A Projection onto the trunk

Anterior view.

The stomach is intraperitoneal and located in the left upper quadrant (epigastrium).

Note the transpyloric plane (halfway between the superior border of the pubic symphysis and superior border of the manubrium, see p. 360). It serves as an important anatomical landmark: the pylorus is located at or slightly below the transpyloric plane. Unlike other parts of the stomach, the pylorus hardly moves at all since it is connected to the duodenum, which is retroperitoneal (and thus relatively immobile).

B Topographical relationships

Transverse section at approximately the T 12/L 1 level. Viewed from above.

Note the relationship of the stomach to the spleen, pancreas, liver, and omental bursa: The greater curvature extends to the spleen; the left lobe of the liver extends in front of the stomach and into the left upper quadrant. When the abdomen is opened, very little of the stomach is visible as most of it is obscured by the liver. Posterior to the stomach lies a narrow peritoneal space called the omental bursa. Its posterior wall is largely formed by the pancreas. Due to its peritoneal covering, the stomach is very mobile relative to the neighboring organs. This is important for facilitating the stomach’s peristaltic movements. Due to its embryonic placement in the ventral and dorsal mesogastrium (see p. 42), the stomach has direct peritoneal attachments to the spleen and liver.

C The stomach in situ

Anterior view of the opened upper abdomen. The liver has been retracted superolaterally, and the esophagus has been pulled slightly downward for better exposure. The arrow points to the omental foramen, the opening in the omental bursa behind the lesser omentum. Peritoneal adhesions are visible between the liver and the descending part of the duodenum. The lesser omentum is visibly subdivided into a relatively thick hepatoduodenal ligament (transmitting neurovascular structures to the porta hepatis) and a thinner hepatogastric ligament, which is attached to the lesser curvature of the stomach. A hepatoesophageal ligament can also be identified. The greater curvature of the stomach is closely related to the spleen in the left upper quadrant (LUQ). The greater omentum is a duplication of peritoneum that covers the transverse colon and drapes over the loops of small intestine (not visible here).

A Muscular layers

Anterior view of the anterior stomach wall with the serosa and subserosa removed. The muscular coat of the stomach has been windowed at several sites. The entire stomach wall ranges from 3 mm to approximately 10 mm in thickness (see B for individual layers). Most of its muscular coat consists not of two layers (as in other hollow organs of the gastrointestinal tract) but of three muscular layers:

• An outer longitudinal layer, which is most pronounced along the greater curvature (greatest longitudinal expansion)

• A middle circular layer, which is well developed in the body of the stomach and most strongly developed in the pyloric canal (anular sphincter, see p. 229)

• An innermost layer of oblique fibers, which is derived from the circular muscle layer and is clearly visible in the body of the stomach.

The three-layered structure of its muscular wall enables the stomach to undergo powerful churning movements. The muscles can forcefully propel solid food components against the stomach wall in the acidic gastric juice, breaking the material up into particles approximately 1 mm in size that can pass easily through the pylorus. The longitudinally-oriented rugal folds (reserve folds that disappear when the stomach is distended) form channels, called gastric canals, that rapidly convey liquids from the gastric inlet to the pylorus.

C Wall structure and duct orifices

Anterior view. Most of the duodenum has been opened. The pyloric orifice (here greatly dilated) opens to a luminal diameter of only about 2–3 mm for the passage of chyme. The duodenum has basically the same wall structure as the other hollow organs of the gastrointestinal tract (see B, p. 231). The structure of the mucosa is shown in F. The descending part of the duodenum has two small elevations along its inner curve: the minor duodenal papilla, which bears the orifice of the accessory pancreatic duct, and the major duodenal papilla (called also the papilla of Vater), which has a common orifice for the pancreatic duct and common bile duct. Thus, the release of bile and pancreatic juice to aid digestion takes place in the upper part of the duodenum.

F Histological structure

Longitudinal section through the duodenal wall. The duodenum has basically the same histological structure as the other hollow organs of the gastrointestinal tract (see B, p. 231), with some notable differences such as the presence of duodenal (Brunner) glands (secrete mucins and bicarbonate to neutralize the acidic gastric juice) and valves of Kerckring (specialized circular folds). Other features that distinguish the duodenum from the jejunum and ileum are its more prominent mucosal folds, which diminish in size toward the end of the small intestine.

