Hello everyone! My name is Megan, and welcome to another anatomy tutorial. In this tutorial, we will be covering the mesentery which is found in the abdominal cavity. I'm going to also cover some of the related structures and organs.
Before we begin talking about the mesentery, I quickly want to go through a brief overview of the peritoneum. The peritoneum is a double layer of serous membrane or mesothelium. It lines the walls of the abdominal cavity and most of the viscera. Throughout the abdominal cavity, there are peritoneal folds that connect organs together or connect these organs to the abdominal wall. Some of these folds also contain vessels and nerves that supply the organs. Their function is two-fold as they also stabilize and help maintain the position of the organs.
The subdivisions of the peritoneum include the omenta, the mesenteries, and the peritoneal ligaments. In this image, we can see the male pelvis. We can see the peritoneum highlighted in green. It's covering the superior aspect of the bladder and continues over the superior surface of the rectum.
In the next illustration, we can see the female pelvis. In the female pelvis, the peritoneum also covers the bladder. It then covers the uterus to form the broad ligament of the uterus. Once it has done that, it then continues to run over the rectum.
Now that we know what the peritoneum is, we can look at the two different types of peritoneum. The peritoneum consists of the parietal and visceral peritoneum. So, as I said before, the parietal peritoneum is a mesothelial layer and covers the internal surface of the abdominal walls and is supplied by the regional vasculature whereas the visceral peritoneum covers the viscera inside of the peritoneal cavity. So, let's start with the mesenteries.
The mesenteries are peritoneal folds that contain the viscera and attach them to the posterior abdominal wall. The most important mesentery is the mesentery proper. It is a fan-shaped double layer of peritoneum that connects the jejunum and the ileum to the posterior abdominal wall. Its superior attachment is at the junction between the duodenum and the jejunum. This attachment then travels obliquely downwards and to the right where it ends at the ileocecal junction. The two peritoneal layers of the mesentery contain fat and the arteries, veins and nerves that supply the jejunum and ileum. The lymphatic vessels that drain the jejunum and the ileum are also in the mesentery proper.
In the next illustration, we have a better view of the superior aspect of the mesentery proper. The reason we can see it clearer is because the large intestine and the transverse mesocolon have been cut away. Here, we can see where the stomach could have been. It then continues as the duodenum in a C-shaped loop before it gets to here which is the junction between the duodenum and the jejunum. We can see the superior aspect of the mesentery proper here. It then continues as a fan downwards.
The transverse mesocolon is also a double layer of peritoneum. It connects the transverse colon to the posterior abdominal wall. Its two layers leave the posterior abdominal wall from the anterior surface of the head and body of the pancreas. We can see that here highlighted in green. It then passes outwards to surround the transverse colon. Similar to the mesentery proper, the transverse mesocolon contains the arteries, veins, nerves and lymphatic vessels for the structure it surrounds.
The next part of the mesentery is the sigmoid mesocolon. It's an inverted V-shaped peritoneal fold that attaches the sigmoid colon to the abdominal wall. The apex of the V is near the division of the left common iliac artery into its internal and external branches. The sigmoid and superior rectal vessels along with the nerves and lymphatics associated with the sigmoid colon pass through the sigmoid mesocolon. In the next illustration, we have an isolated view of the sigmoid mesocolon so that we can see it better.
We've now covered the mesenteries so let's look at another fold of peritoneum that is visible from this perspective which is the greater omentum. The greater omentum is also a double fold of peritoneum. It hangs down from the greater curvature of the stomach and reflects back on itself to attach to the transverse colon. In this image here, you can see the transverse colon and its associated mesentery underneath the greater omentum. The greater omentum contains variable amounts of fat. It also contributes to the immune system through its stored macrophages and it's important in the isolation of wounds and infections in the peritoneal cavity. The vascular supply to the greater omentum comes from the left and right gastroepiploic arteries which are branches of the splenic and gastroduodenal arteries.
So, on a clinical note, the greater omentum is an important site for the metastasis of tumors. Direct omental spread of cancer cells via the transcoelomic route is common for cancer of the ovaries. As the metastasis develops within the greater omentum, it becomes significantly thickened. The thickened greater omentum is then referred to as an omental cake in the CT scan. In this image, we can see the greater omentum hanging from the edge of the greater curvature of the stomach and covering up the transverse colon.
