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The Digestive System consists of a number of organs that work together to accomplish the physical and chemical activities associated with the digestion process. Vertebrates ingest large particles of food that contain proteins, fats & oils, carbohydrates, and nucleic acids. The digestive system, with its muscles and numerous enzymes, physically and biochemically breaks down food substances into small molecules such as amino acids, fat components, glucose, fructose, galactose, and nucleotides that can be readily absorbed into the bloodstream through the small intestinal lining. Most of these molecules are small, water-soluble molecules that are easily transported in the vertebrate blood since it is a water-based medium. Some of the small fat components have to be packaged and absorbed a special way in order to make it into the body.
The Digestive System consists of the alimentary canal and some accessory organs. The alimentary canal is a continual tube beginning at the mouth and terminating at the anus. There are also several accessory organs associated with the alimentary canal. The bulk of the digestive organs are situated in the abdominal cavity, but there are several digestive structures associated with the head as well. You will examine the organs of this system from an anterior to posterior direction.
Prior to the identification of these structures, the abdominal cavity must be opened. Make a cut as shown below.
1.
Greater omentum
—The first thing you will probably see when you open up the abdominal cavity is the greater omentum. Find the stomach and identify the greater omentum that connects to the greater curvature of the stomach. The greater omentum contains adipose and lymph nodes that help fight infection within the abdominal cavity.
(Do not remove this yet.)
2.
Parietal peritoneum
—With the abdominal cavity open, identify the parietal peritoneum of the abdominal cavity.
Q1: What organs are located behind the parietal peritoneum against the dorsal wall?

3.
Visceral peritoneum
—Identify a visceral peritoneum. This is also known as the outer serosa layer of an organ.
4.
Mesentery/Mesocolon
—The mesentery/mesocolon looks like “cobwebs” connecting the organs to other organs and the abdominal wall. Identify the mesentery and mesocolon.
5.
Lesser Omentum
—Identify the lesser curvature of the stomach and its associated lesser omentum.
6.
Liver —This vital organ is the largest gland in the vertebrate body weighing three-four pounds in the adult human. It consists of five lobes in the cat but only four in the human.
Identify the right and left medial lobes, right and left lateral lobes, and the single caudal lobe. The liver lobules filter a tremendous blood volume from the hepatic portal system, removing some substances from the blood while adding others. a.
The falciform ligament is a fold of the peritoneum that is located between the left and right lobes of the liver. Its function is to connect the ventral side of the liver to the abdominal wall. Identify the falciform ligament on the cat’s liver.
Q2: Blood received by the liver through the hepatic portal system is loaded with glucose shortly after a meal. Explain what the liver does with the excessive blood sugar with the help of the pancreas.

Q3: During times of fasting, the blood sugar level drops. Explain what the liver does with the help of the pancreas to raise the blood sugar level.
Q4: The liver also stores fat or adipose tissue. What does this have to do with vitamins A, D, E, and K?
Q5: Bacteria in the intestines and many body cells produce toxic ammonia (NH
3
) that gets into the bloodstream. What does the liver do with this biochemical hazard and other toxins?
Q6: The spleen, red bone marrow, lymph nodes and the liver are all capable of destroying old worn-out erythrocytes. Hemoglobin is broken down into a chemical pigment called bilirubin and placed into the bloodstream. What does the liver do with bilirubin?
7.
Spleen
—The spleen is a long slender dark-colored organ that is situated laterally to the stomach and extends towards the ventral side of the cat. The spleen is not part of the digestive system, but instead . Locate this structure.
8.
Gall bladder —Locate the gall bladder embedded ventrally in the right medial lobe of the liver. Then see if you can find and identify the following structures: a.
Hepatic ducts —Identify the common hepatic duct before it joins with the cystic duct. b.
Cystic duct
—Locate this structure that connects to the gall bladder. c.
Common bile duct
— This is actually very difficult to locate. Locate where the pancreas comes in close contact with the duodenum and see if you can find the duct.
9.
Pancreas —This organ is not as easy to spot but has a ventral lobe suspended in the duodenal mesentery and a dorsal lobe found dorsal to the greater curvature of the stomach. Locate both lobes and leave the pancreas intact, but separate the pancreas from the small intestine and stomach.
Q7: This gland functions both as an exocrine and an endocrine gland. Acinar cells border minute ducts that lead to the major pancreatic duct. These cells secrete the pancreatic juice that act upon specific food substances when they reach the duodenum. This is an exocrine function. What does exocrine mean?

