Third Stage 1 PDF - Cross-Sectional Anatomy of the Upper Abdomen

Cross-sectional anatomy of the upper abdomen Axial section through upper liver and spleen Body wall The ribs and intercostal muscles are sectioned obliquely around most of the perimeter of the section. Anteriorly the costal cartilages forming the costal margin are separated by the two rectus abdomin...

Cross-sectional anatomy of the upper abdomen Axial section through upper liver and spleen Body wall The ribs and intercostal muscles are sectioned obliquely around most of the perimeter of the section. Anteriorly the costal cartilages forming the costal margin are separated by the two rectus abdominis muscles and the midline linea alba between. Superficial to the ribs and intercostal muscles lie the serratus anterior muscles laterally and the latissimus dorsi posterior-laterally. The erector spinae muscles lie posterior to the vertebra on each side The diaphragm The domes of the diaphragm can be distinguished from the liver and other intra-abdominal viscera only where these are separated by fat. The lung bases lie posteriorly, separated from the liver and spleen by the diaphragm. The crura are visible anterior to the vertebral bodies With the adjoining diaphragm these form linear structures extending from the posterior aspect of the liver and spleen to the anterior surface of the abdominal aorta. The retrocrural space so formed is the lowest recess of the mediastinum. In addition to the aorta it contains the azygos vein on the right side and the hemiazygos vein on the left, the thoracic duct on the right posterior to the aorta (not usually visible on CT scans unless dilated), lymph nodes and fat The IVC remains on the abdominal side, that is, lateral to the crura below T8 level, where it pierces the diaphragm. Liver The liver occupies most of the right half of this section Segments II and III are distinguished from segments IVa and IVb by the fissure for the ligamentum teres on the anterior surface of the liver. The fissure for the ligamentum venosum on the visceral surface separates the caudate lobe (segment I) posteriorly from segments II and IV anteriorly. Branches of the portal veins and bile ducts may be seen especially on contrast- enhanced scans. This level is above the porta hepatis. Spleen Lying posteriorly on the left, close to the diaphragm and the ribs, the spleen has a smooth diaphragmatic surface and a concave visceral surface in axial section Other viscera The stomach lies deep to the left lobe of the liver. The transverse colon may be seen anterior to the stomach, although its position is variable. The splenic flexure is found anterior to the spleen. Axial section at the level of the porta of the liver Body wall This is as for the level above. The gap between the costal margins becomes progressively wider in lower cuts. Diaphragm As for above level. Usually only the crura and adjacent diaphragm are visible. Liver The falciform ligament is intrahepatic at this level. On this section, a line dividing the liver that passes through the gallbladder and the IVC defines the morphological right and left lobes (division of segment IV from segments V and VI) (This line has to be extrapolated back from the gallbladder, which is seen on lower sections) That part of the anatomical right lobe left of this line and anterior to the porta is the quadrate lobe and that part posterior to the porta is the caudate lobe At the porta hepatis the portal veins lie posteriorly with the branches of the hepatic artery and of the bile ducts anteriorly. Adrenal glands The right adrenal gland is a linear structure directly behind the IVC between the crura and the liver. The left adrenal gland extends deeper in front of the left kidney than does the right. Nearby splenic vessels may be confused with an adrenal abnormality. Spleen This section may pass through the splenic hilum. Vessels are seen dividing into branches just medial to the spleen. The splenic vein passes from the spleen posterior to the pancreas and is often visible throughout its length on one or two sections. The splenic artery has a tortuous course and only short segments are visible on axial sections. Other viscera The aorta lies in the retrocrural space and its coeliac branch may be seen at this level. The IVC lies close to the liver. The stomach and colon are seen as on the higher section Axial section at the level of the body of the pancreas Body wall The ribs and intercostal muscles. The latissimus dorsi is more lateral in position than before and the serratus anterior muscle is seen interdigitating with the external oblique muscle. The internal oblique muscle is seen arising from