Alimentary system

17 Alimentary system





ANTERIOR ABDOMINAL WALL





Muscles of the anterior abdominal wall


A knowledge of the anatomy of the muscles of the abdominal wall (Fig. 17.1) is a prerequisite to understanding the basis of abdominal incisions. The abdominal wall consists principally of three sheets of muscle which are fleshy laterally, and aponeurotic in front and behind. As the aponeuroses pass forward they ensheath the rectus abdominis muscle.










ANATOMY OF ABDOMINAL INCISIONS


The anatomy of the common abdominal incisions only will be described.








INGUINAL CANAL


The inguinal canal (Fig. 17.3) is an oblique passage in the lower part of the abdominal wall which transmits the spermatic cord and ilio-inguinal nerve in the male, and the round ligament of the uterus and the ilio-inguinal nerve in the female. It is approximately 4 cm long and passes downwards and medially from the deep inguinal ring to the superficial inguinal ring lying above and parallel to the inguinal ligament.






FEMORAL CANAL


The femoral artery and femoral vein enter the femoral triangle deep to the inguinal ligament within a prolongation of fascia termed the femoral sheath. This is derived from the transversalis fascia anteriorly, and posteriorly from the fascia covering iliacus. The medial part of the femoral sheath is occupied by the femoral canal. The upper opening of the femoral canal is called the femoral ring and will just admit the tip of the little finger in the male. In the female the pelvis is wider and the canal, therefore, is larger, and femoral herniae are consequently more common in the female. The boundaries of the femoral ring are:






The femoral canal contains fat, some lymphatics and a lymph node (Cloquet’s node). The canal functions as a dead space for expansion of the femoral vein and secondly as a pathway for lymphatics from the lower limb to the external iliac nodes. The femoral ring is narrow, and the lacunar ligament forms a sharp medial border. Because of this, irreducibility and strangulation occur commonly with femoral hernias. Also, femoral hernias are likely to be of the Richter’s type.




PERITONEAL CAVITY


The peritoneum is the serous membrane of the abdominal cavity. It consists of a parietal layer lining the abdominal and pelvic walls, and a visceral layer which more or less covers the contained organs. In the male the peritoneal cavity is a closed sac, but in the female the free extremities of the uterine tubes open into the cavity, constituting a possible pathway of infection from the exterior. The peritoneal cavity is subdivided into a main cavity, the greater sac, and a small cavity, the lesser sac (omental bursa). The greater sac is further divided by the transverse colon into a supracolic and infracolic compartment. The connection between the greater and lesser sac is known as the epiploic foramen or the foramen of Winslow.


The attachments of the peritoneum are complicated. It is convenient to start at the umbilicus and work down. Below the level of the umbilicus, the parietal peritoneum is smooth apart from some folds (Fig. 17.4). These are the median umbilical fold onthe median umbilical ligament (which is due tothe obliterated urachus passing from the bladder to the umbilicus), the medial umbilical folds on the obliterated umbilical arteries, and the lateral umbilical folds which are further lateral and contain the inferior epigastric arteries. The peritoneum of the pelvis is continuous with that of the abdominal cavity. It completely encloses the sigmoid colon, forming the pelvic mesocolon. It is applied to the front and side of the upper third of the rectum and to the front only of the middle third of the rectum. It is then reflected in the male onto the base and upper part of the bladder, forming the rectovesical pouch. In the female the peritoneum is reflected from the side and front of the rectum, to the upper part of the posterior wall of the vagina and then over the posterior upper and anterior surface of the uterus to the bladder. Between the uterus and the rectum is the recto-uterine pouch (of Douglas). The peritoneum passes off the lateral margins of the uterus to the pelvic wall, forming the broad ligaments, the upper borders of which contain the uterine tubes. The free upper margins of the broad ligament lateral to the uterine tubes form the infundibulopelvic fold.