Leave behind:
Liver
Gall Bladder
Pancreas
Spleen
10.
Stomach
—In order to study the gastrointestinal tract more easily, you will now remove the entire GI tract as one unit. On the posterior end of the esophagus, make your first cut, and at the posterior end of the rectum, make your second cut.
Carefully lift the entire GI tract out of the abdomen, cutting mesentery where necessary and taking care not to destroy the other organs you are leaving behind.
In particular, be careful when separating the duodenum from the pancreas, leaving the connection between the duodenum and the common bile duct intact. (Cut the common bile duct above this point.)
Q8: Little groups of pancreatic cells called “Islets of Langerhans” secrete insulin and glucagon to help control blood sugar. This is an endocrine function.
What does endocrine mean?
Q9: Describe the influence on sugar metabolism provided by the pancreatic hormones listed below.

Insulin

Glucagon a.
Cardiac sphincter
—
Identify the location of the cardiac or gastroesophageal sphincter where the esophagus joins the anterior end of the stomach.
Q10: What is regurgitation? b.
Slice the stomach open lengthwise and remove any contents. Throw contents in trash and rinse stomach out if necessary. Locate the rugae if possible. If not, observe another group’s cat that has rugae.
Q11: Did your cat’s stomach contain chyme? c.
The stomach wall has four layers of tissue. Cut into the ventral stomach wall and remove a ¼ square inch from it with you scalpel. Use the dissecting scope try to identify the four layers from inside out.
Mucosa

Submucosa

Muscularis

Serosa

d.
Pyloric sphincter—This involuntary, circular muscle provides a valve between the stomach and the duodenum at the gastro-duodenal junction. Locate this structure.
11.
Small Intestine
—
Duodenum a.
This is the very first portion of the small intestine just medial to the pyloric sphincter. Identify it in the cat where it is about two inches long. In the human, it is about ten inches long. This portion of the intestine receives the common bile duct into which the cystic, hepatic, and pancreatic ducts drain. b.
Sphincter of Oddi —Within the duodenum, chyme is mixed with bile and pancreatic enzymes. Carefully slice the duodenum open lengthwise and look for the Sphincter of Oddi. If visible, it would be a small opening near the pyloric sphincter.
12.
Small Intestine
—
Jejunum
This is the second part of the small intestine measuring about three feet long in the human and comprises about 2/5 of the small intestinal length in most vertebrates. This tube receives the partially digested food mass from the duodenum and secretes several enzymes to complete chemical digestion. Some absorption of digested food molecules does take place through its wall into the blood. a.
Remove the mesentery of the small intestine to stretch it out. Carefully slice a portion of the jejunum and ileum open lengthwise. Using a dissecting scope examine the cut edge of its walls and also an internal surface view of its mucosa.
Q12: Does the jejunum of the cat contain plicae circulares? b.
Identify the villi under a dissecting scope.
Q13: Explain why each villus has an arteriale and a venule.
Q14: What are lacteals and why are they needed? c.
Now look at a prepared microscope slide containing a section of the jejunum.
Identify the microvilli, which are sometimes called the brush border, and the goblet cells which make mucus.
13.
Small Intestine
—
Ileum .
This last portion of the small intestine is about six to seven feet long in a human and constitutes about 3/5 of the small intestinal length in most vertebrates. The ileum absorbs digested food molecules into the bloodstream.

