A Projection onto the trunk

Anterior view.

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

D Shape and anatomical divisions

Anterior view of the paries anterior. The body (corpus) of the gaster is the largest part of the gaster. It terminates blindly at the fundus gastricus, which in the standing patient is the highest part of the gaster and is usually filled with air (visible on radiographs as the “gastric bubble”).

Note: The cardia is the area of the gastric inlet where the oesophagus opens into the gaster (at the ostium cardiacum). While the oesophagus is invested by adventitial connective tissue (tunica adventitia), the gaster has a visceral peritoneal covering or tunica serosa. The transition from tunica adventitia to tunica serosa is sharply defined, and occasionally the tunica serosa continues a short distance onto the lower end of the oesophagus.

The part of the gaster that opens into the duodenum, the pars pylorica, consists of a broad antrum pyloricum, a narrow canalis pyloricus, and the pylorus itself (ostium pyloricum). The stratum circulare of the tunica muscularis of the gaster is markedly thickened at the end of the canalis pyloricus to form the m. sphincter pyloricus (not visible here), which produces a visible external constriction of the canalis pyloricus.

E Interior of the gaster

Anterior view of the gaster with the paries anterior removed. For clarity, small portions of the oesophagus and duodenum are also shown. The gastric tunica mucosa forms prominent folds (plicae gastricae) that serve to increase its surface area. These folds are directed longitudinally toward the pylorus, forming “canales gastrici”. The plicae gastricae are most prominent in the corpus gastricum and along the curvatura major and diminish in size toward the pyloric end. The tunica mucosa imparts a glossy sheen to the stomach lining. Note: The ostium pyloricum is quite large in this dissection. Normally, the orifice usually opens to a luminal diameter of only 2–3 mm.

A Muscular layers

Anterior view of the paries anterior gastricus with the tunica serosa and tela subserosa removed. The tunica muscularis of the gaster 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 tunica muscularis consists not of two layers (as in other hollow organs of the gastrointestinal tract) but of three muscular layers:

• An outer stratum longitudinale, which is most pronounced along the curvatura major (greatest longitudinal expansion)

B Structure of the gaster wall and gll. gastricae

a The structure of the gaster wall illustrates the layered wall structure that is typical of the hollow organs throughout the gastrointestinal tract. The gaster is unique, however, in that its tunica muscularis consists of three rather than two layers (see A).

Note: The tunica serosa (visceral layer of the peritoneum) and tela subserosa (connective-tissue layer giving attachment to the tunica serosa and transmitting neurovascular structures for the tunica muscularis) are present only in areas where the organ in question is covered by peritoneum viscerale. In wall areas that lack a peritoneal covering (e.g., large portions of the duodenum and colon), the tunica serosa and tela subserosa are replaced by a fibrous tunica adventitia, which connects the wall of the organ to the connective tissue of surrounding structures.

The tunica mucosa contains specialized cells that are aggregated into glandulae (visible microscopically). The glandular orifices open at the base of the foveolae gastricae (see b). In the corpus and pylorus of the gaster these glands extend down to the muscular layer of the mucosa, the lamina muscularis mucosae (deeper glands = more cells = higher secretory output). The tela submucosa (layer of connective tissue transmitting neurovascular structures for the tunica muscularis) contains the plexus submucosus for visceromotor and viscerosensory control of the hollow organs in the gastrointestinal tract. This plexus, like the plexus myentericus (located in the tunica mucosa for visceromotor control of the visceral muscle, not shown here), is part of the enteric nervous system (plexus entericus), which contains, in total, millions of scattered ganglion cells.

b Structure of the gll. gastricae (after Lüllmann-Rauch) (simplified schematic of a gland from the corpus gastricum). Several types of cells are distinguished in the fundus and corpus gastricum:

• Surface epithelial cells: cover the surface of the tunica mucosa and secrete a mucous film.

• Neck cells: produce mucin to strengthen the mucous film (make it more anionic).

• Parietal cells: produce HCl and intrinsic factor, which is necessary for vitamin B₁₂ absorption in the ileum.

