acs

Pa r t i : o v e r v i e w o f b lo o d s u p ply, ly m p h at i c s , and inner vation

Arteries

The small and large intestines develop from the midgut and hindgut. Therefore, they receive their blood supplies from branches of the superior and inferior mesenteries arteries.

• The superior mesenteric artery (SMA) arises from the abdominal aorta behind the neck of the pancreas at L-1, slightly below the celiac trunk. It immediately crosses in front of the left renal vein. As it continues downward it is sandwiched between pancreatic tissue—the neck of the pancreas in front and the uncinate process behind. Below the pancreas, it passes anterior to the third part of the duodenum, then enters the mesentery of the small intestine. The branches of the SMA can be organizedin three categories:

• (1) an artery to the pancreas and duodenum—the inferior pancreaticoduodenal artery. This short artery divides into anterior and posterior branches that supply the head of the pancreas and the duodenum distal to the major duodenal papilla. These branches anastomose with similar branches from the superior pancreaticoduodenal artery (a celiac trunk branch—Chapter 15). Should the celiac trunk become blocked, blood from the SMA could therefore flow retrograde to the rescue.
• (2) branches to the large intestine, from the right side of the SMA, listed in the order they branch:
• Middle colic artery: Enters the transverse mesocolon. As it reaches the transverse colon, it splits into left and right branches
• Right colic artery: Retroperitoneal, it reaches the ascending colon where it splits into ascending and descending branches

Figure 1.  AGUR & DALLEY, GRANT’S ATLAS OF ANATOMY, 15TH ED. FIGURE 4.44.

• Ileocolic artery: Retroperitoneal, it runs alongcthe root of the mesentery proper toward thecileocecal junction. Provides branches to theccecum, a branch to the distal ascending colon,and branches to the terminal ileum. It also gives off the appendicular artery.
• (3) a series of arteries to the jejunum and ileum, from the left side of the SMA, as it forms an arc between the distal duodenum and ileocecal junction. These are the jejunal and ileal arteries, about 20 in number, given off at regular intervals within the mesentery proper. These arteries run parallel to and anastomose with one another. They are described in more detail later in this chapter.
• The distal SMA, upon reaching the ileocecal junction, joins (anastomoses) with the ileocolicartery—a rare case where an artery terminates by joining one of its own branches!

 SUMMARY The superior mesenteric artery supplies midgut organs. It is directed toward the right side of the abdomen, spanning from the distal duodenum to the ileocecal junction. Its branches supply the distal duodenum, lower half of the pancreatic head, the entire jejunum, ileum,appendix, cecum, and ascending colon, and most of the transverse colon.

• The inferior mesenteric artery (IMA) arises from the aorta at the level of L-3, a few centimeters superior to the aortic bifurcation. It emerges below the horizontal (3rd) part of the duodenum—thus,this part of the duodenum is within a vascular sandwich, with the SMA above it and the IMA below.The IMA is short and runs vertically to the left of the aorta, behind the peritoneum. It has fewer branches than the SMA.
• The left colic artery passes retroperitoneally to the descending colon, dividing into ascending and descending branches.
• Several sigmoid arteries branch from the IMA, reaching the sigmoid colon within the sigmoid mesocolon.
• The superior rectal artery is the terminal branch of the IMA—it is really the direct continuation of the IMA, changing its name after it has passed over the pelvic brim into the pelvic cavity.

SUMMARY The inferior mesenteric artery supplies hindgut organs. It is directed toward the left side of the abdomen. Its branches supply the distal transverse colon, descending colon, sigmoid colon, and most of the rectum.

Figure 2. AGUR & DALLEY, GRANT’S ATLAS OF ANATOMY, 15TH ED. FIGURE 4.46.

• The marginal artery. The large intestine is supplied on the right by branches of the SMA and on the left by branches of the IMA. All these branches are joined in a longitudinal anastomotic channel called the marginal artery (artery of Drummond), spanning along the inner border of the large intestine, from cecum to rectum. This anastomosis provides collateral blood supply to all parts of the large intestine should their primary source of blood be obstructed or surgically compromised. The marginal artery is composed of these branches:
• Ascending branch of ileocolic artery
• Ascending and descending branches of right colic
• Right and left branches of middle colic
• Ascending and descending branches of left colic
• Sigmoid artery branches
• Superior rectal artery branches
• In the pelvic cavity below, the marginal artery connects to branches of the middle rectal artery (a branch of the internal iliac artery)

Figure 3. DRAKE, GRAY’S ATLAS OF ANATOMY, 3RD EDITION.