the deep aspect of the ribs. liver The left lobe of the liver is smaller or not seen at this level. The gallbladder is visible below the porta hepatis. Pancreas The tail and body lie higher than the head of the pancreas The tail of the gland is related to the splenic hilum The body of the pancreas is seen anterior to the splenic vein and the aorta Other viscera The adrenal glands are still visible on this section The upper pole of the left kidney (higher than that of the right) is visible The first part of the duodenum lies superior to the pan- creatic head The lower pole of the spleen lies behind the colon and lateral to the left kidney Axial section at the level of the pancreatic head and renal hila Body wall The abdominal wall proper is now visible with the recti on each side of the linea alba and the three muscle layers of the abdominal wall – the internal and external oblique and transversus abdominis muscles – between the recti and the lower ribs. Liver The lower part of the right lobe is seen A smaller left lobe, which may be completely separate from the right lobe, may be seen. The neck of the gallbladder (its highest part) may be seen. The spleen is not usually seen at this level. Pancreas The head of pancreas is seen at this level The pancreatic duct may be identified as a thin, hypodense linear structure The common bile duct and gastroduodenal artery are seen in cross- section The bile duct lies in the lateral aspect of the head, close to the duodenal surface The gastroduodenal artery (a branch of the hepatic artery) lies lateral and more anterior In slightly lower sections the head of the pancreas is seen to be separated from the uncinate process by the superior mesenteric vessels (the vein lies right of the artery) Kidneys and renal vessels This section includes one or both renal hila (the left hilum is slightly higher than the right) The left renal vein is seen passing to the IVC from the left hilum anterior to the aorta. The renal arteries have frequently divided already into anterior and posterior branches and may lie both anterior and posterior to the vein and pelvis Renal medullary pyramids can be demonstrated in their full length at the level of the hilum, whereas above and below this they are intersected at various planes to their long axis because of their orientation towards the hilum The aorta and the inferior vena cava The aorta remains between the crura Its superior mesenteric branch is visible at this level The IVC is separated from the aorta by the crura and, at this level, is free from the liver. Axial section below the kidneys Body wall The second part of the duodenum lies lateral to the pancreatic head The loops of small intestine lie to the left of the head of the pancreas. Vessels and fat are seen in its. The crura of the diaphragm may be seen as muscular columns mesentery that are not connected to one another, depending on level (not seen below L3). The right crus is usually larger than the left and extends further caudally. The psoas muscle lies along the lateral aspect of the vertebral bodies and, although thin here, it increases in bulk in lower cuts Other muscles are as described in previous sections. The duodenum The third part of the duodenum is seen passing between the aorta and the superior mesenteric artery. The colon and jejunum The transverse colon is seen close to the anterior abdominal wall, with the ascending and descending colon laterally. Loops of jejunum and their mesentery occupy most of the anterior part of this section. The ileocaecal valve The distal ileum opens into the medial and posterior aspect of the large intestine at the junction of the caecum and the ascending colon Two horizontal crescentic folds of mucosa and circular muscle project into the lumen on the colonic side These folds are extended laterally as the frenula of the valve Some thickening of the circular muscle of the ileum at the junction acts as a sphincter. Radiological features of the ileocaecal valve Plain films of the abdomen Gaseous distension of the colon is seen proximal to a site of colonic obstruction In some of these cases the ileocaecal valve remains competent, so that marked distension of the caecum can occur with or without distension of the small intestine In other patients the valve is incompetent and there is distension of both large and small intestine without excessive distension of the caecum. Barium-enema examinations The ileocaecal valve may present a filling defect in the postero- medial wall of the caecum This may be polypoid or bilabial, depending on the state of contraction of the valve. In the contracted valve, barium may fill a narrow slit between