Dissection Overview: Skin Incisions and Superficial Fascia
The contents of the abdominal cavity are protected by the anterolateral abdominal wall.
The organization of the layers forming the anterolateral abdominal wall is illustrated in Figure 4.4. The superficial fascia is unique in this
region in that it has a superficial fatty layer called Camper's fascia and a deep membranous layer called Scarpa's fascia. The membranous layer is noteworthy, because it is continuous with the Dartos and Colles' fasciae in the perineum.
Dissection Instructions: Skin Incisions and Superficial Fascia
- Make a midline skin incision from the xiphisternal junction (C) to 3 cm below the pubic crest (E), encircling the umbilicus.
- Make an incision from the xiphoid process (C) along the costal margin to a point on the midaxillary line (V).
- Make a skin incision beginning 3 cm below the pubic crest (E). Extend this incision laterally, 3 cm inferior to the inguinal ligament to a point 3 cm below the anterior superior iliac spine (ASIS). Continue the incision laterally, 3 cm below the iliac crest to a point on the midaxillary line (F).
- Reflect the skin flaps and superficial fascia laterally; BUT LEAVE a "button" of superficial fascia adhered to the aponeurosis of the external abdominal oblique. The "button" should be about 7.5 cm lateral to the midline and should be 10 cm (3-4 in) in diameter. The superficial epigastric artery and vein are in the superficial fascia in this area. If you see them, fine, but do not make a special effort to find them.
- Observe that the deep surface of your "button" is fibrous connective tissue containing relatively little fat (Scarpa's fascia) and the more superficial part is composed entirely of fat (Camper's fascia) (Figure 4.6, arrow 1)..
- As you reflect the skin flaps at the midline, you may come upon the anterior cutaneous branches of intercostal nerves (T7 to T11), the subcostal nerve (T12), and the iliohypogastric and ilioinguinal nerves (L1) that enter the superficial fascia about 2 to 3 cm lateral to the midline. Preserve one or two examples of anterior cutaneous branches if found, but do not make a special effort to find them. Consult a dermatome chart and recall that:
- T7 innervates the skin overlying the tip of the xiphoid process.
- T10 innervates the skin of the umbilicus.
- T12 innervates the skin superior to the pubic symphysis.
- L1 innervates the skin overlying the pubic symphysis.
- Lateral to the incision, use your fingers to separate the superficial fascia from the external abdominal oblique muscle (Figure 4.6, arrow 2). As you near the midaxillary line, palpate the lateral cutaneous branches of the intercostal and subcostal nerves entering the superficial fascia.
- Remove the superficial fascia in an inferior direction until the lower border of the external abdominal oblique muscle is exposed (approximately 2.5 cm into the proximal thigh).
IN THE CLINIC: Superficial Veins of the Abdominal Wall
The superficial epigastric vein anastomoses with the lateral thoracic vein in the superficial fascia. This is an important collateral venous channel from the femoral vein to the axillary vein. In patients who have an obstruction of the inferior vena cava or hepatic portal vein, the superficial veins of the abdominal wall may be engorged, and may become visible around the umbilicus (caput medusae).
Dissection Overview: Muscles of the Anterolateral Abdominal Wall
Three flat muscles (external abdominal oblique, internal abdominal oblique, and transversus abdominis) form most of the anterolateral abdominal wall. The rectus abdominis muscle completes the anterior abdominal wall near the midline. The three flat muscles have fleshy proximal attachments (to the ribs, vertebrae, and pelvis) and broad, aponeurotic distal attachments (to the ribs, linea alba, and pubis). Each of the three flat muscles contributes to the formation of the rectus sheath and the inguinal canal.
The order of dissection will be as follows: The three flat muscles of the abdominal wall will be studied. The composition and contents of the rectus sheath will be explored. The anterior abdominal wall will be reflected.
Dissection Instructions: External Abdominal Oblique Muscle
- Reflect or remove any remnants of the superficial fascia from the surface of the external abdominal oblique muscle and place them in the tissue container (keep your "button" on one side for peer teaching, if possible).