Note: The muscularis externa of all portions of the intestine, unlike that of the stomach, consists of only two layers: an inner layer of circular muscle fibers and an outer layer of longitudinal muscle fibers.

A Parts of the small intestine: overview (a) and anatomical constrictions (b)

Anterior view. The large intestine surrounds the loops of small intestine like a frame. Because the small bowel loops are intraperitoneal and therefore very mobile, it is not possible to define their location by reference to skeletal landmarks. If the intestinal loop rotates normally during embryonic development (see p. 46), the duodenum lies behind the transverse colon. If the intestinal loop rotates in the wrong direction, the duodenum will come to lie in front of the transverse colon.

Note the following normal anatomical constrictions:

• Junction of the pylorus and duodenum (luminal diameter of the pyloric orifice is only about 2–3 mm)

• Duodenojejunal flexure

• Ileocecal orifice

Swallowed foreign bodies may become lodged at these sites, obstructing intestinal transit and causing mechanical intestinal paralysis (mechanical ileus, a life-threatening condition that is an absolute indication for surgical treatment).

B Wall structure of the jejunum and ileum

The wall layers of the small intestine are displayed in a “telescoped” cross-section. The mucosal layer has been incised longitudinally and opened. The jejunum and ileum have basically the same wall structure as the other hollow organs of the gastrointestinal tract (see B, p. 231), but local differences are observed in the circular folds (see C) and vascular supply (see p. 268).

C Differences in the wall structure of the jejunum and ileum

Macroscopic views of the jejunum (a) and ileum (b), which have been opened longitudinally to display their mucosal surface anatomy.

Note: The transversely oriented circular folds in the jejunum are spaced much closer together than in the ileum. Lymphatic follicles are particularly abundant in the wall of the ileum (from the lamina propria to the submucosa) for mounting an immune response to antigens in the intestinal contents (“aggregated lymph nodules,” Peyer’s patches).

Anterior view, large intestine. The terminal part of the ileum and portions of the transverse and sigmoid mesocolon are shown. The ascending and descending colon have been rotated to display their taeniae. Note: Colorectal cancer, which has become one of the most common cancers in industrialized countries, has a special predilection for the rectosigmoid junction and the rectum itself (i.e., sites distal to the left colic flexure see p. 248).

The various colon segments possess all the morphological characteristics of the large intestine (haustra, taeniae, epiploic appendices, see B). Typically these features disappear past the rectosigmoid junction. As the taeniae disappear, they are replaced on the rectum by a continuous layer of longitudinal muscle fibers. Instead of haustra, the rectum has three permanent constrictions that are produced by internal transverse folds (see p. 241). The peritoneal reflection on the anterior rectal wall represents the site where the peritoneum is reflected onto the posterior wall of the uterus (in the female) or onto the upper surface of the bladder (in the male).

Note: The ascending and descending colon are (secondarily) retroperitoneal and therefore, unlike the sigmoid and transverse colon, they do not have a mesocolon and are covered only anteriorly by peritoneum. The rectum is extraperitoneal in the lesser pelvis, lacks a “suspensory ligament,” and bears other unique features.

B Ileocecal Orifice

Anterior view of a longitudinal coronal section of the cecum and ileum. The ileocecal orifice hermetically seals the terminal ileum from the cecum and prevents the reflux of contents from the large intestine (structural constriction, see A, p. 234). At the ileocecal orifice, the end of the ileum evaginates the circular muscle layer of the large intestine into the cecal lumen. All layers of the ileal wall except the longitudinal muscle and peritoneum contribute to the structure of the ileocecal orifice. The circular muscle layers of the ileum and cecum function as a sphincter, which periodically opens the orifice. This allows the contents of the small intestine to enter the large intestine while effectively preventing reflux. The function of the sphincter is similar to that of the pylorus.

D Wall structure of the colon and cecum

Longitudinal section through the bowel wall. All the typical wall layers of the gastrointestinal canal are present: the mucosa, submucosa, muscularis externa, and serosa (or adventitia in the retroperitoneal parts of the colon, see B, p. 231). There are several features, however, that distinguish the wall structure of the colon and cecum from that of the stomach and small intestine:

• The mucosa is devoid of villi (i.e., the total surface area is not enlarged as much as in the small intestine). Instead of villi, there are large numbers of deep crypts (Lieberkühn crypts), more numerous than in the small intestine.

• The epithelial layer of the mucosa contains large numbers of goblet cells (for clarity, not shown here).