the folds like a linear ulcer. Radiology pearl The ileocaecal valve is at the site of the fi rst completely transverse haustral fold. The thickened posterior ends of this haustra are the frenula of the valve. Computed tomography Fat accumulation around the ileocaecal valve makes it easily visible in many abdominal CT scans This can be very marked in some individuals, particularly elderly women The appendix The appendix arises at the convergence of the taenia coli on the posteromedial wall of the caecum about 2.5 cm below the ileocaecal valve. It is a thin structure containing lymphoid tissue Its length is very variable – between 12 and 24 cm long. It has its own mesentery – a triangular fold from the lower border of the ileum – and as a result is mobile Its position is variable and according to most authors the retrocaecal position is commonest. The possible positions are: retrocaecal, pelvic, pre-ileal (anterior to ileum), post-ileal (behind ileum), subcaecal (inferior to caecum). When the appendix lies behind the caecum it is quite free if the caecum is completely invested in peritoneum and is itself mobile Occasionally it lies beneath the peritoneal covering of the caecum and may be fused to the caecum or the posterior abdominal wall. The lumen of the appendix is wide in the infant and obliterated after mid-adult life. Acute appendicitis, which is usually caused by obstruction of the lumen, is therefore rare in the extremes of life. Radiology pearl The appendix is supplied by the appendicular artery, which reaches it in the mesoappendix from the ileocolic artery. This is its sole supply, and if infection causes thrombosis of this artery, gangrene and perforation of the appendix results (compare with the gallbladder, which receives a rich collateral supply from the liver in the gallbladder bed and in which gangrene and perforation are rare). Lymph drainage is to the paracolic nodes along the ileocolic artery to the SMA group. Radiological features of the appendix Plain abdominal film Faecoliths or fluid levels of the appendix may be visible on plain films of the abdomen in the right iliac fossa in approximately 10% of individuals. Ultrasound The appendix is identified as a blind-ended tube arising from the posterior aspect of the caecum Unlike nearby loops of ileum, it does not display peristalsis Its position is variable, with the subcaecal appendix being least likely to be obscured by caecal gas If in a retrocaecal position, visualization of the appendix is aided by compression of the caecum Radiology pearl The appendix can be found by finding the junction of the terminal ileum with the medial aspect of the colon in the right lower quadrant by sequentially scanning the colon from superior to inferior and then by scanning carefully just below this level. Compression using the ultrasound probe is used to reduce obscuring gas in overlying bowel and to push overlying bowel loops out of the way. Barium enema If the lumen of the appendix is patent, it may fill on barium- enema examination. The lumen is often obliterated in patients past mid-adulthood. To fill the appendix the patient should be supine because its orifice is on the posterior aspect of the caecum. Some elevation of the head is also helpful CT and MRI The normal appendix can usually be identified arising from the caecum inferior to the insertion of the terminal ileum It can be followed on sequential images and may have a long and somewhat tortuous course. Appendiceal abscess Because the appendix is on a mesentery and mobile, pus from an infected appendix may cause abscess formation in a variety of locations. Pus may travel inferiorly to the pelvic peritoneum to the rectovesical (or rectouterine) pouch. Pus may also travel superiorly in the right paracolic gutter to the subhepatic spaces (see section on the peritoneal spaces of the abdomen) The large intestine The length of the large intestine is very variable, with an average length of 1.5 m. It is wider in diameter than the small intestine, with a maximum diameter of the caecum at 9 cm and the transverse colon at 5.5 cm. As far as the rectum the colon is marked by taeniae coli. These are three flattened bands of longitudinal muscle that represent the (incomplete) longitudinal muscle layer of the colon. The taeniae converge on the appendix proximally and the rectum distally, and these two structures have a complete longitudinal muscle layer. The taeniae coli are about 30 cm shorter than the colon and cause the formation of sacculations along its length. On radiographs these give rise to the appearance of incomplete septa, called haustra. The caecum is a blind pouch of large bowel proximal (inferior) to the ileocaecal valve. It is approximately 6 cm long and usually has its own mesentery, making it mobile and easily distensible. The ascending colon runs from the ileocaecal valve to the inferior surface of the liver, where it turns medially into the hepatic flexure. The transverse colon runs from the hepatic flexure across the midline to the splenic flexure. The descending colon runs from the splenic flexure inferiorly to the sigmoid colon. The abdominal aorta The aorta enters the abdomen through the aortic hiatus of the diaphragm between the crura at T12 vertebral level. It lies anterior to L1–L4 vertebral bodies, close to the left psoas muscle. The left lumbar veins pass behind it Anteriorly it is related to the pancreas, which separates it from the stomach, to the third part of the duodenum and to the coils of small intestine. The abdominal aorta is 12 cm long and ends by dividing into the right and left common iliac arteries at L4 vertebral level. Branches of the abdominal aorta These are as follows: • Three unpaired anterior branches: coeliac trunk at T12/L1 superior mesenteric artery at L1 inferior mesenteric artery at L3 • Three lateral paired visceral arteries: adrenal arteries at L1 renal arteries at L1/L2 gonadal arteries at L3 • Five lateral paired parietal branches: inferior phrenic arteries at T12 four pairs of lumbar arteries • Terminal arteries: the common iliac arteries median sacral artery The inferior vena cava The inferior vena cava (IVC) is formed by the union of the right and left common iliac veins at L5 vertebral level behind the right common iliac artery It passes on the right of the aorta as far as T12 level, where it is separated from the aorta by the right crus of the diaphragm. The IVC pierces the diaphragm at T8 level, passes through the pericardium and enters the right atrium. An incomplete semilunar valve is found at its entry to the atrium Otherwise the IVC has no valves. The IVC lies on the bodies of the lumbar vertebrae Part of the right adrenal gland and the right inferior phrenic, right adrenal, right renal and right lumbar arteries pass posterior to it. It is related anteriorly from below upwards, to coils of small intestine and the root of the mesentery, the third part of the duodenum, the head of the pancreas and the common bile duct, the first part of the duodenum and the epiploic foramen, with the portal vein anterior to it. It then passes in a deep groove in the liver (sometimes a tunnel) before piercing the diaphragm and entering the heart. Tributaries of the inferior vena cava These are as follows: • third and fourth lumbar veins (upper two to the azygos vein, the fifth to the ileolumbar vein) • right gonadal vein • right renal vein • right adrenal vein • small veins from right and caudate lobes of liver • right, middle and left hepatic veins • right inferior phrenic vein (left drains to left adrenal vein) • left renal vein (which has already received the left gonadal and adrenal veins) The Pelvis The pelvis is a bony ring consisting of paired innominate bones, the sacrum and coccyx. The innominate bones articulate with each other anteriorly and with the sacrum posteriorly. Each innominate bone is composed of three parts, which fuse at the acetabulum. The ilium is a flat curved bone and bears the iliac crest superiorly. The anterior and posterior superior iliac spines are on either end of the iliac crest, with the anterior and posterior inferior iliac spines below them. The inner surface of the bone is smooth and has a sharp crest at its base– the arcuate line – running from the sacroiliac joint to the iliopectineal eminence. This line extends anteriorly to the pubic tubercle as the anatomic iliopectineal line. The pubic bone consists of a body, and inferior and superior rami. The body of the pubic bone articulates with its fellow at the symphysis pubis. It bears the iliopectineal eminence on its superolateral aspect and the pubic tubercle on its superomedial aspect. The articular surfaces of the symphysis pubis are covered in hyaline cartilage with a fibrocartilaginous disc between them. The pubic bone is strengthened on all sides by dense ligaments. The ischium is composed of a body and an inferior ramus, which joins the inferior pubic ramus. The body bears the ischial tuberosity inferiorly and a spine posteriorly. The ischial spine defines the greater and lesser sciatic notches above and below. The obturator foramen is bounded by the body and rami of the pubic bone and the body and ramus of the ischial bone. The sacrum Five fused vertebrae comprise this triangular bone, which is curved posteriorly. The anterior part of its upper end is