GROSS ANATOMY OF THE FOREGUT, MIDGUT, AND HINDGUT REGIONS (Grant's Dissector (16th Ed.) pp. 110-131) TODAY’S GOALS: 1. Identify the coverings and contents of the scrotum, spermatic cord, and testis, and the anatomy related to a vasectomy procedure 2. Explore the abdominal contents in situ 3. Understand the important peritoneal relationships

I. Peritoneal Reflections

DISSECTION NOTES: Reflect the anterior (Dissector p. 111, Fig. 4.16) to enable access to the and its contents. Make a vertical incision through the linea alba just to the left of the midline from the xiphoid process to the pubis. Separate the anterior wall from the abdominal contents with your hand. Carefully cut the abdominal wall along the right costal margin and continue the incision inferiorly in the midaxillary line to the iliac crest. Note the falciform as you begin reflecting this flap from the right upper quadrant. It connects the abdominal wall from the umbilicus to the surface of the . Cut it close to the abdominal wall and reflect the right abdominal wall inferiorly. This preserves the relationships of the iliac vessels and structures passing to the umbilicus from the pelvis. On the left, cut the abdominal wall just superior to the inguinal ligament and reflect the wall laterally.

On the inner surface of the right anterior abdominal wall flap note the following peritoneal folds (Dissector p. 112, Fig. 4.17): • Median umbilical fold/ligament: contains the urachus, a remnant of the allantoic duct; extends from the apex of the bladder to the umbilicus • Medial umbilical fold/ligament: contains the obliterated umbilical artery (paired) • /ligament: consists of covering the inferior epigastric vessels (paired) Examine the area just lateral to the lateral umbilical fold and identify a slight depression in the peritoneal lining. This marks the location of the deep inguinal ring.

II. Inspection of Abdominal Cavity and Viscera (Inspect. Do not dissect at this time.) The abdominal and pelvic cavities, like other body cavities, are lined by serous membranes that secrete a clear, serous fluid to permit movement of organs. This membrane is a continuous sheet that is given different terms depending on structures it covers. Peritoneum that covers the inner surface of the abdominal and pelvic walls is called parietal peritoneum; that which covers surfaces of organs is called visceral peritoneum. The potential space between these layers containing a small amount of serous fluid is the .

Identify the peritoneal folds: • (Dissector p. 114, Figs. 4.18, 4.19): this peritoneal fold forms a broad fat-filled apron that hangs from the borders of the greater curvature of the and transverse colon, covering most of the intestines

• Lesser omentum (Dissector p. 118, Fig. 4.24): a peritoneal extension between the inferior surface of the liver and the first part of the and lesser curvature of the stomach. Based on these connections, it is divided into two : the and the .

13 Understand the ‘sac within a sac’ concept. (Dissector p. 115, 116, Fig. 4.20, 4.22)

• Greater peritoneal sac: the main part of the peritoneal cavity, i.e., the space first entered when the abdominal wall flaps were reflected • Lesser peritoneal sac: also called the omental bursa; a small sac within the peritoneal cavity behind the lesser omentum • Epiploic foramen (foramen of Winslow): the “entrance” to the omental bursa just posterior to the free edge of the hepatoduodenal ligament

Abdominal Viscera In Situ (Dissector p. 114, Figs. 4.18, 4.19) Understand the difference between retroperitoneal and peritoneal structures.

Starting at the abdominal end of the esophagus, follow the gastrointestinal tract distally and identify its components (Dissector 131, Fig. 4.44).