A nice exercise here is to draw yourself a sketch of the box-shaped colon and all the branches of the SMA and IMA that produce the marginal artery. Give it a try!

Venous drainage

As mentioned in Chapter 15, all veins that drain digestive organs in the abdomen (except the hepatic veins) are tributaries of the portal vein = they are part of the portal system of veins.

• The inferior mesenteric vein (IMV) drains the hindgut = descending colon, sigmoid colon, and a good portion of the rectum. It ascends retroperitoneally and vanishes behind the pancreas, where it normally drains into the splenic vein (although it often joins the superior mesenteric vein). Since the IMV is much longer than the inferior mesenteric artery (IMA), terminates higher up than the IMA,and does not directly accompany the IMA, it may be puzzling to identify in the cadaver upon first blush.
• The superior mesenteric vein (SMV) drains the midget = head of the pancreas, distal duodenum, jejunum, ileum, cecum, ascending colon, and most of the transverse colon.

Veins with the same names as arterial branches of the SMA are tributaries of the SMV. The SMV accompanies the SMA, lying on its right side. Behind the pancreas the SMV joins the splenic vein to form the portal vein.

Figure 4. DRAKE, GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIGURE 4.132.

Porto-systemic anastomoses (also called porto-caval anastomoses)

The hepatic portal vein carries all the products of digestion from the gut to the liver, as well as blood from the spleen, pancreas, and gallbladder. The portal vein is formed by the union of the splenic and superior mesenteric veins. The inferior mesenteric vein usually joins the splenic vein. There is no celiac vein. Instead, veins that drain foregut organs (e.g., superior pancreaticoduodenal vein, right and left gastric veins) are direct tributaries of the portal vein. The left gastric vein is especially large. As the portal vein nears the liver, it splits into left and right branches (a rare case where a vein has branches!). The cystic vein joins the right branch. Small veins that drain the abdominal wall and diaphragm (called para-umbilical veins) course through the falciform ligament to join the left branch. The portal vein and its tributaries have no valves. Blockage of the portal vein can therefore cause stasis and reverse flow of venous blood.

Figure 5. Sites of major porto-systemic anastomoses. ADAPTED FROM DRAKE, GRAY’S ANATOMY FOR STUDENTS, 4TH EDITION, FIG. 4.19.

In certain locations, venous tributaries of the portal vein anastomose (connect) with venous tributaries of the systemic circulation (superior and inferior venae cavae = SVC and IVC). If portal circulation through the liver were blocked because of liver disease or obstruction by a tumor, blood from the GI tract could still reach the heart by flowing in a reverse direction through these anastomoses to the IVC and SVC. These porto-systemic venous anastomoses are in the submucosa of certain organs and in the anterior abdominal wall. They have clinical importance because pressure (portal hypertension) in these small vessels can cause their engorgement (varices) and a risk of rupture and bleeding. The major porto-systemic anastomoses are listed in Table 1.

Table 1.

Lymphatics

The principles of lymphatic drainage of digestive organs in the abdomen were laid out in Chapter 15.

Figure 6. Scheme of lymph drainage of small intestine (right side of diagram). MOORE, ET AL., CLINICALLY ORIENTED ANATOMY, 8TH ED., FIGURE 5.50.

Lymphatic vessels draining the intestines follow blood vessels and they usually percolate through several sets of nodes = peripheral nodes near the intestines themselves and then through larger central nodes situated near the aorta.

• Lymph from midgut organs (organs supplied by the SMA) ultimately flows to superior mesenteric nodes —the central nodes in front of the aorta.

• Before reaching central nodes, lymph from the small intestine flows through a vast number of peripheral mesenteric nodes in the mesentery proper. Histology note: the villi in the small intestine contain a large central lymph vessel called a lacteal. These vessels perform a special function = absorption of emulsified fat. They are called lacteals because the fat gives them a “milky” white appearance.
• Before reaching central nodes (superior mesenteric nodes), lymph from the appendix, cecum, ascending colon, and transverse colon flows through several sets of peripheral nodes = adjacent to the organ walls (paracolic nodes) and along their arteries (ileocolic, right colic and middle colic nodes).
• Efferent lymph vessels from the central superior mesenteric nodes join vessels from the celiac nodes to form the intestinal lymph trunk. This trunk then flows into the cisterna chyli, located behind the aorta, just below the diaphragm (see Chapter 15). Recall that the cisterna chyli is the commencement of the thoracic duct.