termed the sacral promontory. It articulates with the lumbar spine superiorly and with the coccyx inferiorly. Anteriorly the sacrum has four pairs of sacral foramina, which transmit nerves from the sacral canal Lateral to these are the lateral masses or alae of the sacrum. The sacrum also bears four pairs of posterior sacral foramina and the canal ends posteriorly in the sacral hiatus – a midline opening that transmits the fifth sacral nerves. The coccyx This is composed of three to five fused vertebrae. The first segment is often separate It articulates at an acute angle with the sacrum. The Sacroiliac Joints The sacroiliac joints are covered with cartilage. The anterior aspect of the joint is lined with synovium. The joint surface is flat and uneven, and this irregularity helps lock the sacrum into the iliac bones. Ligaments support the front and back of the joint. 1-There are dense interosseous sacroiliac ligaments which further lock the sacrum to the iliac bones, limiting movement in all planes. 2-The sacrospinous ligament runs from the ischial spine to the sides of the sacrum and coccyx. It defines the inferior limit of the greater sciatic foramen. 3-The sacrotuberous ligament runs from the ischial tuberosity to the sides of the sacrum and coccyx It defines the posterior limit of the lesser sciatic foramen 4-The iliolumbar ligament runs from the transverse process of L5 to the posterior part of the iliac crest, further stabilizing the joint. The piriformis muscles pass obliquely from the anterior aspect of the sacrum exiting the pelvis through the greater sciatic foramen behind the acetabulum to insert into the greater trochanter of the femur. The aponeurosis of the abdominal wall muscles inserts into the superior surface of the pubic bone. A thickening of the aponeurosis is the inguinal ligament, which runs from the pubic tubercle to the anterior superior iliac spine. All the muscles of the anterior, lateral and posterior abdominal walls insert, to some degree, into the iliac crest, inguinal ligament and pubic bone. The gluteal muscles arise from the external surface of the iliac bone and the iliac crest and insert into the upper femur. Gluteus maximus is the largest, the most superficial and the most posterior gluteal muscle, covering the posterior part of the ilium and the sacroiliac joints. Gluteus medius and minimus are more anteriorly placed, gluteus minimus being the smallest and the most deeply placed. The pelvic floor A sling of muscles closes the floor of the pelvis. The urethra and rectum and the vagina in the female pierce the pelvic floor. The floor is composed of two muscular layers, the levator ani/ coccygeus complex and the perineum. The levator ani muscle is the principal support of the pelvic floor. It provides muscular support for the pelvic organs and reinforces the urethral and rectal sphincters. Levator ani arises in a line from the posterior aspect of the superior ramus of pubis to the ischial spine. Between these bony points, the muscle arises from the fascia covering the obturator internus muscle on the inner wall of the ilium in an arc known as the tendinous arch or white line. Its fibers sweep posteriorly, inserting into the perineal body (a fibromuscular node behind the urethra in males or the urethra and vagina in females), the anococcygeal body (a fibromuscular node between the anus and coccyx) and the lowest two segments of the coccyx. The midline raphe of the levator ani anterior to the coccyx is also known as the levator plate. The fibres of levator ani sling around the prostate gland or vagina and rectum, blending with the external anal sphincter. The components of levator ani are named according to their attachments: pubococcygeus is the main part of levator ani It arises from the inner surface of the body of pubis and tendinous arch running posteriorly to the sacrum and coccyx. Pubo-rectalis is the thickest, most medial aspect of the muscular sling, arising from the inner surface of the pubic bone and forming asling behind the anorectal junction. Iliococcygeusis the posterior part of the muscle and runs from the posterior tendinous arch and the ischial spine to coccyx. The coccygeus muscle is in the same tissue plane as levator ani. It arises from the ischial spine and sacrotuberous ligament and inserts into the side of the coccyx and lower sacrum. It aids levator ani in supporting the pelvic organs The perineum is the diamond-shaped space between the pubis, the ischial tuberosities and the coccyx. It is divided into two compartments by the transverse perineal muscles, which arise from the ischial tuberosity and run medially to insert into the perineal body.