• The colonic mucosal surface undulates in large-scale, crescent-shaped, semilunar folds (see C).

• The muscularis externa consists of an inner circular layer and an outer longitudinal layer, which is concentrated in three longitudinal bands, the taeniae (see p.236).

E Variants in the position of the vermiform appendix

Disturbances in the rotation of the embryonic gut can result in numerous positional variants of the cecum and vermiform appendix. The appendix may even come to lie in the left side of the abdomen. The inflammation of an appendix in the typical position is characterized by tenderness at two points:

• McBurney point: Position on a line connecting the umbilicus and the right anterior superior iliac spine. The McBurney point is one-third of the distance along this line from the iliac spine.

• Lanz point: Position on a line connecting the anterior superior iliac spines. The Lanz point is one third of the distance along this line from the right spine.

Although very useful, these are not definitive clinical signs. Tenderness may be felt at other abdominal sites, especially if the appendix is in an atypical position.

F Wall structure of the vermiform appendix

The vermiform appendix has the typical wall structure of an intraperitoneal intestinal tube. One striking feature is the abundance of lymphatic follicles in the submucosa (also present in the colon and cecum, but in much smaller numbers). With its high degree of immunological activity, the appendix has been characterized as the “intestinal tonsil.” The mucosa has numerous deep crypts that are in intimate contact with the lymphatic follicles in the lamina propria and the submucosa (crypts and lymphatic follicles are not visible here). Since the vermiform appendix is intraperitoneal, it possesses a small mesentery, the mesoappendix, which transmits neurovascular structures.

A Location and curves of the rectum

Anterior view (a) and left anterior view (b). The rectum is 15–16 cm long and extends approximately from the superior border of the third sacral vertebra to the perineum. It is “straight” only in the frontal projection (as shown in a); it presents two flexures in the sagittal projection (see b): the sacral flexure (retroperitoneal) and the perineal flexure (extraperitoneal), which represents the start of the anal canal and is already extraperitoneal. The sacral flexure—conforming to the shape of the os sacrum—is concave anteriorly. The perineal flexure is an important functional component of rectal continence (see p. 242f).

B The rectum in situ

Coronal section of the female pelvis, anterior view, with the rectum opened from about the level of the middle transverse inferior rectal fold. The taeniae of the sigmoid colon are not continued onto the rectum. The constrictions in the outer wall of the rectum correspond to the transverse folds on the inner wall. The rectum (which would appear in this form only if the ampulla were full) is shown in a slightly raised position. Below the levator ani muscle is the powerful external anal sphincter, the muscular component of the rectal continence organ. The pararectal connective tissue below the peritoneal cavity contains numerous vessels that supply the rectum.

This drawing was made from the dissection of a female cadaver. Thus, the peritoneum would be reflected from the anterior wall of the rectum onto the posterior wall of the uterus. Although both the anterior rectal wall and uterus are not visible here (anterior to this plane of section), parts of the rectouterine folds are still visible.

• Rectal ampulla: the lowest portion of the rectum between the middle transverse rectal fold (Kohlrausch fold) and the anorectal junction. The rectal ampulla is the most distensible part of the rectum and, contrary to popular opinion, does not serve as a reservoir for holding stool but is usually empty (see mechanism of defecation, p. 237). The middle transverse fold, which projects into the rectum from its right posterior wall, is approximately 6–7 cm from the anus and can just be reached with the palpating finger. Rectal tumors located below the Kohlrausch fold may therefore be palpable.

• Anal canal: located below the anorectal junction at the distal perineal flexure (see A). It is approximately 4 cm long and normally kept closed by the anal sphincter muscles. The clinically important “surgical anal canal” begins at the level of the anorectal junction and extends to the anocutaneous line, which is also palpable. It is a groove located between the margins of the internal and external anal sphincters (intersphincteric groove) at the junction of the anoderm (white zone), a region with very dense somatic innervation, and the pigmented perianal skin (see p. 242 B). Above the anoderm are located 8–10 longitudinal mucosal folds (anal columns), produced by the arterial cavernous body of the rectum (hemorrhoidal plexus), located in the submucosa (see p. 242). The distal ends of the mucosal folds are connected by valve-like transverse folds (anal valves). All anal valves together form the dentate line, which is an important landmark because it is visible. Behind the anal valves are pocket-like depressions (anal sinuses or pouches of Morgagni), into which empty 6–8 outflow ducts of the rudimentary mucus-secreting anal glands (proctodeal glands). The most common site of these glands is the posterior commissure (approximately at 6 o’clock in the lithotomy position), either in the submucosa or intersphincteric (between the internal and external anal sphincter) space, so that the outflow ducts partially transverse the internal anal sphincter.