• Lymph from hindgut organs (organs supplied by the IMA) ultimately reaches the inferior mesenteric nodes (the central nodes) near the aorta, passing first through peripheral nodes near the gut and along its arteries (left colic nodes, for example). Lymph vessels leaving the inferior mesenteric nodes enter the left lumbar lymph trunk. The left and right lumbar trunks join the intestinal trunk to form the cisterna chyli (Chapter 15).

Figure 7. Lymph nodes and drainage scheme of large intestine. STANDRING S. (2019), THE ANATOMY OF THE LARGE INTESTINE. IN: PARKER M., HOHENBERGER W. (EDS) LOWER GASTROINTESTINAL TRACT SURGERY: VOL. 1, LAPAROSCOPIC PROCEDURES. SPRINGER SURGERY ATLAS SERIES.

Innervation

The innervation scheme of the entire gut was outlined in Chapter 15. The intestines receive parasympathetic, sympathetic, and visceral afferent nerve fibers.

General rules:

• Parasympathetic innervation stimulates peristalsis and glandular secretion, relaxes sphincters, and empties the rectum (Parasympathetic = pouring out secretions, peristalsis, passing feces). It activates,opens, and empties the gut.
• Sympathetic innervation on the other hand inhibits peristalsis, closes sphincters, and constricts gut blood vessels. It closes the gut and delays its emptying.
• Most afferent pain fibers from the intestines follow sympathetic pathways in reverse to reach the spinal cord = that is, they follow splanchnic nerves. This is because most of the gut is above the pelvic pain line.

Specifics:

• The proximal part of the intestines (midgut = distal duodenum to distal transverse colon) are supplied with parasympathetic fibers from the vagus nerves.Sympathetic fibers are derived from greater, lesser, and least splanchnic nerves (T-8 to T-12). Pain can be referred to the umbilical region of the abdominal wall.
• The distal part of the intestines (hindgut = distal transverse colon to rectum) are supplied with parasympathetic fibers from the pelvic splanchnic nerves (S-2 to S-4). Sympathetic fibers are from lower thoracic and lumbar splanchnic nerves.

Pain from parts of the hindgut above the pelvic pain line may be referred to the hypogastric region (between umbilicus and pubis). Pain from the rectum (below pelvic pain line) refers to the perineum (S-2 to S-4 dermatomes).

Pa r t i i : i n f r a co l i c o r g a n s – t h e i n t e s t i n e s

Small intestine

The small intestine is composed of three parts: duodenum, jejunum, and ileum. In the living body, the small intestine is 5 to 6 meters in length (~20 feet). Holy cow!

For convenience, the entire duodenum was described in Chapter 15. Since the second (descending) part of the duodenum is crossed horizontally by the transverse mesocolon, the proximal half of the duodenum is actually in the supracolic region. The lower part of the duodenum(distal portion of second part, as well as parts three and four) is in the infracolic region. The duodenum is short, thick, and mostly retroperitoneal and immobile (except for part one).

Figure 8. DRAKE, GRAY’S ATLAS OF ANATOMY, 3RD EDITION.

The remaining parts of the small intestine (jejunum and ileum) are long, coiled, mobile, and specialized for absorption. The loops of small bowel occupy the “box” created by the large intestine and a good portion of them fill the pelvic part of the peritoneal cavity. The jejunum and ileum are intraperitoneal—they are supported by the mesentery of the small intestine (mesentery proper). Recall from Chapter 14 that the mesentery proper is fluted and shaped like a folded hand fan = wide along its intestinal attachment and narrow at its base. Indeed, the 20-foot-long small intestine is suspended by a mesentery whose base is only about 6 inches long and attached obliquely along the posterior abdominal wall, from just to the left of L-2 above, to the right iliac fossa below. Excluding the duodenum, the jejunum is said to comprise 2/5 of the small intestine and the ileum 3/5. Although there are clear histological differences between the proximal jejunum and distal ileum, their transition is gradual and the junction between the two is imperceptible grossly. The small intestine contains both liquid and gas. Peristalsis moves these substances distally and this activity can produce audible gurgling noises called bowel sounds (the medical term is borborygmi), which can be heard with a stethoscope or with the naked ear.