Stomach

• lesser curvature with attached lesser omentum • greater curvature with attached greater omentum • also find the gastrocolic and gastrosplenic (gastrolienal) ligaments • pylorus – junction between the stomach with the duodenum

Small Intestine

• duodenum – its first part is mobile (intraperitoneal); its 2nd, 3rd, and most of the 4th portions are secondarily retroperitoneal; the distal end of the 4th part is mobile near the duodenojejunal flexure (location of the ligament of Treitz, a surgical landmark) • jejunum – intraperitoneal. Is suspended by the of the small intestine, which extends from the posterior abdominal wall to the ileocecal junction • ileum – intraperitoneal. Is continuous with the jejunum and also suspended by the mesentery of the small intestine; it connects to the ascending colon at the cecum

Large Intestine

• cecum – a blind-ended pouch and the first part of ascending colon. It is located at the junction with the ileum and from it extends a narrow fingerlike projection, the vermiform appendix; the appendix is attached to the posterior abdominal wall by the mesoappendix • ascending colon – secondarily retroperitoneal. It is continuous with the transverse colon at the right colic or hepatic flexure • transverse colon – intraperitoneal (via transverse mesocolon) and very mobile; extends from the right colic flexure to the left colic or splenic flexure; it is also attached to the greater omentum • descending colon – secondarily retroperitoneal. It is continuous distally with the sigmoid colon • sigmoid colon – intraperitoneal ( via sigmoid mesocolon which connects the sigmoid colon to the posterior abdominal wall). The sigmoid colon is a mobile, “S”-shaped portion of the large intestine that descends into the as the rectum

14 Liver (Fig. 4.21)

• – a double layer of parietal peritoneum that spans the space between the inner surface of the anterior abdominal wall and the anterior surface of the liver. It contains the ligamentum teres (round ligament of the liver), a remnant of the left umbilical vein in its inferior free margin. It divides the liver into right and left anatomic lobes.

Gallbladder

• Emerges from under the cover of the inferior border of the liver at the junction of the right costal margin and rectus abdominis muscle

Peritoneal Gutters

1. Passages formed by folds of peritoneum around abdominal viscera 2. Paracolic gutters – lateral gutters formed between the ascending and descending parts of the colon and the lateral walls of the 3. Mesenteric gutters – formed by the oblique course of the root of the small intestine mesentery from the duodenojejunal flexure to the distal ileum

Q. What is located or normally present in the peritoneal cavity, or for that matter, in the pleural or pericardial cavities?

PEER TEACHING GUIDE: Organize your narrative to minimally include: 1. Demonstration and/or discussion of the fascial coverings and contents of the spermatic cord. 2. Demonstration and/or discussion of the coverings of the testis, including the two layers of the tunica vaginalis. 3. Demonstration of the internal anatomy of the testis, including the tunica albuginea, septae/lobules, seminiferous tubules, and the different parts of the duct system. 4. Demonstration of the peritoneal folds on the inner surface of the anterior abdominal wall and discussion of the contents contained within each. 5. Provide a “tour” of the abdominal viscera in situ and a demonstration of the following peritoneal structures: greater and lesser omentum, falciform ligament, the mesentery, sigmoid mesocolon, and their relevant relationships. Demonstration of the greater and lesser peritoneal sacs and the passageway (i.e., epiploic foramen) connecting them.

15 CELIAC TRUNK AND FOREGUT DERIVATIVES (Grant's Dissector [16th Ed.] pp. 117-122)

Today’s Goals: 1. Inspect the stomach and its parts 2. Dissect the structures (portal triad) in the hepatoduodenal ligament 3. Dissect the branches emanating from the celiac trunk (one of the unpaired arteries of the abdominal aorta) 4. Inspect the liver, its surfaces and lobes 5. Mobilize the spleen and observe its surfaces 6. Inspect the gallbladder

DISSECTION NOTES: Before proceeding with dissection, refer to the Dissector p. 117, Fig, 4.23 and review the parts of the stomach: Greater and Lesser Curvatures Cardia Fundus Body Pylorus and the liver (Dissector p. 121, Fig, 4.29): Right lobe Separated by the falciform ligament Left lobe Diaphragmatic surface Visceral surface and (gateway for nerves, vessels, and lymphatics to/from the liver) Inferior border

A. Portal Triad Structures and Celiac Trunk (Dissector p. 118, Figs. 4.24 and 4.25) Access to the celiac trunk and its branches is best achieved by initially dissecting through the anterior peritoneal layer of the hepatoduodenal ligament. Locate the hepatoduodenal ligament (right free border of the lesser omentum). Q. What are the contents of the hepatoduodenal ligament and their spatial relationships to one another?