- The external abdominal oblique muscle forms the most superficial portion of the inguinal canal (Figure 4.8A). The proximal attachments of the external abdominal oblique muscle are the external surfaces of the lower ribs (ribs 5 to 12). The distal attachments of the external abdominal oblique muscle are the linea alba, pubic tubercle, and anterior half of the iliac crest. Observe that the fibers of the external abdominal oblique muscle course from superolateral to inferomedial.
- Clean off the surface and define the medial borders of the external abdominal oblique muscle. Gentle scraping motions with a dull scalpel blade yield good results. To avoid damage to the inguinal canal, work at and above the intercristal line. This is to protect the spermatic cord (or round ligament of the uterus) where it emerges from the superficial inguinal ring.
Dissection Instructions: Internal Abdominal Oblique Muscle
The internal abdominal oblique muscle lies deep to the external abdominal oblique muscle. The internal abdominal oblique muscle forms the intermediate layer of the inguinal canal (Figure 4.8B). To expose the internal abdominal oblique muscle, the external abdominal oblique muscle must be transected and reflected. Perform this transection bilaterally.
- At the intersection of the midaxillary line and transumbilical plane (point A in Figure), use scissors and forceps to separate the fibers of the external abdominal oblique muscle along their directional line. Identify the fibers of the underlying internal abdominal oblique muscle coursing in the opposite direction. Insert your finger into the opening and direct it superiorly to create space between the external abdominal oblique muscle and the underlying internal abdominal oblique muscle. Then use scissors to make a transverse cut that extends from the midaxillary line to the medial border (i.e. semilunar line) of the external abdominal oblique muscle (Cut 2 in Figure).
- Use blunt dissection to separate the fibers of the external abdominal oblique muscle from the underlying internal abdominal oblique muscle. Note that your fingers cannot pass medial to the semilunar line because the external abdominal oblique aponeurosis is fused to the internal abdominal oblique aponeurosis to form the anterior lamina of the rectus sheath (described later).
- Use scissors to cut superiorly following the medial border of the external abdominal oblique muscle (point B in Figure) and reflect it superolaterally. Separate the external abdominal oblique muscle from the internal abdominal oblique muscle as needed, to sufficiently expose the internal abdominal oblique muscle.
Dissection Instructions: Transversus Abdominis Muscle
The transversus abdominis muscle lies deep to the internal abdominal oblique muscle. The transversus abdominis muscle contributes to the deepest layer of the inguinal canal (Figure 4.8C). In the inguinal region the transversus abdominis muscle has attachments and fiber directions that are similar to the internal abdominal oblique muscle.
Use an illustration to study the proximal attachments, distal attachments, and fiber direction of the transversus abdominis muscle. The proximal attachments of the transversus abdominis muscle are the internal surfaces of the lower costal cartilages (of ribs 7 to 12), the thoracolumbar fascia, the iliac crest, and the lateral third of the inguinal ligament. The distal attachments of the transversus abdominis muscle are the linea alba, the pubic crest, and the pecten pubis.
- Superiorly, just inferior to the ribs, a blunt probe can be used to separate the fibers of internal abdominal oblique to view the transversus abdominus muscle. Open the incision with your fingers and with blunt dissection in the plane between these two muscles, separate them as far medially and inferiorly as possible (without enlarging the incision).
Dissection Instructions: Rectus Abdominis Muscle
The rectus sheath is formed by the aponeuroses of the three flat abdominal muscles. The rectus sheath contains the rectus abdominis muscle, the superior and inferior epigastric vessels, and the terminal ends of the ventral rami of spinal nerves T7 to T12.
- Reposition the internal abdominal oblique and external abdominal oblique muscles. The following cuts should be made bilaterally.
- Use scissors to make a transverse incision through the anterior wall of the rectus sheath (through the aponeuroses superficial to the rectus abdominis muscle) at the level of the umbilicus (Figure 4.12, cut 1). Begin the cut approximately 2.5 cm lateral to the umbilicus and continue it laterally as far as the semilunar line.
- Use scissors to cut the rectus sheath along the medial border of the right rectus abdominis muscle (Figure 4.12, cut 2). This incision should extend in a superior direction, about 2.5 cm from the midline. Stop at the costal margin.
