces vomiting, loss of weight, and a palpable lump in the infant's abdomen. The hypertrophied sphincter can be incised surgically. The gastroduodenal junction is often normally marked by a prominent prepyloric vein.
The greater and lesser curvatures extend between the gastro-esophageal and pyloric openings. The greater is on the left and is convex and longer; the lesser is on the right and is concave and shorter. The lesser curvature usually has an angular notch (incisura angularis) at its most dependent point, and this is commonly visible in radiographs of the barium-filled stomach of a subject in the erect position. The stomach, which is sometimes J-shaped when empty, is very variable in shape, capacity, and position. The front of the organ faces the greater sac; the back forms the anterior border of the lesser sac. The stomach lies on a variable visceral bed that includes the diaphragm, pancreas, and transverse mesocolon. Posterior gastric ulcers may involve the pancreas and the splenic artery, resulting in severe pain and bleeding.
The gastro-esophageal orifice is the most fixed part of the stomach. The fundus fits into the curve of the left dome of the diaphragm. The pyloric part is very mobile. The greater curvature may even enter the true pelvis.
The stomach can be examined by radiography, using a barium meal (figs. 27-2 and 27-3) and either plane x-rays or CT scanning. It's inside may be directly visualized through a fiber-optic tube passed down the esophagus (gastroscopy).
The food in the stomach is transformed into a liquid termed chyme, which, by rhythmic muscular contractions (peristalsis) of the pyloric part, is emptied into the duodenum. Reflux is prevented by the pyloric sphincter.
The liver is connected to the lesser curvature by the lesser omentum, the two layers of which surround the stomach and leave the greater curvature as the greater omentum (see fig. 26-5B). The stomach is covered entirely by peritoneum except for a small posterior "bare area" near the cardiac opening.
Blood Supply (fig. 27-4).
The stomach is supplied by the celiac trunk: (1) the right gastric (from the hepatic) and left gastric arteries run along the lesser curvature; (2) the right gastro-epiploic (derived from the hepatic) and left gastro-epiploic and short gastric (from the splenic) arteries course along the greater curvature. The veins empty directly or indirectly into the portal vein. The connections between the left gastric and esophageal veins are important portal-systemic anastomoses.
Lymphatic drainage (fig. 27-5).
Plexuses drain into regional nodes that accompany the arteries and end ultimately in the thoracic duct. Carcinoma can spread (1) to the liver, (2) to the pelvis by retroperitoneal lymphatics, and (3) to the rest of the body by veins and by the thoracic duct.
Innervation (fig. 27-6).
The stomach receives innervation from several sources: (1) sympathetic fibers via the splanchnic nerves and celiac ganglion (synapse) supply blood vessels and musculature, (2) parasympathetic fibers from the medulla travel in the gastric branches of the vagi, and (3) sensory vagal fibers include those concerned with gastric secretion.
The major part of digestion occurs in the small intestine, which extends from the pylorus to the ileocecal junction and includes the duodenum, jejunum and ileum. The Greek word enteron, meaning "gut," refers to the intestine ("enteritis" is inflammation of the intestine) and is used for its peritoneal attachment in the term "mesentery." The small intestine, which is an indispensable organ, is about 7 m (varying from 5 to 8 m) in length. Its characteristic feathery appearance after a barium meal (see fig. 27-2) is due to permanent circular folds and villi. The entrance of food into the stomach tends to cause the ileum to empty into the large intestine (gastro-ileal reflex).
The duodenum, which is derived from both foregut and midgut, is named for being approximately 12 (L., duodenarius) fingerbreadths in length. It is C-shaped, the concavity enclosing the head of the pancreas. It extends from the pylorus to the duodenojejunal flexure. Smooth muscle and elastic fibers from the right diaphragmatic crus to parts 3 and 4 of the duodenum constitute the suspensory muscle of the duodenum (so-called "Ligament of Treitz"). The suspensory muscle opens out the angle at the duodenojejunal flexure and thereby facilitates the passage of the contents.* The duodenum is described in four parts (see fig. 28-8).