Open the hepatoduodenal ligament and identify the portal triad structures: bile duct, , hepatic portal vein. The bile duct is formed by the union of the cystic duct and common hepatic duct. The common hepatic duct, in turn, is formed by the union of the right and left hepatic ducts. Identify each of these structures by dissecting the tissue around them. The tough “stringy” material you encounter in this process are autonomic nerve fibers to the liver. Lymphatic vessels and hepatic lymph nodes are also contained within the hepatoduodenal ligament.

Clean the hepatic artery proper in the direction of the porta hepatis and observe its branching into the right and left hepatic arteries. Look for the to the gallbladder, which commonly arises from the right hepatic artery (expect to find variations (Dissector p. 119, Fig. 4.27).

Near the origin of the hepatic artery proper from the common hepatic artery, look for a small branch, the right gastric artery, coursing toward the lesser curvature of the stomach (Dissector p. 119, Figs. 4.26). The common hepatic artery also gives rise to the gastroduodenal artery, which descends posterior to the first part of the duodenum. Clean this vessel until you observe it

16 dividing into its right gastro-omental (gastroepiploic) artery and the superior pancreaticoduodenal artery.

As seen in other parts of the body, blood vessels vary from the patterns found in an atlas, so anticipate variations in your donor. Again, recall that vessels are identified by their distribution.

B. Celiac Trunk and Its Branches Continue to clean along the common hepatic artery toward its origin from the celiac trunk and identify the remaining branches of the celiac trunk (Dissector p. 119, Fig. 4.26**-THIS IS AN EXCELLENT DIAGRAM). The celiac trunk is the first unpaired branch off the abdominal aorta, located at the level of the T12 vertebra. It is the artery of supply to embryonic foregut derivatives. Before you get too far along, look for the celiac ganglion, a large prevertebral ganglion surrounding the origin of the celiac trunk from the abdominal aorta. The celiac trunk is short and gives rise to the left gastric, splenic, and common hepatic arteries.

1. Common hepatic artery – courses to the right and divides into the hepatic artery proper and gastroduodenal artery o proper hepatic artery – gives rise to: right gastric artery (to lesser curvature – usually small) right and left hepatic artery cystic artery – usually arises from the right hepatic artery o gastroduodenal artery – descends posterior to the first part of duodenum; divides into: superior pancreaticoduodenal arteries (with anterior and posterior branches) – participate in an arterial anastomosis around the proximal portion of the duodenum and head of pancreas right gastro-omental (= gastroepiploic) artery – enters the greater omentum about a fingers-breadth from the greater curvature of the stomach giving rise to gastric and omental branches 2. Splenic artery – very tortuous; courses along the superior border of the pancreas toward the hilus of the spleen. Gives rise to: o short gastric arteries – to the fundus of the stomach o left gastro-omental (gastroepiploic) artery – courses within the greater omentum along the greater curvature of the stomach; anastomoses with the right gastro- omental artery o dorsal and greater pancreatic arteries, and other branches to pancreas 3. Left gastric artery – ascends towards the esophageal end of the stomach to enter and course along the lesser curvature of the stomach where it anastomoses with the right gastric artery o esophageal branches

C. Hepatic Portal Vein The hepatic portal vein is a large diameter, thin walled vessel located posterior to the bile duct and proper hepatic artery in the hepatoduodenal ligament. It is formed by the union of the superior mesenteric and splenic veins. The portal system of veins carries venous blood from the abdominal portion of the GI tract and accessory organs to the liver. As you proceed in your dissections, confirm the fact that the veins of this system accompany the arteries of the celiac trunk, and the superior and inferior mesenteric arteries. Also, veins of this portal system lack valves, thus enabling reversal of blood flow away from the liver in cases of hepatic portal hypertension to enlarge communications with caval-related veins (i.e., portocaval anastomoses).