- Extend the vertical incision inferiorly along the medial border of the rectus abdominis muscle (Figure 4.12, cut 3). Cut 3 should be about 1.2 cm from the midline and stop at the pubic crest.
- Insert your fingers into the vertical incision and bluntly dissect the anterior wall of the rectus sheath from the anterior surface of the rectus abdominis muscle. Observe that the anterior wall of the rectus sheath is firmly attached to the anterior surface of the rectus muscle by several tendinous intersections (Figure 4.13). Insert scissors between the rectus sheath and the anterior surface of the rectus abdominis muscle and cut the tendinous intersections to free the rectus sheath from the rectus abdominis muscle.
- Observe the rectus abdominis muscle (Figure 4.13). The inferior attachment of the rectus abdominis muscle is the symphysis and body of the pubis. The superior attachment of the rectus abdominis muscle is on the costal cartilages (ribs 5 to 7). The rectus abdominis muscle flexes the trunk.
- Find a number of the branches of six nerves (T7 to T12) that enter the lateral side of the rectus sheath (Figure 4.14). These nerves innervate the rectus abdominis muscle and then emerge as the anterior cutaneous branches of the intercostal and subcostal nerves.
- Use your fingers to mobilize the medial border of the rectus abdominis muscle (Figure 4.13). At the level of the umbilicus, transect the rectus abdominis muscle with scissors. Reflect the two halves superiorly and inferiorly, respectively. Only reflect the inferior half as far as the 3 cm below the intercristal line. If the nerves prevent full reflection of the rectus abdominis muscle, cut them along the lateral border of the muscle.
- Observe two sets of vessels on the deep surface of the rectus abdominis muscle (Figure 4.14):
- Superior epigastric artery and vein - on the superior half of the rectus abdominis muscle - If necessary, look for the superior epigastric artery at the xiphisternal junction.
- Inferior epigastric artery and vein - on the inferior half of the rectus abdominis muscle - This artery may dive into the muscle early and not be visible. Study Figure XX to understand the anastomosis between these arteries. This anastomosis forms a functional interconnection between the arteries that serve the upper and lower limbs.
- Examine the posterior layer of the rectus sheath (Figure 4.15, left), formed by aponeuroses of the internal abdominal oblique and transversus abdominis muscles (Figure 4.15, right).
- Identify the arcuate line (Figure 4.15, left), which is located midway between the pubic symphysis and the umbilicus. The arcuate line is the inferior limit of the posterior layer of the rectus sheath, and it may be indistinct. At the level of the arcuate line, the inferior epigastric vessels enter the rectus sheath.
- Inferior to the arcuate line, the fused aponeuroses of all 3 flat abdominal muscles pass anterior to the rectus abdominis muscle (Figure 4.15, right).
- Inferior to the arcuate line, observe the transversalis fascia. Deep to the transversalis fascia observe a thin layer of extraperitoneal fascia and the parietal peritoneum (Figure 4.15, right).
- In the midline, identify the linea alba. The linea alba is formed by the fusion of the aponeuroses of the right and left flat abdominal muscles (external abdominal oblique, internal abdominal oblique, and transversus abdominis).
IN THE CLINIC: Epigastric Anastomoses
The superior epigastric vessels anastomose with the inferior epigastric vessels within the rectus sheath (Figure 4.14). If the inferior vena cava becomes obstructed, the anastomosis between the inferior epigastric and superior epigastric veins provides a collateral venous channel that drains into the superior vena cava. If the aorta is occluded, collateral arterial circulation to the lower part of the body occurs through the superior and inferior epigastric arteries.
- Use an illustration to review the distribution of the superficial epigastric vessels.
- Review the abdominal distribution of the ventral rami of spinal nerves T7 to L1.
- Replace the muscles of the anterior abdominal wall in their correct anatomical positions.
- Review the proximal attachment, distal attachment, and action of each muscle.
- Review the structures that form the nine layers of the abdominal wall (Figure 4.4).
- Use the dissected specimen to review the rectus sheath at the level of the umbilicus and just superior to the pubic symphysis (Figure 4.15).
- Review the nerve supply to the anterior abdominal wall. Review the blood supply to the anterior abdominal wall.