The jejunum begins proximally at the duodenojejunal flexure on the left side of L-2. This is the sudden kink in the small bowel where the retroperitoneal last part of the duodenum (part four) bends to become the intraperitoneal jejunum. The jejunum is wider, and its mucosa is thicker than that of the ileum. Like the duodenum, the thick mucosa of the jejunum contains circular folds (plicae circulares) and has a velvety appearance due to the intestinal villi. The jejunum mainly occupies the umbilical region of the abdomen. Ancient anatomists discovered that the jejunum was empty in cadavers, thus its name (jejunum = Latin: “empty”).

The ileum is the terminal portion of the small bowel. It ends distally where it connects to the cecum (ileocecal junction) in the right lower quadrant. The name of this organ reflects its highly coiled structure (ileum = Latin: “rolled up”). Loops of ileum are mainly found in the hypogastric region of the abdomen and in the pelvic cavity. The ileum has a thinner wall and narrower lumen than the jejunum. The distal ileum is the narrowest part of the small intestine, so it is a potential spot for obstruction. The presence of circular folds and intestinal villi in the mucosa diminishes distally in the ileum. On the other hand, lymphoid tissue, which is scattered in the submucosa of the jejunum, becomes aggregated into clumps within the ileum, called Peyer’s patches. These are large enough to produce prominent elevations visible through the inner epithelium.

CLINICAL APPLIC ATION

The most common anomaly of the small intestine is the presence of an ileal (Meckel’s) diverticulum. Its development is discussed in Chapter 17. Essentially it is persistence of a structure (vitelline duct) that should have disappeared.When present, it is located about 2 feet proximal to the ileocecal junction, projecting from the antimesenteric border of the ileum (the surface of the bowel not attached to the mesentery proper) toward the umbilicus. Its story can be summed up with the number “2”= about 2 inches long, 2 feet from the cecum, and 2% of folks have one. It is usually benign and asymptomatic and discovered by accident during surgery or imaging. However, problems can arise if it is fibrotic (may lead to volvulus of the bowel) or if it contains ectopic gastric tissue, in which case it can cause ulcers and bleeding.

• Blood supply of small intestine: Superior and inferior pancreaticoduodenal arteries (from celiactrunk and SMA); jejunal and ileal arteries (from SMA).
• The approximately 20 jejunal and ileal arteries are parallel in arrangement and each bifurcates into two arched vessels that join their immediate neighbors, forming what are called arterial arcades. Secondary loops sprout from arcades and continue to replicate, forming a graceful latticework of anastomotic arched arteries. Anatomy IS a beautiful thing! From the last row of arcades (row closest to the bowel), straight arteries (vasa recta) pass to the jejunum and ileum and enter their walls. The pattern of arterial arcades differs by regions:
• Jejunum: arterial arcades are few and simple. Vasa recta are long.
• Ileum: arterial arcades are many and complex. Vasa recta are short.
• Venous drainage: Superior and inferior pancreaticoduodenal veins (to portal vein directly and to SMV, respectively); jejunal and ileal veins to SMV.
• Lymphatic drainage: Duodenum = pancreaticoduodenal nodes  celiac nodes and superior mesenteric nodes  intestinal lymph trunk; Jejunum and ileum = mesenteric nodes (scattered within the intestinal arcades)  superior mesenteric nodes (near aorta)  intestinal lymph trunk

• Innervation: Vagus nerves (parasympathetic—stimulate peristalsis and glandular secretion); Greater,lesser, and least splanchnic nerves (sympathetic—constrict blood vessels, inhibit peristalsis and glandular secretion). Visceral afferent fibers carrying pain follow the greater, lesser, and least splanchnic nerves, with cell bodies in dorsal root ganglia T-8 to T-12.

Figure 10. DRAKE, GRAY’S ANATOMY FOR STUDENTS, 4TH ED., FIGURE 4.69.

Table 2. Summary: Anatomic comparison of jejunum and ileum.