Returning to the umbilicus, the falciform ligament, the sickle-shaped fold of peritoneum, passes upwards and slightly to the right of the midline to the liver. It contains the ligamentum teres, i.e. the obliterated umbilical vein, in its free edge, and this passes into the groove between the quadrate lobe and left lobe of the liver. Traced superiorly the two layers of the falciform ligament diverge from each other, the right limb joins the upper layer of the coronary ligament while the left layer passes to the left to form the anterior layer of the left triangular ligament. Elsewhere on the anterior abdominal wall, above the umbilicus, the peritoneum sweeps upwards and over the inferior aspect of the diaphragm to be reflected onto the liver and onto the right margin of the abdominal oesophagus. Details of the peritoneal reflections of the liver are described in the section on the liver. After enclosing the liver the peritoneum descends from the porta hepatis as a double layer, i.e. the lesser omentum, and then this separates to enclose the stomach. It reforms again at the greater curve and then loops downwards, again turning upwards and attaching to the length of the transverse colon, forming the greater omentum (Fig. 17.5).



The lower leaf of the greater omentum then continues upwards, enclosing the transverse colon within the peritoneum, and then passes upwards and backwards as the transverse mesocolon, a double layer of peritoneum, to the posterior abdominal wall, where it attaches along the anterior aspect of the pancreas. At the base of the transverse mesocolon, this double layer of peritoneum divides once again, the upper leaf passing upwards over the posterior abdominal wall to reflect onto the liver, while the lower leaf passes over the lower part of the posterior abdominal wall to cover the pelvic viscera and to join with the peritoneum of the anterior abdominal wall. However, the peritoneum of the posterior abdominal wall is interrupted as it is reflected along the small bowel from the duodenal jejunal flexures to the ileocaecal junction, forming the mesentery of the small intestine. The lines of peritoneal reflection on the posterior abdominal wall are shown in Fig. 17.6.



The lesser sac (Fig. 17.7) is entered via the epiploic foramen or foramen of Winslow. The lesser sac is a potential space lying behind the lesser omentum and stomach and projecting downwards to the transverse mesocolon. Superiorly is the superior recess, whose anterior border is the caudate lobe of the liver. The left wall of the lesser sac is formed by the spleen and the gastrosplenic and lienorenal ligaments. To the right the sac opens into the main peritoneal cavity via the epiploic foramen.



The epiploic foramen has the following boundaries (Fig. 17.8).









OESOPHAGUS


The oesophagus (Fig. 17.9) extends from the lower border of the cricoid cartilage to the cardiac orifice of the stomach. It is about 25 cm long. It has three parts: cervical, thoracic and abdominal.











STOMACH


The stomach is approximately J-shaped, having two surfaces: the anterior and posterior. It has two curvatures – the greater and lesser curve – and two orifices: the cardia and the pylorus. Initially the stomach projects to the left, the dome-like gastric fundus projecting above the level of the cardia. In the erect living subject the vertical part of the J shape of the stomach represents the upper two-thirds of the stomach. The lesser curvature of the stomach is vertical in its upper two-thirds but then turns upwards and to the right, where it becomes the pyloric antrum. The junction of the body with the pyloric antrum is marked along the lesser curve by a distinct notch termed the incisura angularis. Between the cardia and pylorus lies the body of the stomach, leading to the pyloric antrum which is a narrow area of the stomach immediately before the pylorus. The left margin of the body of the stomach is the greater curvature. In the erect subject this may reach or lie below the umbilicus. It then passes upwards to the right as the lower margin of the pyloric antrum. To the lesser curvature of the stomach is attached the lesser omentum and to the greater curvature the greater omentum, which to the left is continuous with the gastrosplenic ligament. The thickened pyloric sphincter is easily palpable at surgery and surrounds the pyloric canal. The junction of the pylorus with the duodenum is marked by a constant prepyloric vein of Mayo which crosses it vertically at this level. Unlike the cardiac sphincter of the stomach the pyloric sphincter is well marked anatomically.