Dissection Instructions: Spleen
The spleen is the largest hematopoietic organ in the body. Its size and weight may vary considerably depending upon the
blood volume that it contains. The spleen is covered by visceral peritoneum except at the hilum where the splenic vessels enter and leave.
- Use your left hand to retract the fundus of the stomach to the right. Use your right hand to gently pull the spleen
anteriorly (Figure 4.28).
- Observe that the spleen has a smooth diaphragmatic surface. The spleen has sharp anterior, inferior, and superior borders. The superior border of the spleen is often notched.
- The visceral surface of the spleen is related to four organs:
- Left kidney
- Transverse colon (left colic flexure)
- The diaphragmatic surface of the spleen is related (through the diaphragm) to ribs 9, 10, and 11 (Figure 4.28).
IN THE CLINIC: Spleen
The relationship of the spleen to ribs 9, 10, and 11 is of clinical importance in evaluating rib fractures and penetrating wounds. A lacerated spleen bleeds profusely into the abdominal cavity and may have to be removed surgically (splenectomy). It must be emphasized that there is a risk of puncturing the spleen during pleural tap (thoracocentesis).
An enlarged spleen (splenomegaly) may be encountered during physical examination. The spleen is considered enlarged when it can be palpated inferior to the costal margin.
IN THE CLINIC: Anatomical Variation in Arteries
In about 12% of cases the right hepatic artery arises from the superior mesenteric artery. An aberrant left hepatic artery may arise from the left gastric artery. During surgical removal of the stomach (gastrectomy), blood flow to an aberrant left hepatic artery could be interrupted, endangering the left lobe of the liver.
The cystic artery usually arises from the right hepatic artery, but other origins are possible. The cystic artery may pass posterior (75%) or anterior (24%) to the common hepatic duct (Figure 4.27).
- Use an illustration and the dissected specimen to trace the branches of the celiac trunk.
- Review the relationships of the structures in the hepatoduodenal ligament.
- Review the boundaries of the omental foramen.
- Use an embryology textbook to review the development of the liver and the ventral mesogastrium.
- Review the derivatives of the embryonic foregut.
Dissection Instructions: Liver
The liver is the largest gland in the body, comprising about 2.5% of the body weight of an adult. To study the surface features of the liver, it must be detached from the diaphragm.
- Review the falciform ligament and the coronary ligament of the liver (Figure 2.64).
- Use scissors to cut the falciform ligament along its attachment to the anterior abdominal wall, if you haven't done this
already. Extend the cut superiorly and cut the right and left triangular ligaments along the inferior surface of the diaphragm.
- Insert your fingers between the liver and the diaphragm and tear the connective tissue that attaches the liver to the diaphragm. Cut the posterior layer of the coronary ligament.
- Extend the cut previously made in the diaphragm toward the inferior vena cava. Use scissors to cut the inferior vena cava between the liver and the diaphragm. Elevate the inferior border of the liver and cut the inferior vena cava again, as close to the inferior surface of the liver as possible. These two cuts will leave a short segment of the inferior vena cava within the liver (Figure 4.29B).
- The liver should now be freely mobile but attached to the other abdominal viscera by the biliary tree, hepatic artery
proper, and hepatic portal vein. Move the liver carefully to avoid tearing these structures.
- Examine the liver and note that the right lobe is six times larger than the left lobe. The sharp inferior border of the liver separates the visceral surface from the diaphragmatic surface.
- Identify the bare area on the posterior aspect of the diaphragmatic surface. Here, the liver was adjacent to the diaphragm and not covered by peritoneum. Around the bare area, note the cut edges of the coronary ligament.
- Examine the visceral surface of the liver (Figure 4.29A). An H-shaped set of fissures and fossae defines four lobes.
Identify the right lobe, left lobe, caudate lobe, and quadrate lobe.
- Observe that the ligamentum venosum and falciform ligament occupy the left fissure of the "H" (Figure 4.29B; Figure 2.64). The gallbladder and inferior vena cava occupy the fossae that form the right side of the "H."