• Chief cells: produce pepsinogen, which is converted to pepsin (for protein breakdown) in the stomach.

• Enteroendocrine cells: different subtypes producing gastrin (G cells), somatostatin (D cells), or other factors controlling motility and secretion

• Stem cells: reservoir for replenishing the surface epithelial cells and gland cells

C Endoscopic appearance of the gastric tunica mucosa

a, b Healthy gastric tunica mucosa with a glistening surface; c Gastric ulcer.

a View into the corpus gastricum, which has been moderately distended by air insufflation. The tunica mucosa is raised into prominent, tortuous plicae gastricae that form the canales gastrici.

b Inspection of the antrum pyloricum shows less prominent folds than in the corpus gastricum.

c Fibrin-covered gastric ulcer with hematin spots. A gastric ulcer is defined as a tissue defect that extends at least into the lamina muscularis mucosae, but many ulcers extend much deeper into the stomach wall. Most gastric ulcers are caused by infection with Helicobacter pylori, a bacterium that is resistant to stomach acid (from Block, Schachschal and Schmidt: The Gastroscopy Trainer. Stuttgart: Thieme, 2004).

A Projected onto the columna vertebralis

The duodenum is a C-shaped loop of intestinum tenue lying predominantly on the right side of the columna vertebralis in the right upper quadrant (RUQ) and encompassing the L 1 through L 3 vertebrae and occasionally extending to L 4. The concavity of the duodenum normally encloses the caput pancreatis at the L 2 level (see D).

B Parts of the duodenum

Anterior view. The anatomical parts of the duodenum (partes superior, descendens, horizontalis, and ascendens with intervening flexures) have a total length of approximately 12 fingerwidths (L. duodeni = “twelve at a time”). Note the lig. suspensorium duodeni (called also the ligament of Treitz), which often contains smooth-muscle fibers. Mobile loops of intestinum tenue may wrap around this ligament and become entrapped between the ligament and the vessels behind it (most notably the pars abdominalis aortae). This “Treitz hernia” may cause mechanical obstruction of the affected bowel loop and strangulate its blood supply.

C Wall structure and duct orifices

D The duodenum in situ

Anterior view. The gaster, hepar, intestinum tenue, and large portions of the colon transversum have been removed. The retroperitoneal fat and connective tissue, including the capsula adiposa, have been substantially thinned. The caput pancreatis lies in the concavity of the C-shaped loop of the duodenum. The first 2 cm of the pars superior of the duodenum is still intraperitoneal (attached to the hepar by the lig. hepatoduodenale), but most of the duodenum is retroperitoneal. Owing largely to the proximity of the duodenum and the caput pancreatis, lesions of the pancreas (tumors) or malformations (anular pancreas) may cause duodenal obstruction. The peritoneum at the duodeno-jejunal junction forms the recessus duodenales superior and inferior. Mobile loops of intestinum tenue may enter these peritoneal recesses and become entrapped there (internal hernia), causing a potentially life-threatening bowel obstruction.

E Endoscopic view

The endoscope is pointing down into the pars descendens of the duodenum. The papillary region where the ductus biliaris and ductus pancreaticus open into the duodenum is visible on the left side of the image at approximately the 10 o’clock position. The plicae circulares (valves of Kerckring) are typical of those found in the intestinum tenue, diminishing in size in the proximal to distal direction (from Block, Schachschal and Schmidt: Endoscopy of the Upper G I Tract. Stuttgart: Thieme, 2004).

F Histological structure

A Parts of the intestinum tenue: overview (a) and anatomical constrictions (b)

D The jejunum and ileum in situ

Anterior view. The abdominal wall has been opened and the colon transversum has been reflected upward. Coils of jejunum and ileum completely fill the four quadrants of the cavitas peritonealis below the mesocolon transversum and are framed by the colon segments. In this dissection the loops of intestinum tenue have been displaced slightly to the left in front of the colon descendens, hiding it from view. The colon ascendens and caecum are visible along the right flank of the abdomen.