Q15: Peristaltic muscular action occurs about eleven times per minute moving the food through the intestinal tract at about three inches per minute. The circular muscles of the intestinal muscularis also cause segmental action. How does segmentation facilitate digestion in the jejunum and absorption in the ileum?
Q16: Simple passive transport is sometimes responsible for the absorption of digestive food molecules into the bloodstream. Explain what this means.
Q17: Active transport is the most common method of absorption of intestinal nutrients into the blood. Explain what this means.
Ileocaecal valve
—Locate this valve as a constriction at the junction of the ileum and the beginning of the colon. This junction is normally found in the lower right quarter of the abdominal cavity.
14.
Large Intestine
—
Caecum
The caecum is a holding area for the food that needs to pass through the large intestine. In humans, this is also the location of the vermiform appendix. Identify the caecum on the large intestine side of the ileocaecal valve.
Q18: Does the cat have an appendix like the human does?
15.
Large Intestine
—
Colon
—This tubular structure has a greater diameter but a lesser length than the small intestine. Identify its ascending, transverse, and descending portions. It receives a liquid mass from the ileum. The cat lacks a sigmoid colon.
Q19: Does the cat’s colon have haustra like the human colon does?
16.
Large Intestine
—
Rectum
—Identify the last tubular portion of the digestive tract.
Q20: What is its function?
17.
Large Intestine — Anus —This is the opening of egestion or defecation. Identify this structure.

1.
Parotid salivary glands —This pair is one of three major pairs of salivary glands in the cat or human. The parotids are the largest salivary glands situated ventral to the base of the ear and posterior to the masseter muscle. These glands are the ones infected when the mumps virus attacks a human. Remove the skin from this area. Clean the superficial fascia and adipose tissue from this area either on the left or right side of the face. Locate this structure.
2.
Parotid duct — Within the parotid gland, cells line minute ducts and empty a secretion called saliva into ducts that drain it to a single major duct on each side of the head. This parotid duct that drains the parotid gland to the oral cavity. The parotid duct is a transparent tube that passes across the masseter muscle. If possible, locate this structure.
3.
Submandibular (Submaxillary) salivary glands
—Try to locate one of these glands.
This gland is smaller than the parotid and is found ventral and posterior to it. (The
Sublingual salivary gland will not be located in this lab. You would find this gland deeper within the neck.)
You will now open the mouth cavity. Do not destroy the exposed parotid duct in the process. The cheek tissue should be bisected (cut in half) starting at the corners of the mouth and working towards the mandibular joint. The coronoid process of the mandible will need severed from the rest of the mandible on both sides.
4.
Oral cavity . Locate this structure.
5.
Hard palate
—This forms the anterior part of the roof of the mouth. Locate this structure.
6.
Soft palate
—This forms the posterior part of the roof of the mouth. Locate this structure.
Q21: Does the cat have a uvula like the human?
7.
Tongue a.
Lingual frenulum . Locate this structure. b.
Bisect the end of the tongue into right and left halves. Examine the musculature of the tongue.
Q22: Do all the muscle fibers travel in the same direction? Why or Why not? c.
Remove a part of the tongue and look at it under the stereoscope. There are several kinds of papillae found on the human tongue. See if you can identify the papillae that your cat has. Filiform papillae are short threadlike elevations covering most of the tongue surface. In cats and cattle they are heavily cornified to give a rasping tongue. These papillae are mainly to help us grasp food.

Fungiform papillae are knob-like projections of the tongue scattered among the filiform papillae. Circumvallate or vallate papillae are 8 to 12 large circular papillae arranged in a V near the base of the tongue. Both the fungiform and the circumvallate papillae have taste buds associated with them.
Filiform Papillae Circumvallate Papillae
Fungiform Papillae
8.
Epiglottis & Pharynx
—Look past the oral cavity and deep into the larygnopharynx and find the opening to the epiglottis.
9.
Esophagus —On the ventral side of the neck, nudge the trachea to the cat’s right. This should reveal a portion of the esophagus. Follow the esophagus down as far as possible.
If we opened the chest, you would be able to follow the esophagus pass posterior to the heart and lungs and see it pass through the diaphragm. (We will do this at a later date.)

(Look in your textbook or online to find this information.) a.
What is gastritis? b.
What is a peptic ulcer? c.
What is a duodenal ulcer? d.
What is gastroenteritis? e.
Describe diabetes and what causes it? f.
What is jaundice? g.
What is hepatitis? h.
What are hemorrhoids?
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