- Identify the porta hepatis. It forms the horizontal bar of the "H." The structures passing through the hepatoduodenal ligament (biliary tree, hepatic arteries, hepatic portal vein, lymphatics, and autonomic nerves) enter or leave the liver at the porta hepatis.
- Examine the small segment of the inferior vena cava that is attached to the liver. Note that several hepatic veins drain directly into the inferior vena cava (Figure 4.29B).
- Use a textbook to study the two conventions by which the liver may be divided into lobes (Figure 2.64).
The falciform ligament divides the liver into right and left anatomical lobes. The pattern of its bile drainage and
vascular supply are used to divide the liver into right and left functional lobes.
- The liver has a substantial lymphatic drainage. At the porta hepatis, small lymph vessels drain into hepatic lymph nodes. From the hepatic lymph nodes, lymphatic vessels follow the hepatic arteries to celiac lymph nodes located around the celiac trunk.
IN THE CLINIC: Liver
The liver may undergo pathologic changes that could be encountered during dissection. The liver may be enlarged. This happens in liver congestion due to cardiac insufficiency (cardiac liver). In contrast, the liver may be small and have fibrous nodules. Such a finding may indicate cirrhosis of the liver. Because the liver is essentially a capillary bed downstream from the gastrointestinal tract, metastatic tumor cells are often trapped within it, resulting in secondary tumors.
Dissection Instructions: Gallbladder
The gallbladder is a reservoir for the storage and concentration of bile. The gallbladder occupies a shallow fossa on the visceral surface of the liver (Figure 4.29B). The gallbladder is usually stained dark green by bile, which leaks through the wall of the gallbladder after death, staining neigboring structures (Note that, in life, bile is yellow-brown. It is turned dark green by the fixatives).
- Replace the liver into its correct anatomical position.
- Confirm that the gallbladder is located near the tip of the ninth costal cartilage in the midclavicular line (Figure 4.17).
- Reposition the liver to expose the visceral surface. Use blunt dissection to remove the gallbladder from its fossa.
- Identify the parts of the gallbladder (Figure 4.30):
- Review the course of the cystic artery. The cystic artery may be stained green by bile and is often fragile and difficult to dissect.
- Use scissors to make a longitudinal incision through the wall of the gallbladder, beginning at the fundus and continuing
through the neck. If gallstones are present, remove them. The spiral fold is a fold in the mucosal lining of the neck that continues into the cystic duct.
IN THE CLINIC: Triangle of Calot
An important anatomical landmark used by surgeons performing cholecystectomies (removal of the gallbladder) is the Triangle of Calot (cystohepatic triangle).
The triangle was first described by Jean Francois Calot in 1891 as bounded by the cystic duct, common hepatic duct, and the cystic artery. Over the years many surgeons
have expanded the triangle such that its borders are the cystic duct, common hepatic duct, and the inferior border of the liver. The precise definition is now
controversial (e.g., our surgery clerkship faculty are divided in their opinion). Nevertheless, the Triangle of Calot is an important hepatobiliary surgical landmark.
PROSECTION OF THE UPPER GI
Prosection Overview: The Digestive System
The digestive system includes all of the organs concerned with gustation and olfaction, ingestion, mastication (chewing), deglutition (swallowing), digestion and absorption of food and the excretion of the unabsorbed residue. The basis of the digestive system is the alimentary canal extending from the oral to the anal opening. The successive parts of this tube are the mouth (oral cavity), pharynx, esophagus, stomach, small and large intestines and the anal canal. The mouth and pharynx are used for respiration as well as digestion, so the alimentary canal proper actually begins at the upper end of the esophagus. In addition a number of salivary glands (parotid, submandibular, sublingual) empty their secretions via ducts into the oral cavity. Secretions from the liver and pancreas empty into the duodenum.
You are dissecting the digestive system from the abdominal esophagus to the anal opening. The upper part of the digestive system will be studied on a skull and some prosections.