A Projection of the intestinum crassum onto the skeleton

Because of the embryonic rotation of the primary intestinal (midgut) loop, the intestinum crassum typically forms a frame encompassing the intestinum tenue. The position and length of the colon segments may vary, however, depending on the course of intestinal rotation. For example, when the intestinal loop rotates normally, the colon ascendens acquires a “normal” length (as shown here). If intestinal rotation is incomplete, the colon ascendens is shortened. The colon transversum is particularly mobile owing to its mesocolon, while the cola ascendens and descendens are less mobile because they are fixed to the posterior wall of the cavitas peritonealis. The flexura coli sinistra usually occupies a somewhat higher level than the flexura coli dextra due to the space occupied by the large lobus hepatis dexter. Also, the colon descendens is usually more posterior than the colon ascendens.

B Distinctive morphological features of the intestinum crassum

There are four morphological features—three visible externally and one internally—that distinguish the intestinum crassum from the intestinum tenue. It should be noted that these features do not occur equally in all parts of the intestinum crassum and are absent in the caecum, appendix vermiformis, and rectum.

C The three taeniae of the colon

Sagittal section, viewed from the left side. The three taeniae are named for their position on the colon:

• Taenia libera (free taenia)

• Taenia omentalis (the taenia at the attachment of the omentum majus)

• Taenia mesocolica (the taenia at the attachment of the mesocolon)

D Anatomical divisions of the intestinum crassum

E Intestinum crassum: Segments, shape, and distinctive features

Anterior view, intestinum crassum. The pars terminalis of the ileum and portions of the mesocola transversum and sigmoideum are shown. The cola ascendens and transversum 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 flexura coli sinistra).

A Caecum and pars terminalis ilei

Anterior view. The caecum is unique in its end-to-side connection with the pars terminalis of the intestinum tenue (ileum) and the presence of the appendix vermiformis. As a result, there are two openings in the wall of the caecum: the ostium ileale on a small papilla (papilla ilealis) and, below that, the ostium appendicis vermiformis. The ostium ileale is approximately round in the living individual but is often slit-like in the postmortem condition. It is bounded by superior and inferior flaps or “lips,” the labrum ileocolicum (labrum superius) and the labrum ileocaecale (labrum inferius). Both are continued as a narrow ridge of mucosa, the frenulum ostii ilealis.

Note: Inflammation of the appendix vermiformis (appendicitis) is one of the most common surgically treated diseases of the gastrointestinal tract. If acute appendicitis goes untreated, the inflammation may perforate into the free cavitas peritonealis (a “ruptured appendix” in popular jargon). This creates a route by which bacteria in the bowel lumen can enter the cavitas peritonealis and gain access to the large peritoneal surface, quickly inciting a life-threatening inflammation of the peritoneum (peritonitis).

B Ostium ilealis

C Interior of the colon

The interior of the colon is marked by transversely oriented folds called plicae semilunares. They are formed by the shortness of the muscular taenia of the colon wall and are visible externally as anular constrictions. The sacculations between the folds are the haustra coli. The plicae semilunares are inconstant features that depend on the muscular tension in the taenia. The plicae and haustra move slowly down the colon with waves of peristaltic activity.

D Wall structure of the colon and caecum

A Location and curves of the rectum

D Rectum and canalis analis: Divisions, internal surface and wall structure

Anterior view of the rectum in coronal section with the anterior wall removed. Instead of plicae semilunares, the rectum contains three permanent plicae transversae. The distal portion of the rectum, also known as the anorectum, is recognizable by a palpable protrusion (m. puborectalis sling or junctio anorectalis) which is visible on the mucosal surface. The anorectum is divided into two segments, the ampulla recti and the canalis analis.

A Components of the continence apparatus

Midsagittal section at the level of the canalis analis 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 flexura perinealis and continues along the canalis analis:

• Distensible hollow organ:

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

– anus with distensible skin in the canalis analis (somatosensory innervation);

• Muscular continence:

– m. sphincter ani internus (visceromotor innervation)

– m. sphincter ani externi (somatomotor innervation)

– m. levator ani, especially the m. puborectalis (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 nn. splanchnici pelvici, n. pudendus and plexus rectales.