Nerve supply


The clinically important nerve supply (Fig. 17.14) of the stomach is the vagus nerves. The anterior and posterior vagus nerves enter the abdomen through the oesophageal hiatus. The anterior vagus nerve liesclose to the wall of the oesophagus and upper part of the stomach, but the posterior nerve is at a little distance from it. The anterior vagus runs caudally and supplies the anterior surface and lesser curve of the stomach. Before it reaches the stomach, it gives off a hepatic branch which passes in the lesser omentum to the liver and gall bladder and the pyloric branch to the pyloric sphincter. The posterior vagus nerve gives off a coeliac branch which passes to the coeliac plexus before sending a gastric branch to the posterior surface of the stomach. The gastric divisions of both anterior and posterior vagi reach the stomach at the cardia and descend along the lesser curve between the anterior and posterior peritoneal attachments of the lesser omentum. These nerves are referred to as the anterior and posterior nerves of Latarjet.



The vagus nerves used to be divided in operations for peptic ulceration. However, with the advent of H2 receptor antagonists and proton pump inhibitors and the discovery of the role of H. pylori in the aetiology of peptic ulceration, these operations are performed less and less. However, it is necessary to understand the role of the vagus, as vagotomy is still required in surgery for bleeding peptic ulcer, and also a knowledge of the oesophageal hiatus and the relations of the vagus nerve is required so that these nerves are not inadvertently damaged in repair of hiatus hernia. The vagus nerve constitutes both the motor and secretory nerve supply for the stomach, i.e. it is responsible for motility and control of gastric secretions. When the nerve is divided in the operation of vagotomy, acid secretion is cut down in the stomach, but so is motility, so that the stomach empties through an intact pylorus only with difficulty. Because of this, total vagotomy (truncal vagotomy) must always be accompanied by some form of drainage procedure: either a pyloroplasty to destroy the pyloric sphincter or a gastrojejunostomy to bypass the pyloric sphincter. In the operation of highly selective vagotomy (proximal selective vagotomy) it is possible to avoid the drainage procedure, as the nerve of Latarjet remains intact and this maintains the innervation of the pyloric antrum and hence its propulsive activity.



Structure of the gastric mucosa


The surface of the gastric mucosa is covered by columnar epithelial cells that secrete mucus and alkaline fluid that protects the epithelium from mechanical injury and from gastric acid. The surface of the mucosa is studded with gastric pits, each pit being the opening of a duct into which the gastric glands empty. The gastric mucosa can be divided into three areas. The cardiac gland area is the small segment located near the gastro-oesophageal junction. Histologically it contains principally mucus-secreting cells, although occasionally a few parietal (oxyntic) cells are present. The remainder of the stomach is divided into the acid-secreting region (oxyntic gland area) and the pyloric gland area. The oxyntic gland area is the portion containing the parietal (oxyntic cells) and the chief (zymogen) cells. The pyloric end area constitutes the distal 30% of the stomach and contains G cells that produce gastrin. In this region there are few oxyntic and peptic cells, mucus-secreting cells predominating. As in the rest of the gastrointestinal tract the muscular wall of the stomach is composed of an inner circular layer and an outer longitudinal layer. However, in addition there is an incomplete inner layer of obliquely situated fibres which is more prominent near the lesser curvature. Figure 17.15 shows the histological featuresof the mucosa in the oxyntic gland area. Each gastricpit drains between three and seven tubular gastric glands. The neck of the gland contains many mucus cells, oxyntic cells being most numerous in the midportion of the glands and chief cells predominating in the basal portion.




DUODENUM


The duodenum is C-shaped. It curves around the head of the pancreas and is approximately 25 cm long. The first 2–3 cm of the first part of the duodenum is completely covered with peritoneum, but then the duodenum becomes retroperitoneal. The duodenum is divided into four parts.








SMALL INTESTINE


The length of the small intestine is variable, averaging some 6 m in length. The upper half of the small intestine is termed the jejunum, the remainder being termed the ileum, although the distinction between the two is not sharply defined. The jejunum and ileum lie in thefree edge of the mesentery. The mesentery of the small intestine is about 15 cm long and is attached across the posterior abdominal wall. It commences at theduodeno-jejunal juncture to the left of the second lumbar vertebrae and passes obliquely downwards to the right sacroiliac joint. From left to right the root of the mesentery crosses anterior to the following structures:









The mesentery contains the superior mesenteric vessels which enter the mesentery anterior to the third part of the duodenum, the lymph nodes draining the small intestine, and autonomic nerve fibres.


At surgery it is necessary to distinguish between the jejunum and the ileum. The following factors serve to distinguish the jejunum from the ileum.







