Rat Anatomy - The Thoracic Organs
Procedure: Cut through the abdominal wall of the rat following the incision marks in the picture. Be careful not to cut too deeply and keep the tip of your scissors pointed upwards. Do not damage the underlying structures. Once you have opened the body cavity, you will need to rinse it in the sink.
1. Locate the diaphragm, which is a layer of muscle that separates the thoracic from the abdominal cavity. 
2. The heart is centrally located in the thoracic cavity. The two dark colored chambers at the top are the atria (single: atrium), and the bottom chambers are the ventricles. The heart is covered by a thin membrane called the pericardium. (We will come back to the heart later.)
3. Locate the thymus gland, which lies directly over the upper part of the heart. The thymus functions in the development of the immune system and is much larger in young rats than it is in older rats.
4. The lungs are spongy organs that lie on either side of the heart and should take up most of the thoracic cavity.
The Abdominal Organs
1. The coelom is the body cavity within which the viscera (internal organs) are located. The cavity is covered by a membrane called the peritoneum, which is very thin and web-like, you may need to use forceps to remove some of this membrane to see the organs clearly.
2. Locate the liver, which is a dark colored organ suspended just under the diaphragm. The liver has many functions, one of which is to produce bile, which aids in digesting fat. The liver also transforms wastes into less harmful substances. Rats do not have a gall bladder, which is used for storing bile in other animals. There are four parts to the liver:
median or cystic lobe - located at the top, there is an obvious central cleft
left lateral lobe - large and partially covered by the stomach
right lateral lobe - partially divided into an anterior and posterior lobule, hidden from view by the median lobe
caudate lobe - small and folds around the esophagus and the stomach, seen most easily when stomach is raised
3. The esophagus pierces the diaphragm at a spot called the hiatus and moves food from the mouth to the stomach. It is easiest to locate where it enters the stomach.
4. Locate the stomach on the left side just under the diaphragm. The functions of the stomach include food storage, physical breakdown of food, and the digestion of protein.
The outer margin of the curved stomach is called the greater curvature, the inner margin is called the lesser curvature. You can make a slit in the stomach and see what is inside it. Most of the contents should be partly digested rat food.
At each end of the stomach (on the inside) is muscular valve. The opening between the esophagus and the stomach is called the cardiac sphincter. The opening between the stomach and the intestine is called the pyloric sphincter.
5. The spleen is about the same color as the liver and is attached to the greater curvature of the stomach. It is associated with the circulatory system and functions in the destruction of blood cells and blood storage. A person can live without a spleen, but they're more likely to get sick as it helps the immune system function.
6. The pancreas is a brownish, flattened gland found in the tissue between the stomach and small intestine. The pancreas produces digestive enzymes that are sent to the intestine via small ducts (the pancreatic duct). The pancreas also secretes insulin, which is important in the regulation of glucose metabolism.
7. The small intestine is a slender coiled tube that receives partially digested food from the stomach (via the pyloric sphincter). The coils of the small intestine are held together by a membrane called the mesentery.
The small intestine has three sections: duodenum, jejunum and ileum, (Listed in order from the stomach to the large intestine.) The duodenum is the first stretch of the intestine leading from the stomach, it is mostly straight. The jejunum and ileum are both curly parts of the intestine, with the ileum being the last section before the small intestine becomes the large intestine.
8. Locate the colon, which is the large greenish tube that extends from the small intestine and leads to the anus. The colon is also known as the large intestine. Food entering the colon from the small intestine is controlled by the ileocecal valve. The colon is where the finals stages of digestion and water absorption occurs and it contains a variety of bacteria to aid in digestion. The colon consists of five sections:
cecum - large sac where the small and large intestine meet (the ileocecal valve regulates passage of materials)
ascending colon – food travels upward.
transverse colon – a short section that is parallel to the diaphragm
descending colon – the section of the large intestine that travels back down toward the rectum.
rectum - the short, terminal section of the colon that leads to the anus. The rectum temporarily stores feces before they are expelled from the body.
Procedure:
Pin the organs of the digestive cavity.
Checkpoint: Test Your Knowledge
1. Lies under the stomach and secretes insulin
2. The section of large intestine between the ascending and descending colon:
3.
Connects the mouth to the stomach:
4. Thin membrane that covers the heart:
5. Muscle that separates the abdominal cavity from the thoracic cavity:
6. Destroys old blood cells and lies within the folds of the small intestine:
7. The lobe of the liver that has an obvious central cleft:
8. Another name for the large intestine: _
9. Organs of the respiratory system that lie on either side of the heart:
10. Large organ of the thoracic cavity that lies just under (posterior) to the diaphragm:
11. The last section of the colon, storage of feces:
12. The pouch of the colon that is found just where the small intestine joins it:
13. Valve that regulates the passage of food from the stomach to the small intestine:
14. Thin membrane that covers the organs of the abdominal cavity:
15. The first section of the small intestine:
16. The section of large intestine that is parallel to the diaphragm:
17. Structure related to the immune system, lies at the top of the heart:
18. Valve the regulates passage of materials from the small to the large intestine:
19. The opening in the diaphragm where the esophagus passes through:
20. Section of small intestine that comes after the duodenum:
Other Resources on the Rat
Rat External Anatomy | Head | Abdominal Cavity
Circulatory System | Urogenital System
Other Resources:| Virtual Rat Dissection | Rat Dissection with Photo Submission