17 Spleen The spleen is a hemopoietic organ and located in the upper left quadrant of the abdomen. Reach behind the fundus of the stomach and pull the spleen out from under the diaphragm. It is covered by visceral peritoneum except at its hilus where the vessels enter and exit. Note the clinical correlation in the Dissector on p. 120 concerning its location and potential for enlargement (= splenomegaly).

Liver (Dissector p. 121, Fig. 4.29) To adequately view the features and lobes of the liver, it should be mobilized from its attachment to the diaphragm. • Cut through the falciform ligament and continue cutting through the anterior layer of the which reflects onto the inferior surface of the diaphragm. • Insert your fingers between the liver and diaphragm and proceed to dissect its posterior surface from the diaphragm • Cut the inferior vena cava between the liver and the diaphragm and again where it is related to the inferior surface of the liver. The liver should now be mobilized, but attached to the portal triad structures. Proceed to examine the following structures: • Right Lobe (considerably larger than the left lobe and separated from the left lobe by the falciform ligament) • Bare area of the liver – an area of the diaphragmatic surface of the liver devoid of visceral peritoneum and enclosed by the layers of the coronary ligament. • Visceral surface – the surface of the liver that contains the porta hepatis. Note an unequal “H- shaped” set of fissures and fossae. A pair of fissures contain the falciform ligament anteriorly and the posteriorly, while a pair of fossae are occupied by the gallbladder anteriorly and the inferior vena cava posteriorly. The “crossbar” connecting these fissures and fossae is the porta hepatis for vessels, ducts, autonomic nerves, and lymphatics entering and leaving the liver. Two small lobes, the quadrate anteriorly and the caudate posteriorly, are interposed between these structures.

Gallbladder (Dissector p. 122, Fig. 4.30) Bile produced by the liver is stored and concentrated in the gallbladder, which occupies a fossa on the visceral surface of the liver. Occasionally, gallstones may be palpated if present. It consists of the following parts: • Fundus – projects from the inferior border of the liver • Body • Neck – gives rise to the cystic duct

PEER TEACHING GUIDE: Organize your narrative to minimally include: 1. Demonstration of the parts of the stomach and a review of the relationships of the lesser and greater omentum to the lesser and greater curvatures of the stomach. 2. Demonstration of the hepatogastric and hepatoduodenal ligaments and portal triad structures within the hepatoduodenal ligament. 3. Demonstration of the celiac trunk, its branches, and the course and relationships of these branches to the organs they supply. 4. Demonstration of the portal vein and the veins that form it. 5. Demonstration of the lobes, surfaces, fissures and fossae and contents therein, of the liver. 6. Demonstration of the gallbladder, its parts, and the duct system of the biliary tree.

18 SUPERIOR AND INFERIOR MESENTERIC ARTERIES AND MIDGUT AND HINDGUT DERIVATIVES (Grant's Dissector [16th Ed.] pp. 123-131)

TODAY’S GOALS: 1. Identify the branches of the superior and inferior mesenteric arteries and their supply to the small and large intestines 2. Identify the tributaries to the superior and inferior mesenteric veins and their communication with the hepatic portal venous system 3. Identify distinctive external and internal features of the stomach, small and large intestines, and accessory organs of the digestive system

DISSECTION NOTES: This lab session is largely a dissection of the branches of the superior and inferior mesenteric arteries, observation of the veins tributary to the superior and inferior mesenteric veins, the hepatic portal vein, and relevant external and internal features of portions of the GI tract and accessory organs of digestion.

Superior mesenteric artery (SMA) (Dissector p. 123, Fig. 4.31) – the second unpaired branch off the abdominal aorta and the artery of supply to embryonic midgut derivatives (supplies duodenum, jejunum, ileum, cecum, appendix, ascending colon, and right half of transverse colon). It arises from the aorta about 1 cm below the celiac trunk at the L1 vertebral level. It initially descends posterior to the neck of the pancreas and then emerges and courses anterior to the uncinate process of the pancreas and the third part of the duodenum before entering the mesentery of the small intestine. To its right courses the superior mesenteric vein.

