Abdominal cavity

Abdominal cavity 

Abdominal cavity is the largest cavity. It encloses the peritoneal cavity between its parietal and visceral layers.Parietal layer clings to the wall of parities while visceral layer is intimately adherent to viscera concerned. So their vascular supply and nerve supply are same as the parities and viscera respectively. There are very lengthy organs in the peritoneal cavity. These had to be disciplined with limited  movements for proper functioning of the gut in particular and the body in general. Infections involving the parietal peritoneum impart protective "board-like rigidity" to the abdominal wall. Referred pain from the viscera to a distant area is due to somatic and sympathetic nerves reaching the same spinal segment. The abdominal cavity is an extensive space which extends upwards, deep to the coastal margin,into the concavity of the diaphragm; and projects downwards and backwards into the body pelvis as pelvic cavity. Thus a considerable part of the abdominal cavity is overlapped by the thoracic cage above, and the bony pelvis below.

Nine regions of abdomen

For the purpose of describing the location of viscera,the abdomen is divided into nine regions by four imaginary planes, two horizental and two vertical. The horizontal palnes are the transpyloric and transtubercular planes. The vertical planes are the right lateral and the left lateral planes. the transpyloric plane of addison passes midway between the suprasternal notch and the pubic symphysis. It lies roughly a hand's breadth below the xiphisternal joint.Anteriorly,it passes through the tips of the ninth costal cartilage;and posteriorly through the body of vertebra L1 near its lower border. The transtubercular plane passes through the tubercles of the iliac crest and the body of vertebra L5 near its upper border. The right and left lateral planes correspond to the midclavicular or mammary lines. Each of these vertical planes passes through the midinguinal point and crosses the tip of the ninth costal cartilage. The nine regions marked out in this way are arranged in three vertical zones, median,right and left. From above downwards, the median regi-
ons are epigastric, umbilical and hypogastric. The right and left regions, in the same order, are ypochondriac, lumbar and iliac.Position of many organs is shown. Liver chiefly occupies the right hypochondrium. Stomach and spleen occupy the left hypochondrium. Duodenum lies in relation to posterior abdominal wall.Coils of jejunum and ileum fill up the umbilical, lumbat and iliac regions.Large intestine lies at the periphery of abdominal cavity, caecum, ascending colon on right side, descending colon on left side and transverse colon across the cavity.

Peritoneal cavity

The viscera which invigilate the peritoneal cavity completely fill it so that the cavity is reduced to a potential space separating adjacent layers of peritoneum. Between these layers there is a thin film of serous fluid secreted by the mesothelial cells. This fluid performs a lubricating function and allows free movement of one peritoneal surface over another.Under abnormal circumstances there may be collection of fluid called as cites,or of blood called haemoperitoneum,or of air called pneumoperitoneum within the peritoneal
cavity. The peritoneal cavity is divided broadly into two parts. The main, larger part is known as the greater sac, and the smaller part situated behind the stomach, the lesser omentum and the liver, is known as the lesser sa. The two sacs communicate with each other through the epiploic foramen or foramen of winslow or opening into the lesser sac. Small pockets or recesses of the peritoneal cavity may be separated from the main cavity by small folds of peritoneum. These peritoneal recesses or fossac are of clinical importance. Internal hernia may take place into these recesses.

Peritoneal folds : The folds can be best understood by recapitulating the embryology of the gut. The developing gut is divisible into three parts, foregut, midgut and hindgut. Each part has its own artery which is a ventral branch of the abdominal aorta. The coeliac artery supplies the foregut;the superior mesenteric artery supplies the midgut;and the inferior mesenteric artery supplies the hindgut                
Apart from some other structures the foregut forms the oesophagus, the stomach, and the upper part of the duodenum up to the opening of the common bile duct.The midgut forms the rest of the duodenum, the jejunum, the ileum,the appendix, the caecum,the ascending colon,and the right two-thirds of the transverse colon, the descending colon, the sigmoid colon,proximal upper part of the rectum. The anorectal canal forms distal part of rectum and the upper part of the anal canal up to the pectinate line. The abdominal part of the foregut is suspended by mesenterirs both ventrally and dorsally.The ventral mesentery of the foregut is called the ventral mesogastrium, and the dorsal mesentery is called dorsal mesogastrium.The midgut and hingut have only a dorsal mesentery,which forms the mesentery of jejunum and ileum,the mesoappendix,the transverse mesocolon and the sigmoid mesocolon.The mesenteries of the duodenum, the ascending colon the descending colon and the rectum are lost during development.