Large intestine

This portion of the GI tract begins at the ileocecal junction and terminates at the “end of the road” = the anus. From proximal to distal it includes the cecum and appendix, ascending, transverse, and descending colons, rectum, and anal canal. The distal parts (rectum and anal canal) will be discussed with the pelvis and perineum in later chapters. The large intestine is about 4-5 feet long in the living. Its major function is to absorb water and electrolytes,transforming the liquid contents delivered from the small intestine into semi-solid feces. Although the word “colon” is often used interchangeably with large intestine, the colon technically is the part of the large intestine between cecum and rectum.

The ascending and descending colons and rectum are retroperitoneal and fixed. The transverse and sigmoid colons are mobile due to their attachments to mesenteries (transverse and sigmoid mesocolons, respectively). The cecum and appendix are in between—they are somewhat mobile since they are surrounded by peritoneum and hang freely from the bottom of the ascending colon (the appendix does have a small mesentery = the meso-appendix). The large intestine forms a picture frame-like box = ascending and descending colons along the sides, transverse colon above, and sigmoid colon below. Loops of small intestine fill the picture frame. Now, that’s a work of art!

Distinguishing features of the large intestine:

• It is considerably wider than the small intestine, although the lumen diameter of an empty descending or sigmoid colon can be narrower than that of the small bowel in some individuals.
• The large intestine lacks circular folds and villi—but does contain an abundance of goblets cells, to lubricate feces.
• The longitudinal layer of smooth muscle in the cecum and colon is incomplete and does not surround the large intestine as it does in the small intestine. Instead, this layer is gathered up into three concentrated longitudinal ribbons of smooth muscle called the teniae coli. These are spaced equidistantly 120 degrees apart. All three teniae commence at the appendix proximally. Distally they merge with the complete longitudinal smooth muscle layer of the rectum.
• Since the teniae are shorter than the colon itself, their contraction causes the bowel walls to be“bunched” up into a series of rectangular sacs called haustra—kind of like a drawstring around the waistband of your comfy pair of sweatpants. Haustra give the large intestine the appearance of a chain of small sacs, especially when seen in radiographic images, like CT colonography.
• Fat globs (50–100 in number) project externally from the colon, often along the course of one of its three tenia. These are called omental appendices and are part of the organ since they are subserosal = surrounded by the colon’s visceral peritoneum. Their function is unknown—they may act as protective cushions, store blood, serve as energy sources, or provide defense against infection similar to the greater omentum. Clinically they can be seen onimaging and can undergo twisting (torsion), producing pain.

Figure 11. The large intestine. DRAKE, GRAY’S ANATOMY FOR STUDENTS, 4TH EDITION, FIGURE 4.79.

Figure 12. Supine scout image of CT colonography—colon inflated with air—haustra are shown as contiguous rectangular pouches. HTTPS://WWW.RESEARCHGATE.NET/.

Figure 13. DRAKE, GRAY’S ANATOMY FOR STUDENTS, 4TH EDITION, FIGURE 4.70.

Cecum

The first part of the large intestine is the cecum, a pouch that hangs below the entrance of the ileum (ileocecal junction). It communicates above with the ascending colon. The cecum has no mesentery but is surrounded by peritoneum and hangs freely into the peritoneal cavity, so it is considered intraperitoneal.

Located in the right lower quadrant of the abdomen, the cecum is just superior to the lateral part of the inguinal ligament. Its anterior surface touches the parietal peritoneum of the anterior abdominal wall and when full of feces, it is palpable here. The right iliacus muscle is behind the cecum, separated from it by the retrocecal recess, a peritoneal space that is part of the greater sac. Inside the cecum on its medial wall is the ileocecal orifice. This defines the junction of the cecum (below) and the ascending colon (above). Flaps of mucosa surround the ileocecal orifice and contain smooth muscle, forming the ileocecal valve. It is thought that the valve prevents reflux of feces into the ileum when the cecum contracts.

Appendix

The appendix (aka – vermiform appendix = “worm-shaped” appendage), is a hollow, blind diverticulum of the cecum. In newborns, the appendix projects inferiorly from the bottom of the cecum. Differentialgrowth of the cecum displaces the appendix to its posteromedial left wall, where its orifice opens internally just below the ileocecal orifice. The wall of the appendix is thick since its submucosa contains an abundance of lymphoid tissue, making it the “abdominal tonsil”. The appendix probably has an immune function and a role in maturation of lymphocytes. It may also serve as a reservoir for beneficial gut bacteria.