LARGE INTESTINE


The large intestine extends from the ileocaecal junction to the anus. It is approximately 1.5 m in length on average. It consists of the caecum, ascending colon, transverse colon, descending colon, sigmoid colon and rectum. The caecum is a dilated blind-ended pouch situated in the right iliac fossa and is usually completely covered with peritoneum. The ileocaecal valve lies on the left side of the junction between the caecum and ascending colon. Tumours may grow to a large size in the caecum without causing any obstruction until they encroach on the ileocaecal junction. The appendix arises from the posteromedial aspect of the caecum about 2.5 cm below the ileocaecal valve. The taenia coli, three flattened bands of longitudinal muscle which pass from caecum to rectosigmoid, converge at the base of the appendix. The taenia are shorter than the length of the bowel hence the sacculated appearance of the large bowel.


The ascending colon extends from the caecum to the undersurface of the liver where, at the hepatic flexure, it turns left to become the transverse colon. It is covered on its anterior and lateral aspect by peritoneum. Posterior relations include iliacus, quadratus lumborum, and the perirenal fascia over the lateral aspect of the kidney.


The transverse colon passes to the left, where it becomes the descending colon at the splenic flexure. It is attached to the anterior border of the pancreas by the transverse mesocolon. Superiorly it is related to the liver, gall bladder, greater curvature of the stomach and the spleen. Inferiorly are coils of small intestine. Anteriorly lie the anterior layers of the greater omentum. Posteriorly lie the right kidney, second part of the duodenum, pancreas, small intestine and the left kidney.


The descending colon passes from the splenic flexure to the sigmoid colon. Peritoneum covers its anterior and lateral surfaces. Between the splenic flexure and the diaphragm is a fold of peritoneum, the phrenicocolic ligament. Posteriorly to the descending colon lies the left kidney, quadratus lumborum and iliacus. Anteriorly lie coils of small bowel.


The sigmoid colon commences at the pelvic brim and extends to the rectosigmoid junction. It has a mesentery which occasionally is extensive allowing the sigmoid colon to hang down into the pelvis. The root of the sigmoid colon crosses the external iliac vessels and left ureter. The sigmoid loop rests on the bladder in the male and is related to the uterus and the posterior fornix of the vagina in the female. Hence the development of vesicocolic and vaginocolic fistulae in diverticular disease of the sigmoid colon. The taenia coli extend from the base of the appendix to the rectosigmoid junction. There are no taenia on the appendix or rectum. The colon, but neither the caecum, the appendix, nor rectum, possesses fat-filled peritoneal tags scattered along its surface. These are called appendices epiploicae and are most numerous in the sigmoid colon.




RECTUM


The rectum is about 12 cm long, commencinganterior to the third segment of the sacrum and ending about 2.5 cm in front of the coccyx, where it bends sharply backwards to become the anal canal. It isextraperitoneal on its posterior aspect in its upper third and extraperitoneal on its posterior and lateral aspect in its middle third. The lower third is completely extraperitoneal lying below the pelvic peritoneum. The rectum is curved to follow the contour of the sacralhollow. There are three lateral inflexions projected to the left, right and left again from above downwards. Each inflexion is capped by a valve of Houston.


The relations of the rectum are important in the understanding of a digital rectal examination and also in the spread of rectal cancer. Anteriorly in the male lies the rectovesical pouch, the base of the bladder, seminal vesicles and the prostate. A layer of fascia (of Denonvilliers) lies in front of the rectum, separating it from the anterior structures, and this is the plane of dissection which must be sought during abdominoperineal excision of the rectum. In the female lies the recto-uterine pouch (of Douglas) and the posterior wall of the vagina. The upper two-thirds of the rectum is covered with peritoneum anteriorly and related to coils of small bowel and the sigmoid colon in the rectovesical or recto-uterine pouch. Posteriorly lie the sacrum, coccyx and middle sacral artery. The lower sacral nerves also lie posteriorly and may be invaded by rectal cancer spreading posteriorly and resulting in sciatic pain. Laterally, below the peritoneal reflection, lie the levator ani and coccygeus.


Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Alimentary system
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