Dissection 

Expose the extensive abdominal cavity. Identify the paritoneum, adherent to the parieties or walls of the abdominal cavity. Trace it from the walls to from various double-layered folds which spread out to enclose the viscera as the visceral peritoneum. Identify and lift up the greater omentum. See its continuity with the stomach above and the transverse colon fused with its posterior surface a short distance inferior to the stomach. Cut through the anterior layers of the greater omentum 2-3 cm inferior to the arteries to open
the lower part of the omental byrsa sufficiantly to admit a hand. Explore the bursa. Pull the liver superiorly and lift its inferior margin anterior to expose the lesser omentum. Examine the right free margin of lesser omentum, containing the bile duct, hepatic artery and portal vein. This free margin forms the anterior boundary of the opening into the lesser sac,i.e epiploic foremen. The posterior boundary is the inferior vena cava.Superior to opening into the lesser sac is the caudate process of liver and inferiorly is the first part
of duodenum. Remove the anterior layer of peritoneum from the lesser omentum along the lesser curvature of the stomach. Find and race and left gastric vessels along the lesser curvature of stomach. Trace the oesophageal branches to the oesophague. Trace the right gastric artery to the proper hepatic artery and the vein to the portal vein.Expose the proper hepatic artery and trace its branches to the porta hepatis. Trace the cystic duct from the gall bladder. Follow the common hepatic duct to the porta hepatic and the bile duct till it passes posterior to the duodenum.

Vertical tracing : Peritoneum lining the anterior abdominal wall and diaphragm,layer 1.Peritoneum lining upper part of posterior abdominal wall and diaphragm, layer 3. Layers 1 and 3 enclose most of the liver.The two layers get reflected at porta hepatis to from the lesser omentum. Lesser omentum encloses the stomach and two layers pass downwards covering the intestines, where these fold upon themselves.First layer becomes fourth layer and second layer becomes the third layer.Third and fourth layers enclose transverse mesocolon. Third layer lines the structures in the upper part of posterior abdominal wall.The fourth layer passes around the small intestine to from the mesentery of small intestine. Peritoneum lines the structures in the posterior abdominal wall and descends into the true pelvis in fornt of the rectum. The subsequent tracing is different in the male and in the female. The female, it passes from the front of rectum to the uterus forming rectouterine pouch and from the uterus to the urinary bladder forming the vesicouterine pouch.

Special regions of the peritoneal cavity : From a surgical point of view the peritoneal cavity has two main parts, the abdomen proper and the pelvic cavity. The abdominal cavity is divided by the transverse colon and the transverse mesocolon into the supracolic and infracolic compartment is subdivided by the reflection of peritoneum around the liver into a number of subphrenic spaces. The infracolic compartment is also subdivided, by the mesentery, into right and left parts. Further, the right paracolic gutter lies along the lateral side of the ascending colon, and the left paracolic gutter along the lateral side of the descending colon.

Subphrenic spaces/supracolic compartment classification
The intraperitoneal spaces are : The left anterior space, the left posterior, the right anterior space, the right posterior space.The left anterior space or the left subphrenic space lies between the left lobe of the liver and the diaphragm,in front of the left triangular ligament. Inferiorly, it extends to the front of the lesser omentum and of the stomach. Towards the left it reaches and spleen. As abscess may from in this space following  operations on the stomach, the spleen, the splenic flexure of the colon, and the tail of the pancreas. The left posterior space or the left subhepatic space is merely the lesser sac which has already been described.The right anterior space or right subphrenic space lies between the right lobe of the liver and the diaphragm, in front of the superior layer of the coronary ligament and of the right triangular ligament. Infection may spread to this space from the gall bladder, or the vermiform appendix; or may follow operations on the upper abdomen. The right posterior space or right subhepatic space is also called the hepatorenal pouch of morison. It is described below.