So next, we're going to look at the viscera that come into contact with the mesentery. First, we're gonna look at the jejunum. The jejunum is the middle segment of the small intestine between the duodenum and the ileum. It is approximately two and a half meters long. The internal surface of the jejunum is lined by villi which increase the surface area and improve absorption of nutrients. The wall of the jejunum is composed of two muscular layers – a circular and a longitudinal layer of smooth muscle. These are responsible for peristalsis. Sympathetic innervation to the jejunum is supplied by the celiac? (6:40) ganglia. Parasympathetic innervation on the other hand is supplied by the vagus nerve. In the previous image, we saw the start of the jejunum and then the mesentery but in this image, we can see the jejunum suspended from the mesentery in the abdominal cavity.
The other portion of the small intestines we can here is the ileum. The ileum is the terminal portion of the small intestine. It measures approximately three and a half meters in length. It is a continuation of the jejunum and ends at the ileocecal valve. This ileocecal valve separates it from the cecum. The inner lining of the ileum is also covered by villi that increases surface area for absorption. The main function of the ileum is to absorb bile salts, vitamin B12 and any other nutrients that haven't been absorbed yet. The ileum receives its vascular supply from the ileal arteries which are branches of the superior mesenteric artery. It receives sympathetic innervation from the superior mesenteric ganglion and parasympathetic innervation from the vagal trunk.
In this illustration, we can see the ileum highlighted in green. We can also see that it's continuous with the jejunum.
Now that we've looked at the small intestine from this perspective, let's move on to the large intestine. The large intestine begins with the ileocecal valve I mentioned before. The first part of the large intestine is the cecum. This is a pouch just below the ileocecal valve. The next part is the ascending colon which starts immediately after the cecum and travels up to the right colic flexure. This flexure which is known as the hepatic flexure because of its proximity to the liver marks the end of the ascending colon. In these illustrations, we can see the cecum, the ascending colon and the right colic flexure highlighted in green.
The next part of the large intestine is the transverse colon. The transverse colon – highlighted here in green – is the most mobile part of the large intestine. It's lifted up here to show the mesentery. The transverse colon travels across the abdomen to the other colic flexure, the left or splenic colic flexure. It's called the splenic flexure for – as you might have guessed – its proximity to the spleen.
The next part of the large intestine is the descending colon. It's located on the left side of the abdomen and it travels straight down to the sigmoid colon. The sigmoid colon is highlighted in green here. This segment of the large intestine is intraperitoneal which means it's completely surrounded by peritoneum. It is a continuation of the descending colon. The sigmoid colon transports feces from the descending colon into the rectum. It is also responsible for the absorption of water and nutrients and vitamins. The final part of the large intestine is the rectum. When the rectum pierces the pelvic floor, it becomes the anal canal.
Now that we've seen the various parts of the large intestine, let's look at the intestines as a whole and look at some of their features.
On the external surface of the large intestine, there are these whitish bands of tissue. They are called taenia coli. They are three thickened bands of longitudinal muscle. They measure approximately 1 centimeter in width and they are found on the outer surface of the colon. They converge at the root of the appendix. On the inner surface of the muscles that make up the anterior abdominal wall, there is an artery called the inferior epigastric artery. This artery arises from the external iliac artery. It anastomoses with the superior epigastric artery, the umbilical artery and the lower intercostal arteries.
So, in the next illustration, we have an isolated view of the inferior epigastric artery. We can see the origin of this artery from the left external iliac artery. We can then see it passing upwards to anastomose or connect with the superior epigastric artery, the umbilical artery and the lower intercostal arteries.
Accompanying the inferior epigastric artery, there is an inferior epigastric vein. You can see them here running right next to the artery. The inferior epigastric vein arises from the superior epigastric vein and drains into the external iliac vein. Again, we're gonna look at the inferior epigastric vein in isolation. We can see that it's following an extremely similar pattern to the artery and that it drains into the external iliac vein here.
Next, we're going to look at the medial umbilical folds. They are paired structure that cover the medial umbilical ligaments and they are in close relation to the two vessels that we've just talked about. The medial umbilical ligaments are the remnants of the fetal umbilical arteries.
Before we end this tutorial, there is one last structure we need to talk about – the median umbilical fold. It's an unpaired structure that extends from the apex of the bladder to the umbilicus and covers the median umbilical ligament. This ligament is a remnant of the urachus, a canal that connects the bladder to the umbilicus during fetal development.
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Now, good luck everyone, and I will see you next time.