The first part runs to the right and posteriorward, close to the vertebral column. Anteriorly are the liver and gallbladder; posteriorly are the bile duct, portal vein, and pancreas. The beginning of the first portion is mobile; i.e., it is not attached to the posterior abdominal wall. It lacks circular folds, giving the appearance of a "duodenal cap" on radiography (see fig. 27-2).
The second part descends anterior to the right renal vessels and posterior to the transverse colon.
The third part runs horizontally to the left across the inferior vena cava and aorta. The superior mesenteric vessels and the root of the mesentery lie anterior to the duodenum.
The fourth part ascends on the left of the aorta and then turns anteriorly as the duodenojejunal flexure.
The duodenal cap, which is often at the level of the second lumbar vertebra in vivo, is attached to the liver by the hepatoduodenal part of the lesser omentum and is mobile. The rest of the duodenum, however, is secondarily retroperitoneal and relatively fixed (see fig. 26-6).
The descending (second) part of the duodenum receives the bile, pancreatic, and accessory pancreatic ducts. The bile and pancreatic ducts frequently unite and form a short hepatopancreatic ampulla (Vater), which opens into the greater duodenal papilla, a small projection in the interior of the posteromedial aspect of the concavity of the duodenum (see fig. 28-6). In other cases, the ducts meet but open separately on the papilla. Each duct usually has a sphincter, the sphincter of the bile duct frequently being termed the choledochal sphincter. A sphincter of the hepatopancreatic ampulla may also be present (Oddi). The accessory pancreatic duct empties into the lesser duodenal papilla, which is situated on the anteromedial aspect of the descending part of the duodenum, about 2 cm above the greater papilla. It is frequently absent.
The proximal half of the duodenum is supplied by branches of the celiac trunk, the distal half by branches of the superior mesenteric artery (see fig. 27-4). The arteries approach the duodenum at its concavity. An incision along the right edge of the second part of the duodenum would mobilize the duodenum and the head of the pancreas without endangering their blood supply.
The jejunum and ileum are the continuous coiled part of the small intestine. In contrast to the ileum, the jejunum is shorter and typically emptier, more vascular (redder in vivo), and more thickly walled. Moreover, its mesentery shows translucent areas between the vessels, owing to the absence of fat. However, it is often difficult to distinguish the distal part of the jejunum from the proximal part of the ileum.
Occasionally a remnant of the vitelline (vitellointestinal) duct persists in the adult as an ileal (Meckle's) diverticulum, which is situated near the ileocolic junction. It may contain gastric or pancreatic tissue, and inflammation of the diverticulum may simulate acute appendicitis.
The jejunum and ileum are suspended from the posterior abdominal wall by the mesentery (see fig. 26-4); they are highly mobile and occupy much of the abdominal and some of the pelvic cavity.
The root of the mesentery (see fig. 26-6), about 15 cm long, runs inferiorly and to the right from the duodenojejunal flexure to the ileocolic junction, which is approximately at the level of the right sacro-iliac joint. Between the two layers of the mesentery are the branches of the superior mesenteric vessels, as well as nerves, lymph nodes and vessels, and some fat.
Blood Supply (fig. 27-7).
The jejunum and ileum are supplied by the superior mesenteric artery through a series of arcades within the mesentery.
After a fatty meal, the mesenteric lymphatics contain emulsified fat in a creamy lymph termed chyle. Lymph nodes and vessels are abundant in the mesentery. Carcinoma of the intestine can spread to the liver through the portal vein as well as through lymphatics.
Innervation (fig. 27-8).
The small intestine is supplied by autonomic and sensory fibers from the celiac and superior mesenteric plexuses. The sensory fibers include those for pain, the intestine being sensitive to distension ("cramps").