First, orient yourself to the small intestine and its mesentery. Locate the duodenojejunal junction where the 4th part of the duodenum joins the jejunum (Dissector p. 124, Fig. 4.34). This junctional area is a surgical and anatomic landmark and the site of the suspensory ligament of the duodenum (clinically referred to as the ligament of Treitz). This is a fibromuscular sling that connects this junction of small intestine to the right crus of the diaphragm. Work your fingers along the small intestine distally until it joins the ascending colon at the ileocecal junction (site of the ileocecal valve). Appreciate that the root of the mesentery (extending from the duodenojejunal junction to the ileocecal junction) is relatively short [~6 inches] compared to its intestinal border [~18 ft]. Proceed to remove the peritoneal layers of the mesentery to locate and identify the following branches of the superior mesenteric artery (you’ll also encounter corresponding veins, mesenteric lymph nodes, and autonomic nerve fibers): • Inferior pancreaticoduodenal artery – usually the first branch off the SMA as it passes through the pancreas. It contributes anterior and posterior branches to the inferior portion of the head of the pancreas and distal part of the duodenum and forms an anastomosis with similar branches of the superior pancreaticoduodenal artery. • 15-18 Intestinal (jejunal and ileal branches) – these arise from the left border of the SMA and course in the mesentery to the jejunum and ileum. As they near their surfaces, they give rise to arterial arcades or loops, from which arise straight (vasa rectae) terminal branches to the intestine. Note the differences in numbers of arcades and length of vasa rectae associated with the proximal jejunum compared to the distal ileum. • Ileocolic artery – to the cecum and appendix; gives rise to the appendicular artery • Right colic artery – arises from the right side of the SMA or the ileocolic artery and supplies the ascending colon ; variable in its origin • Middle colic artery – lift up the transverse colon and look for this artery coursing through the transverse mesocolon to the midpoint of this segment. It gives rise to a right and left branch and

19 supplies the transverse colon. Near the left colic flexure it anastomoses with the left colic branch of the inferior mesenteric artery (IMA).

Note the tributaries to the superior mesenteric vein and follow this vessel posterior to the pancreas where it unites with the splenic vein to form the hepatic portal vein (Fig. 4.39).

Inferior mesenteric artery (IMA) (Dissector p. 125, Fig. 4.35) – the third unpaired branch off the abdominal aorta (about L3 vertebral level) and the artery of supply to embryonic hindgut derivatives. It gives branches to the descending colon, sigmoid colon, and superior portion of the rectum. • Left colic artery – ascends and courses toward the left colic flexure to supply the left portion of transverse colon and the descending colon; it anastomoses with a branch from the middle colic artery • Sigmoid arteries – usually 4 branches that course through the sigmoid mesocolon to the sigmoid colon • Superior rectal artery – the most medial branch of IMA; it descends into the pelvic cavity to supply the proximal part of the rectum

Note the marginal artery (of Drummond) that frames the inner border of the large intestine and forms an anastomosis between branches of the SMA and IMA.

Inferior mesenteric vein. It receives tributaries corresponding to the above arteries and drains into the splenic vein, the superior mesenteric vein, or at the junction of these veins.

Internal Surfaces of the Gastrointestinal Tract [Do NOT follow the Dissector instructions to remove the GI tract]

Stomach (Dissector p. 129, Fig. 4.40) Make an incision along the anterior surface of the stomach to the first part of the duodenum. Clean the mucosa as needed and observe the following: • Gastric folds or rugae • Pyloric canal/area • Pyloric sphincter • Ampulla of the duodenum

Small Intestine (Dissector p. 130, Figs. 4.41, 4.42) First, observe the four parts of the duodenum (Dissector p. 128, Fig. 4.38). • Superior (first) part – intraperitoneal and attached to the hepatoduodenal ligament. Its initial dilated portion is the ampulla or duodenal cap or bulb • Descending (second) part – retroperitoneal. It borders the head of the pancreas and receives the bile duct and pancreatic ducts (main and accessory) • Horizontal (third) part – retroperitoneal. It borders the head and uncinate process of the pancreas and is crossed anteriorly by the superior mesenteric artery and vein. • Ascending (fourth) part – mostly retroperitoneal, except where the distal end of the duodenum joins the jejunum at the duodenojejunal junction.