Because of its thick wall, the lumen is narrow and may become occluded with feces (especially in older people), predisposing to infection. The appendix has a variable length, anywhere from one to nine inches! It has a small mesentery (meso-appendix), an extension of the mesentery proper, that attaches to the distal ileum.

The appendix has a base (attached to the cecum) and tip. The position of the appendix is variable. Most often it is tucked up into the retrocecal recess (retrocecal appendix). However, it may curve around thebottom of the cecum (subcecal), loop to the left toward the terminal ileum, or hang down into the pelvic cavity. Although the tip is variable in location, its base is constant. This can be mapped to the anterior body wall at the junction of the middle and lateral thirds of a line drawn from the right anterior superioriliac spine to the umbilicus (McBurney’s point). In the case of open appendectomy (not nearly as common these days with the use of laparoscopy), this is a useful surgical landmark. The blood supply of the appendix is via the appendicular artery, a branch of the ileocolic artery. It reaches the appendix through the meso-appendix. The appendicular artery is often an “end artery” (no anastomoses with other vessels). Occlusion of the artery due to inflammation of the appendix could therefore lead to necrosis and possible perforation of the organ. In the bad old days prior to surgery, appendicitis was usually fatal.

Figure 14. Variations in appendix position. Retrocecal is most common. DRAKE, GRAY’S ANATOMY FOR STUDENTS, 4TH EDITION, FIGURE 4.84.

Figure 15. Location of McBurney’s point. DRAKE, GRAY’S ATLAS OF ANATOMY, 3RD EDITION.

CLINIC AL APPLIC ATION

Acute appendicitis is a common abdominal pathology. Inflammation of the appendix distends its walls, stretching the visceral peritoneum. This visceral pain is poorly localized and usually referred to the central abdominal region around the umbilicus since the pain pathway from the appendix enters the spinal cord at T-10. If inflammation spreads to the parietal peritoneum on the body wall, the pain often moves to the right lower quadrant. The location and nature of the pain of appendicitis is influenced by the position of the appendix.

• An inflamed subcecal appendix (curled below the cecum) would likely produce a physical sign of rebound tenderness = eliciting pain by slowly pressing on the abdominal wall over the cecum, then releasing. This maneuver stretches the irritated parietal peritoneum on the body wall.
• A pelvic appendix, if it lies close to the ovary, Fallopian tube, or uterus, could mimic pathologic conditions in these organs.
• An inflamed retrocecal appendix could irritate the nearby ureter (causing flank pain) orthe iliacus or psoas muscles. Pain in the right lower quadrant produced by stretching these muscles is known as a psoas sign. This is elicited by having the patient lie on their left side while the examiner extends their right hip. [Note: pain could of course also be due to pathology of the muscles themselves, such as muscle strain—so, the psoas test does not necessarily mean appendicitis—however, this test is good application of anatomic knowledge in the case of a retrocecal appendix!]

Colon: The colon has four parts

The ascending colon begins at the ileocecal junction and moves upward over the muscles of the posterior abdominal wall and the inferior pole of the right kidney. Just below the liver, it bends sharply to the left, creating the right colic flexure (also called the hepatic flexure because the liver overlaps it). Distal to this the ascending colon becomes the transverse colon. The ascending colon is secondarily retroperitoneal—“glued” to the posterior abdominal wall by fusion fascia (described in Chapter 14). The peritoneum on the posterior abdominal wall passes over the anterior and lateral surfaces of the ascending colon and then makes a sharp posterior indentation along its right lateral border before folding back along the inside of the anterior abdominal wall. This peritoneal crease along the right border of the ascending colon is the right paracolic gutter. Blood and lymphatic vessels supplying the ascending colon reach it beneath the peritoneum on its left side. Therefore, surgeons cut through the peritoneum along the avascular right paracolic gutter should the ascending colon and its vessels need to be mobilized.