Large intestine (fig. 27-9)
The large intestine comprises the cecum and appendix; the ascending, transverse, descending, and sigmoid colon; and the rectum and anal canal (see fig. 27-7). Air and gas bubbles are often seen in the large intestine radiographically. Most of the large intestine is characterized by a series of shifting sacculations termed haustra (see fig. 27-12). Multiple diverticula may develop in the colon (diverticulosis) and may become inflamed (diverticulitis).
The large intestine is concerned with the formation, transport, and evacuation of feces and is associated with the absorption of water and the secretion of mucus. The large intestine is distensible and mobile.
The large intestine is supplied successively by the superior and inferior mesenteric arteries. The branches form a long marginal artery that usually extends from the cecum to the sigmoid colon. The venous drainage is mostly into the portal vein ultimately. The lymphatic drainage is similar to that of the small intestine.
Innervation (fig. 27-11).
Autonomic and sensory fibers travel in continuations of the celiac and mesenteric plexuses that accompany the colic arteries. The parasympathetic supply to the sigmoid colon and to at least a part of the descending colon, however, is from the pelvic splanchnic nerves by way of the hypogastric nerves and the inferior hypogastric plexuses. Distension activates pain fibers in the splanchnic nerves.
The cecum lies below the level at which the ileum joins the large intestine at the iliocolic (iliocecal) opening (fig. 27-12B). The cecum lies in the right iliac fossa, may reach the pelvic brim, and is usually surrounded by peritoneum (see fig. 26-7).
The outer longitudinal layer of muscle of the cecum and colon is thickened as three bands known as teniae coli (fig. 27-12). The anterior tenia serves as a guide to the appendix.
Appendix (fig. 27-12).
The vermiform appendix, often about 9 or 10 cm in length, arises typically from the posteromedial aspect of the cecum at the junction of the three teniae coli and about 1 or 2 cm below the ileum. Its mucosa is laden with lymphoid tissue.# The appendix usually possesses a peritoneal fold (its "mesentery" or mesoappendix; see fig. 26-7), which contains the appendicular artery (a branch of the ileocolic artery).
The appendix is variable in position and may be classified as (1) anterior, with ileal or pelvic (the commonest) positions, or (2) posterior, with subcecal, retrocecal, or retrocolic positions. Appendices may also be free or fixed. A free appendix may point in any direction, whereas a fixed appendix is usually either retrocecal or retrocolic. An inflamed appendix goes into spasm, becomes distended, and causes discomfort referred to the periumbilical area. Inflammation of the adjacent parietal peritoneum causes pain in the lower right quadrant of the abdomen, and the overlying muscles often show reflex spasm. The point of maximum tenderness to pressure may be anywhere in the right lower quadrant. Local pain may be minimal in retrocecal appendicitis, because the parietal peritoneum is not involved.
The ileum enters the large intestine usually posteromedially. The ileocolic (or ileocecal) opening presents two lips that form a so-called valve, which is of slight mechanical importance.
The surface of the colon presents small masses of fat, enclosed in peritoneum and termed appendices epiploicae.
The colon begins at the ileocolic junction and ascends in the right iliac fossa and on the posterior abdominal wall to the right colic flexure, which lies in front of the right kidney. An ascending mesocolon is said to be rare but perhaps may be more frequent than previously believed.
The part of the colon between the right and left flexures is derived from both the midgut and hindgut. Most of the transverse colon sags down, often below the level of the iliac crests and even into the true pelvis (see fig. 26-1). The transverse mesocolon is attached to the pancreas and is adherent to, but separable from, the greater omentum (see fig. 26-5B). The left colic flexure is usually higher, more acutely angled, and less mobile than the right, and it may be anchored to the diaphragm (by the phrenicocolic ligament).
From the left flexure, the colon descends, usually without a mesocolon, to the pelvic brim, where the sigmoid colon begins.
Below the pelvic brim, the colon acquires the sigmoid mesocolon and forms a variable loop, which, when full, may reach the epigastrium or lie in the pelvis. Feces are usually held in the sigmoid colon until immediately before defecation. The line of attachment of the sigmoid mesocolon may form an inverted V, with its apex in front of the left ureter.
