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Home , Mesentery

73: BODY CAVITIES and

We've already mentioned that all the organs in the body are wrapped in "bags" made of thin layers of connective tissue. These bags are often inside of other bags, or even inside of several bags. The largest bags define areas that we call body cavities. There are three main cavities: the thoracic cavity, the and the .

The thoracic cavity is subdivided into three smaller cavities: the pleural cavity (containing the lungs), the mediastinum(in the middle), and the pericardial cavity (containing the heart). The pleural cavity is easy to understand because it simply contains the lungs. The pericardial cavity contains not only the heart itself, but the large blood vessels that come out of it, such as the aorta. The pericardial cavity is inside of the third cavity, the mediastinum. ("Media" means "middle" and "stinum" can refer to the "sternum," which is the bone that runs down the center of the ribcage.) The mediastinum contains not only the pericardial cavity but also part of the esophagus and trachea, the thymus (remember this from module 2 on the immune system?), and quite a few and nodes. The thin layers of connective tissues that surround these cavities are made primarily of collagen and elastin (produced by fibroblast cells) but they also contain some very tiny nerves and blood vessels, as well as cells that make serous fluid. As we've seen in the past few lessons, the diaphragm separates the thoracic cavity from the abdominal cavity.

The abdominal cavity contains the , the , the tail of the , the last half of the , the small intestines, most of the large intestines, and the mesentery (thin layers of connective tissue that anchor the intestines to the back wall of the abdominal cavity). There would also be many blood vessels and nerves all through the abdominal cavity.

The pelvic cavity contains the urinary bladder, the , the rectum, and the reproductive organs. As with the other cavities there would also be many blood vessels and nerves in this cavity. Oddly enough, the kidneys and most of the pancreas are not in any of these body cavities. They sit behind the abdominal cavity and are called retroperitoneal organs. ("Retro" means "behind.")

All three of these cavities, the thoracic, the abdominal and the pelvic can be categorized into one very large area called the ventral cavity. ("Ventral" means on the stomach side.) There is also a dorsal cavity, which consists of the cranial cavity, containing the brain, and the spinal cavity, containing the spinal cord. ("Dorsal" means on the back side.)

The mesentery wasn't considered to be an organ until 2012 when anatomists at the University of Limerick, in Ireland, began studying electron microscope images of it. These images allowed them to examine the tissues in great detail. They realized that all the seemingly unconnected sheets of tissue were actually all connected. Since they were all connected and they were all doing basically the same job, this allowed the mesentery tissues to be classified as an organ. The definition of an organ is "a group of tissues that work together to perform a certain function." They felt that the mesentery satisfied this definition and should be added to the list of organs of the body. They presented their research to other scientists and finally everyone agreed to officially declare the mesentery to be an organ. In the past, young students were never taught anything about the mesentery and were thus unaware of its existence. This will change. Now, even elementary students will have to learn the word "mesentery" along with the words "stomach" and "intestines." What does the word "mesentery" mean? "Meso" means "middle," and "entery" is a variation of "entero" which refers to the digestive system, particularly the intestines. The mesentery isn't all in one place like the stomach or the or the pancreas. It is spread throughout the abdominal cavity. The mesentery looks like very thin sheets of strong connective tissue. They are translucent, meaning clear enough to let light through but not perfectly clear. They have two main jobs. The first is to anchor the intestines to the back wall of the abdomi- nal cavity. Just imagine what would happen if nothing was keeping the intestines in place. All those squiggles and loops of intes- tine would slide around and get tangled. Gravity would also tend to pull them down and eventually all your guts would sag down to the bottom. Yuck. Good thing the mesentery keeps them in place! The second thing the mesentery does is to provide a surface through which nerves, blood vessels and lymph vessels can run. We'd have the same tangling and sagging problem with all these vessels if they were not anchored in place. The mesentery is made of two thin layers of stuck together. We've already met serous membrane in several places. We saw it around the around the heart, forming the pericardium, and we also saw it in the lungs, forming the parietal and visceral pleura. Serous membrane is made of a layer of (a single layer of simple squamous epithelial cells that secrete serous fluid) stuck to a layer of connective tissue (collagen and elastin) made by fibroblast cells. It the connective tissue layer that contains the blood vessels and nerves. Serous membranes are called by different names, depending on where they are located. When the serous membranes line the walls of the body cavities they are called parietal . (Remember, "parietal" means "wall.") When the mem- branes wrap around organs they are called visceral peritoneum. ("Visceral" means "organs.") When two membranes are stuck together to form a sheet, they are called mesentery. Serous membrane around the lungs is called pleura, and serous membrane around the heart is called pericardium. There are names for various parts of the mesentery. The part that holds the colon in place is called the mesocolon. There is a sheet of tissue between the liver and stomach that is called the lesser omentum. There is a large sheet of tissue called the that acts like an apron, covering the front of the small intestines.