C Structure of the muscular continence mechanism

a Midsagittal section, viewed from the left side; b M. puborectalis sling and anorectal angle: relaxed muscle (left) and contracted muscle (right).

The complex system of anal sphincters involves both smooth and striated muscles. Whereas the smooth muscles represent the direct continuation of the muscles of the rectal wall, the striated muscles are formed by specialized areas of the pelvic floor muscles. Thus, these muscular continence mechanisms are maintained under both somatic-voluntary and visceral-involuntary control.

Involuntarily innervated smooth muscles:

• M. sphincter ani internus: most significant smooth muscle; as the continuation of the stratum circulare tunicae muscularis of the rectum, it forms a strong circular ring. Sympathetic nerve fibers and the absence of enteric ganglion cells (aganglionosis) allow it to maintain constant tonic activity to help constrict the canalis analis (the internal sphincter is responsible for 70% of fecal continence);

• Lamina muscularis mucosae of the canalis analis: as the continuation of the lamina muscularis mucosae, it extends beyond the hemorrhoidal plexus and ends at the linea pectinata; stabilizes the hemorrhoidal plexus and holds it in place;

• Corrugator ani: as the continuation of the stratum longitudinale of the tunica muscularis of the rectum, the muscle fibers extend beyond the canalis analis, permeate through the subcutaneous part of the m. sphincter ani externus and insert into the perianal skin. M. corrugator ani owes its name to the fact that muscle contraction produces radial wrinkles on the perianal skin.

Voluntarily innervated striated muscles:

• M. sphincter ani externus: cylindrical muscle that encircles the outside wall of the canalis analis, made up of three recognizable parts: partes profunda, superficialis, and subcutanea. Whereas the partes profunda and subcutanea are arranged in circular layers, the pars superficialis extends between the anteriorly located corpus perineale and the posterior lig. anococcygeum and surrounds the canalis analis and serves as a clamp. It is largely composed of type I fibers, which are slow, durable, and fatigue resistant.

• M. puborectalis: as the innermost portion of the m. levator ani, it forms a strong sling of muscle, which loops around the rectum at the level of the junctio anorectalis and is closely aligned to the pars profunda of the m. sphincter ani externus. It arises from the fixed end of the os pubis, so when the m. puborectalis contracts it creates a “kink” between canalis analis and rectum at the anorectal angle.

D Structure of the vascular continence mechanism

A Innervation (after Stelzner)

a Somatomotor and somatosensory innervation; b Visceromotor and viscerosensory innervation.

• Somatomotor: n. pudendus for the m. sphincter ani externus, levator nerves for the m. levator ani (especially the m. puborectalis). They provide active, partially voluntary innervation of the m. sphincter ani externus and m. levator ani.

• Somatosensory: nn. rectales inferiores for the anus and perianal skin. Arising from the n. pudendus, they transmit touch and especially pain sensation. The skin of the anus is extremely sensitive to pain. Even small tears in the anal skin, which often show inflammatory changes, tend to be extremely painful.

• Visceromotor: nn. splanchnici pelvici (S2–S4) for the m. sphincter ani internus. The resting tone of the internal sphincter helps to maintain closure of the canalis analis and inhibits venous drainage from the hemorrhoidal plexus; the corpus cavernosum remains distended, contributing to fecal continence and flatus control. Topographically, the nn. splanchnici pelvici are closely related to the plexus rectales.

• Viscerosensory: nn. splanchnici pelvici (S2–S4) supply the wall of the rectum, particularly the stretch receptors in the ampulla recti. Stretching of the ampulla by the fecal column triggers a subjective awareness of the need to defecate.

B Mechanism of defecation (after Wedel; see right page)

a Filling of the ampulla recti; b Relaxation of the voluntarily controlled sphincters and propulsion of fecal column.

Both defecation and continence are under central nervous system control involving different anatomical structures ranging from the cortex cerebri to the perianal skin, with the anorectum being one of multiple effectors. Directly involved are the diaphragma pelvis, muscles used during squatting, the abdominal press as well as autonomic and sensory nerves along with their higher nerve centers.