27-1 Where are the junctions between the foregut and midgut and between the midgut and hindgut?
27-2 Provide an example of a sphincteric mechanism of the alimentary canal.
27-3 What is a hiatal hernia?
27-4 What is the origin of the terms cardia, pylorus, and duodenum?
27-5 What is congenital pyloric stenosis?
27-6 What is the "duodenal cap"?
27-7 How does the jejunum differ from the ileum?
27-8 What is a Meckle's diverticulum?
27-9 Devise a term for a surgical anastomosis (Gk, stoma, "mouth") between the stomach and jejunum.
27-10 What are the most frequent positions of the vermiform appendix?
27-11 What are the main differences between the small and large intestines?
Figure 27-1 A, The parts of the stomach. B, The location of the glands.
Figure 27-2 The stomach and small intestine. A, The stomach after a barium meal, prone position. Note the duodenal cap, the feathery pattern of the barium in the small intestine, the twelfth rib on the right side of the body, and the pyloric part of the stomach at the level of the L4 vertebra. B, The small intestine 25 minutes after ingestion of a barium meal. Note the duodenal cap, the feathery pattern of the barium in the jejunum, and the ileum in the lower part of the photograph.
Figure 27-3 The stomach. A, After a barium meal, upright position. Some barium has transversed the bile and cystic ducts and is seen in the lower part of the gallbladder. Both the stomach and the gallbladder show a fluid level. The lower part of the stomach extends considerably below the supracristal plane here, and the fundus of the gallbladder is at the level of L4. In B, note the horizontal position of the "steerhorn" stomach. In C, the stomach is coated with barium and shows mucosal folds. (A, Courtesy of A. J. Chilko, M.D., New York, New York.)
Figure 27-4 Arterial supply of the stomach, duodenum, pancreas, and spleen. The stomach and first part of the duodenum have been removed in B.
Figure 27-5 Lymphatic drainage of the stomach, pancreas, and spleen. Each dot represents one or more nodes. The white arrows show the three major directions of spread.
Figure 27-6 Functional components of the nerve supply of the stomach. For simplification, each component is shown as a single fiber. Sympathetic fibers are shown as continuous lines, parasympathetic fibers as interrupted lines, and sensory fibers in blue.
Figure 27-7 Arterial supply of the jejunum, ileum, and colon.
Figure 27-8 Functional components of the nerve supply of the small intestine. Sympathetic fibers are shown as continuous lines, parasympathetic fibers as interrupted lines, and sensory fibers in blue.
Figure 27-9 The large intestine shown by a barium enema. Note the different levels of the transverse colon. A shows the appendix. B shows the ileum and the pattern of the colon. C, Oblique view of the colon and rectum. D, The colon and rectum outlined by a double contrast enema. (A, Courtesy of Maurice C. Howard, M.D., Omaha, Nebraska. C, Courtesy of Robert A. Powers, M.D., Palo Alto, California. D, Courtesy of Eugene E. Ahern, M.D., Minneapolis, Minnesota. C and D are from Medical Radiography and Photography.)
Figure 27-10 A common pattern of the inferior mesenteric artery as viewed in lateral perspective. The descending colon and sigmoid colon have been pulled forward.
Figure 27-11 Functional components of the nerve supply of the colon. Sympathetic fibers are shown as continuous lines, parasympathetic fibers as interrupted lines, and sensory fibers in blue.
Figure 27-12 A, Common relations of the cecum and appendix. A free appendix may hang over the pelvic brim or be retrocecal. B, Positions of the teniae, which meet at the appendix. The region marked by the asterisk may be regarded as ileocolic or ileocecal.
#The vermiform appendix has a rich blood supply and "far from being a vestigial organ, it has actually developed progressively in primates" (G. B. D. Scott, J. Anat., 131:549-563, 1980). Moreover, "the human appendix, at least in children, has the characteristics of a well-developed lymphoid organ, suggesting that it has important immunological functions" (P. Gorgollon, J. Anat., 126:87-101, 1978).