The jejunum (approximately two-fifths of length) and ileum (approximately three-fifths of length) portions of the small intestine are continuous and lack an anatomic junction that marks the end of one and the beginning of the other.

20 Make a longitudinal incision along the anterior surface of the second part of the duodenum and selective incisions in the proximal jejunal and distal ileal segments and observe the following: • Major duodenal papilla – a small projection of mucosa on the posteromedial wall of the second part of the duodenum. This is the combined opening of the main pancreatic duct and bile duct. When present, a minor duodenal papilla draining the accessory pancreatic duct may be located about 2 cm superior to the major papilla. • Jejunum – note the height and density of the circular folds (plicae circulares) for maximum absorption of nutrients in this segment compared to the distal ileum • Ileum – contains fewer and shorter plicae circulares; • Ileocecal junction – open this junctional area and observe the ileocecal orifice and valve (Fig. 4.43)

Large Intestine The colon or large intestine is characterized externally by three features: a) teniae coli (3 narrow bands of longitudinal smooth muscle around the circumference of its wall [give the distinct triangular appearance to cross sections of the large intestine]), b) haustra, which are sacculations or outpouchings of the wall, and c) epiploic appendages (appendices epiploica), are small bags of fat enclosed in visceral peritoneum extending from the surface of the colon.

Make a single longitudinal incision along the anterior surface of the transverse colon and observe semilunar folds of mucosa (plicae semicirculares) and that the mucosal surfaces are smooth.

Pancreas (Dissector p. 127, Fig. 4.37) The pancreas is a secondarily retroperitoneal organ. It consists of a: • Head – which “rests” in the concavity of the duodenum. Posterior to it is the inferior vena cava. The uncinate process is located on the lower margin of the head and is crossed superficially by the superior mesenteric vessels. • Neck – a narrow segment that connects the head to the body • Body – crosses the midline of the body from right to left; posterior to it is the abdominal aorta • Tail – its tapered end, which contacts the hilus of the spleen

Review the blood supply to the head of the pancreas and duodenum (via anterior and posterior branches of the superior and inferior pancreaticoduodenal arteries). The body and tail of the pancreas receive multiple branches from the splenic artery.

Attempt to dissect and observe the main pancreatic duct by removing tissue from the anterior surface of the head of the pancreas (Dissector p. 127, Fig. 4.37). Look for the bile duct which joins it near the wall of the second part of the duodenum.

21 PEER TEACHING GUIDE: Organize your narrative to minimally include: 1. Demonstration of the branches of the superior mesenteric artery and their distribution to the organs they supply, including a comparison of the number of arterial arcades and length of vasa rectae to the proximal jejunum and distal ileum. 2. Demonstration of the branches of the inferior mesenteric artery and their distribution to the organs they supply. 3. Demonstration of the superior and inferior mesenteric veins and their drainage to the portal vein. 4. Demonstration of the different parts (intraperitoneal and secondarily retroperitoneal) of the duodenum and its relationship to the head and uncinate process of the pancreas. 5. Demonstration of the internal features of the GI tract: e.g., rugae and pyloric sphincter in the stomach, ampulla in the first part of the duodenum, the major duodenal papilla in the second part of the duodenum, the height and density of plicae circulares in the jejunum and ileum, the ileocecal valve, and the plicae semilunares associated with the large intestine. 6. Demonstration of the external features of the large intestine: teniae coli, haustra, and epiploic appendages. 7. Identification of the pancreas and its parts.

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