Prosection Instructions - On the skull:
The teeth are set in bony sockets in the upper (maxilla) (Plate 544) and lower (mandible) (Plate 543) jaws. To adjust the arrangement of the teeth to the growth of the jaws we have two sets of teeth, the deciduous (primary, "baby", "milk") and permanent (secondary) teeth. Children have 20 deciduous teeth while adults have 32 permanent teeth. Deciduous teeth erupt from about 6 months to 2 years of age. The deciduous teeth are shed and replaced with permanent teeth from about age 6 to 18. Teeth are grouped as incisors for cutting, canines for piercing, premolars and molars for grinding
* Take time to look in a mirror and identify your own teeth. You may be missing your 3rd molars if you have had your "wisdom teeth" removed.
- Identify the maxilla and the mandible
- Identify on both the upper and lower jaws, starting from the midline (4 quadrants of 8 teeth each):
- 2 incisors
- 1 canine
- 2 premolars
- 3 molars
- Identify the hard palate (formed by the maxillae and palatine bones)
Prosection Instructions - On the bisected head:
- Refer to Netter Plate 57 and Plate 525. Identify parts of the nose and nasal cavity (part of the respiratory system):
- Naris (plural, nares) - External opening of the nose. They are separated from each other by the lower border of the nasal septum.
- Each nasal cavity begins at the naris and extends back to the choana (plural, choanae), the
opening into the nasopharynx
- The nasal septum, comprised of cartilage and bone and covered by mucous membrane, separates the two nasal cavities. Frequently the nasal septum is deviated to one side.
- Identify the hard palate and soft palate, which separate the nasal cavity from the oral cavity (Netter 57).
- In the oral cavity (mouth), identify
- Incisors, if present, embedded in the bony sockets of the maxilla and mandible
- Tongue - tip, body and root. The tongue is highly muscular and covered by a mucous membrane. The anterior two thirds of the tongue is freely movable and attached to the floor of the oral cavity.
- The pharynx extends from the base of the skull to the inferior border of the cricoid cartilage (level of the C6 vertebra).
It is the continuation of the respiratory and digestive systems posterior to the nasal, oral, and laryngeal cavities.
Identify the 3 parts of the pharynx (Netter 57):
- The nasopharynx is posterior to the choanae and superior to the soft palate.
- The oropharynx is posterior to the tongue between the soft palate and the epiglottis.
- The laryngopharynx is continuous with the oropharynx at the upper border of the epiglottis and narrows
rapidly below at the level of the cricoid cartilage to become continuous with the esophagus.
- Identify the larynx anterior to the laryngopharynx extending from the epiglottis superiorly to the lower border of the cricoid cartilage inferiorly where it becomes continuous with the trachea.
Prosection Instructions - On the pharynx/larynx prosections:
- Refer to Netter Plate 60, Plate 552, Plate 539, Plate 61, Plate 553
- On the dorsum of the tongue (Clemente 860), identify
- The tip and body of the tongue, which is separated from the root of the tongue by a V-shaped furrow, the sulcus terminalis.
- A row of papillae containing taste buds lying immediately anterior to the sulcus terminalis; these are the circumvallate papillae
- The pharynx has been opened by a posterior longitudinal incision. Identify the 3 parts of the pharynx (Netter 60; Clemente 893):
- The nasopharynx
- The oropharynx
- The laryngopharynx
- Identify the continuation of the laryngopharynx as the esophagus at the lower border of the cricoid cartilage
- On the prosections where the mucous membranes and connective tissues have been removed (Netter 61; Clemente 895) observe that the pharyngeal walls consist of circularly and longitudinally oriented muscles. The details of the musculature of the pharynx will be studied in the Brain, Mind and Behavior block.
Prosection Instructions - Salivary glands:
- Refer to Plates 536 and 532
- On yourself, or a volunteer, palpate the (Clemente 856)
- Parotid glands in front of the ear between the mandible and the sternocleidomastoid muscle. The parotid ducts empty into the mouth opposite the 2nd upper molar teeth
- Submandibular glands below the body and angle of the mandible. The submandibular ducts open at the sublingual caruncula (Clemente 847) at the side of the frenulum below the tongue.
- With the mouth open and the tip of your tongue touching the hard palate, identify (Clemente 847)
- The sublingual caruncula for the openings of the submandibular ducts
- The sublingual folds within which are the sublingual glands. The ducts of the sublingual glands open all along the sublingual folds.