Filling of the ampulla recti and stimulation of local stretch receptors in the ampullary wall: When the fecal bolus is propelled into the ampulla recti by anterogradely propagating waves, mechanoreceptors detect distension and transmit the information via visceral afferents in the funiculus posterior medullae spinalis to the sensory cortex, which perceives the urge to defecate. Olfactory, visual, or acoustic stimuli can either accelerate or decelerate the perception and subsequent voluntary action, which results in defecation.

Rectoanal inhibitory reflex and relaxation of the voluntarily innervated sphincters: When the ampulla fills with feces, the intrarectal pressure increases and the m. sphincter ani internus relaxes, followed by voluntary relaxation of the m. puborectalis sling and the m. sphincter ani externus. As a result, the anorectal angle straightens and the canalis analis widens.

Propulsion of fecal column: Rectal evacuation is assisted by a direct involuntary increase in pressure in the rectal area and by simultaneous increase in pressure by the contraction of voluntarily innervated muscles: abdominal (abdominal press), perineal (pelvic floor lift), diaphragmatic (diaphragma contraction) and glottic (glottis closure) muscles. The squatting position further increases abdominal pressure (flexor reflex). With the propulsion of the fecal column, the hemorrhoidal cushions are drained and pushed out.

Completion of defecation: After the sphincter apparatus allows the fecal column to pass through, it comes in contact with the highly sensitive anoderm, which perceives the volume, consistency, and location of the stool. This perception initiates the voluntary process of completing defecation. Defecation is completed once the sphincter apparatus contracts and the hemorrhoidal plexus fills up.

A Hemorrhoidal disease

Hemorrhoidal disease is one of the most common proctological disorders. The site of origin is the circular hemorrhoidal plexus of the cavernous body of the rectum located above the linea pectinata. It is largely responsible for the fine adjustment of anal continence. Hemorrhoid is a general term used to describe hyperplasia (enlargement) of a corpus cavernosum with arterial blood supply, a condition which initially does not cause any symptoms. Hemorrhoids become pathological once they become symptomatic (bleeding is bright red from arterial blood, mucus discharge, itching, burning, fecal soiling, etc.) and when they require treatment (hemorrhoidal disease). Most commonly, hemorrhoids result from increased pressure on the anus during defecation, often caused by chronic constipation as a result of a lack of fiber and fluids in the diet. Another cause is impaired venous return due to increased m. sphincter ani tone as this may lead to the hemorrhoidal plexus taking on a gnarled appearance. Diagnosis and classification of hemorrhoids are based on examination, palpation, and proctoscopy of the canalis analis. Depending on the severity of the hemorrhoids and their symptoms, they are divided into four grades:

• Grade I (a): swollen, elastic cushions of tissue that are visible only on proctoscopy (located above the linea pectinata) and may cause painless bright red bleeding (painless because the swollen cushions are located above the anoderm);

• Grade II (b): visibly hyperplastic vascular cushions, which can prolapse inside or outside of the canalis analis during defecation or while pressing but retract immediately after emptying the bowels. Dripping blood and mucus discharge may cause oozing or itching, a condition also known as perianal eczema;

• Grade III (c): during defecation or while the intra-abdominal pressure is increased, hemorrhoids prolapse spontaneously and require manual repositioning. Possible thrombosis or incarceration of the prolapsed knot may cause significant pain;

• Grade IV (d): at this stage, the nodular enlargements and large parts of the canalis analis, including the highly pain-sensitive anoderm, are permanently prolapsed (irreducible) and attached to the anal margin (also known as an anal prolapse).

Note: Unlike in the German medical terminology, the Anglo-American and Swiss terminology distinguish between internal and external hemorrhoids. Internal hemorrhoids originate from the internal plexus venosus rectalis, external hemorrhoids are subcutaneous clots at the margin of the anus (e.g., perianal thromboses). In our assessment, however, external hemorrhoids are simply hyperplastic vascular cushions of the plexus venosus rectalis with their arterial blood supply that have prolapsed to the outside.

B Conditions of the perianal skin with and without hemorrhoidal disease

a Normal anatomy of the perianal area in a 38 year old female patient;