The transverse colon begins at the right colic flexure and spans across the peritoneal cavity in front of the small intestine, ending at the left colic flexure (splenic flexure). The left colic flexure is generally higher, more acute, and more posterior than the right colic flexure, since there is no liver in the way, and it is tethered to the diaphragm (via a phrenicocolic ligament). The transverse colon is the longest and most mobile part of the colon. It is intraperitoneal,being attached to the posterior abdominal wall by the transverse mesocolon. Blood vessels and nerves reach the transverse colon through this mesentery. The two colic flexures are fixed points, so the transverse colon swings downward from them like a jump rope held by two kids on a playground. The position of the transverse colon varies from person-to-person and depends on its contents and body position. An empty transverse colon can be horizontal, whereas when full and the person standing, can curve downward as far as the level of the pelvic brim. Wow!

The descending colon begins at the left colic flexure in front of the left kidney. It passes down the posterior abdominal wall to the left iliac fossa, where it becomes the sigmoid colon. Like the ascending colon, it is secondarily retroperitoneal and covered by peritoneum on its anterior and lateral surfaces. Behind the descending colon are the left kidney and the quadratus lumborum and transversus abdominis muscles. On its lateral side is the left paracolic gutter, a sharp vertical crease in the peritoneum. Blood vessels and nerves reach the descending colon along its right side.

The sigmoid colon (its layout apparently resembles the Greek letter sigma ∑) begins where the ascending colon ends over the left iliacus muscle. Here the colon changes from retroperitoneal to intraperitoneal, as the sigmoid colon hangs from a mesentery, the sigmoid mesocolon. The attachment of the sigmoid mesocolon to the posterior body wall forms an inverted V shape as it crosses over the left psoas major muscle, ureter, common iliac vessels, and then over the pelvic brim into the pelvic cavity, where it attaches to the sacrum as far down as the 3rd sacral vertebra. At S-3 the sigmoid colon loses its mesentery and becomes the rectum (rectosigmoid junction). Omental appendices are especially numerous and large in the sigmoid colon. The teniae coli end at the rectosigmoid junction.

CLINIC AL APPLIC ATION

DIVERTICULAR DISEASE Diverticulosis is the presence of small pouches (diverticula) where the mucosa protrudes through weak areas in the wall of the large intestine. Weak areas occur naturally where blood vessels enter the muscular layer of the colon. Recall that the colon has only one continuous layer of smooth muscle (the circular layer)—the longitudinal layer is not robust, instead it is arranged as the teniae coli. If these pouches become inflamed, diverticulitis results.

Diverticulosis can occur anywhere in the large bowel, but most commonly occurs on the left side (descending or sigmoid colons), probably because this side of the bowel experiences the highest pressure and can contain large accumulations of hardened stool in chronic constipation. Left lower quadrant pain is the most common presenting complaint in patients with diverticular disease.

• Blood supply of large intestine: Superior and inferior mesenteric arteries. Branches of the SMA and IMA to the large intestine are connected by anastomosing branches, forming the marginal artery.
• Ileocolic artery (SMA) supplies cecum and ascending colon (and distal ileum)
• Appendicular artery—from ileocolic artery (SMA) supplies appendix
• Right colic artery (SMA) supplies ascending colon
• Middle colic artery (SMA) supplies transverse colon
• Left colic artery (IMA) supplies descending colon
• Sigmoid arteries (IMA) supply sigmoid colon
• Venous drainage: Superior and inferior mesenteric veins  portal vein
• Lymphatic drainage: Peripheral nodes are near the wall of the large intestine (paracolic nodes) and along its arteries  to superior mesenteric (midgut) and inferior mesenteric (hindgut) nodes (these are the central nodes)
• Innervation:
• Parasympathetic = Vagus nerves supply the cecum, ascending colon and most of the transverse colon(proximal 2/3); Pelvic splanchnic nerves (from spinal cord segments S-2, S-3, and S-4) ascend from the pelvis and supply the distal transverse colon, descending colon, sigmoid colon, and rectum.
• Sympathetic = Lower thoracic and lumbar splanchnic nerves; T-8 to L-2.
• The cecum through sigmoid colon are all above the pelvic pain line, so visceral afferentfibers carrying pain follow sympathetic routes (thoracic and lumbar splanchnic nerves) to the CNS, with cell bodies in dorsal root ganglia T-8 to T-12. Pain may refer to the umbilical (midgut) or hypogastric(hindgut) regions of the abdominal wall.