Note: Bacterial infections of the glands may cause perianal abscesses and anal fistulas, which are difficult to treat (see p. 247).

A Components of the continence apparatus

Midsagittal section at the level of the anal canal in the male, viewed from the left side.

The continence apparatus, or continence organ, controls the closing (continence) and opening (defecation) of the rectum and provides a tight closure before and after evacuation of solid, liquid and gas bowel contents.

It consists of a distensible hollow organ as well as vascular and muscular continence mechanisms, including their neural control. These angiomuscular continence mechanisms are integrated into a structurally narrow segment, which begins at the level of the perineal flexure and continues along the anal canal:

• Distensible hollow organ:

– rectum with stretch receptors, mainly in the rectal ampulla (viscerosensory innervation)

– anus with distensible skin in the anal canal (somatosensory innervation);

• Muscular continence:

– internal anal sphincter (visceromotor innervation)

– external anal sphincter (somatomotor innervation)

– levator ani, especially the puborectalis muscle (somatomotor innervation);

• Vascular continence:

– hemorrhoidal plexus (permanently distended cavernous tissue that subsides only during defecation;

• Neural control:

– visceral and somatic nervous system (mainly from S2–S4) with the pelvic splanchnic nerves, pudendal nerve and rectal plexus.

Functionally, both continence and defecation are the result of a finetuned feedback loop between receptors and effectors of the continence apparatus with involvement of the central nervous system (see p. 244 f).

B Epithelial regions of the anal canal (after Lüllmann-Rauch)

In the anal canal, the unilayered columnar epithelium of the colorectal mucosa at the level of the transition zone is continuous with the stratified squamous epithelium of the anoderm and perianal skin. The transition occurs near characteristic landmarks. The anal canal can be divided into the following epithelial regions:

• colorectal zone between anorectal junction and supratransitional line; homogeneous colorectal mucosa with crypts;

• transition zone at the level of the anal columns (between supratransitional line and dentate line) mosaic patterns of colorectal mucosa, unilayered columnar epithelium and stratified squamous epithelium;

• squamous zone between dentate line and anocutaneous line: evenly covered by stratified, nonkeratinized squamous epithelium, which is intimately attached to the underlying internal anal sphincter, hence its whitish appearance (white zone). Deep sensory innervation with touch, pressure, temperature and mainly pain receptors (clinically: anoderm);

• perianal skin below the anocutaneous line: start of the stratified squamous epithelium of the outer layer of the skin (heavy pigmentation, eccrine and apocrine sweat glands and hair follicles).

Note: Knowledge about the epithelial regions of the anal canal is important mainly for the differentiation between rectal (usually adenocarcinoma) and anal carcinoma (of keratinizing or non-keratinizing squamous epithelium).

D Structure of the vascular continence mechanism

a Longitudinal section of the anal canal with the hemorrhoidal plexus windowed; b and c Hemorrhoidal plexus at rest and during defecation. Above the dentate line at the level of the anal columns in the submucosa lies a cavernous body, the hemorrhoidal plexus. Its elasticity largely ensures liquid- and gas-tight closure of the rectum. The circular configuration of the hemorrhoidal plexus is similar in structure to the cavernous body of the penis but differs in that it is permanently distended. The hemorrhoidal plexus is a network of cavernous tissue and is almost exclusively supplied by three branches of the superior rectal artery (at 3, 7 and 11 o’clock in the lithotomy position), which further divide near the anal columns (see p. 273). Blood reaches the venous drainage system via arteriovenous anastomoses through transsphincteric veins—largely along the internal anal sphincter—and reaches the drainage area of the inferior mesenteric vein (and is carried to the portal vein) but also partially through the middle and inferior rectal veins to the perianal veins of the external venous plexus. When the sphincter apparatus relaxes during defecation, it allows blood to drain from the hemorrhoidal plexus. Note: Abnormal dilation (hyperplasia) of the hemorrhoidal plexus beyond the physiological range leads to hemorrhoidal disease, one of the most common proctological disorders (see p. 246 f).