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                                LAB – Anatomy of the Mink
                               Part Three: Digestive System
The digestive system consists of many organs working together for the major purpose of digesting
food. Digestion is the process of breaking down large organic macromolecules into their building
blocks. The purpose for this is twofold. By breaking down the molecules, they will be small enough
to be absorbed into the bloodstream where their can use them. Also, if one stops to consider an
animal’s food was once another organism, the food molecules are literally the macromolecules once
used by the previous organism. By digesting these molecules into their building blocks, the animal can
use some of these building blocks to construct its own macromolecules. An animal eats not only to get
molecules that can supply energy, but also to get molecules from which it can make its own unique
The digestive system can be divided into two parts. The alimentary canal is the “food tube” through
which the ingested food actually travels. Food normally travels only one way through an alimentary
canal, with different regions of the canal being specialized to do different jobs. The opening through
which food is taken into the alimentary canal is the mouth, and the opening through which the
undigested materials leave the alimentary canal is the anus.
The second part of the digestive system contains all of the accessory organs. These are organs that
are important for digestion, but do not actually have the food pass through them. The best examples of
accessory organs are the many glands that make digestive enzymes and juices that are released into the
alimentary canal. Examples of accessory organs include salivary glands, the liver, pancreas, etc…

Advance Preparation
 1.  Digested food is not used just for energy. What else are the particles used for?
 2.  How are accessory organs different from those of the alimentary canal?
 3.  Where specifically will you find the pancreas in your mink? To what two systems does the
     pancreas belong?
 4.  What is a vestigial organ? Give an example of a vestigial organ in humans.

                                                                      Structures of Interest
                                                                     Esophagus
                                                                     Stomach
                                                                     Small Intestine (duodenum,
                                                                      jejunum, ileum)
                                                                     Large intestine (colon,
                                                                     Liver
                                                                     Gall bladder
                                                                     Spleen
                                                                     Pancreas
1.   The alimentary canal begins with the mouth cavity. It is in this chamber that digestion begins.
     Using bone shears, cut through the jawbones as demonstrated by your instructor. Open the
     mouth and find the tongue and teeth (remember the lower jaw will still be attached by the
     frenulum—the membrane that attaches the tongue to the bottom jaw!). Note the ridged hard
     palate, which is the bony upper roof of the mouth cavity. The teeth, tongue, and hard palate
     all work together to begin mechanically breaking the food down. The soft palate is a
     continuation of the hard palate that extends back toward the throat region. It does not have
     bone beneath the tissue like the hard palate does.
2.   After the food has been crushed and formed into a mass by the tongue, it is swallowed. The
     food passes to the back of the throat (pharynx) where the food then enters the esophagus. The
     esophagus is located just dorsally to the trachea (windpipe) and is collapsed, unless food is in
     the esophagus. The thoracic cavity and the abdominal cavity are separated by a thin muscular
     sheet called the diaphragm.
               Locate the diaphragm that is anterior to the liver and stomach. It runs perpendicular
                to the length of the mink.
3.   Pin the muscular flaps of tissue away from the abdomen so you can see the internal organs.
     You may need to crack the ribs in order to fully pin back the flaps. When you first pull back
     the flaps of tissue, you will see the large, brownish-red liver. The stomach will be just left (the
     specimen’s left!) of the liver.

4.   The organ that follows the stomach in the alimentary canal is the small intestine, where the
     majority of chemical digestion occurs.
        Locate the small intestine where it connects the stomach. Note that the small intestine,
         while it appears to be a jumbled mass, is actually held by the mesentery. Look at the
         mesentery that holds the small intestine. Can you see red blood vessels running in the
         mesentery toward the small intestine? This is the blood vessel into which nutrients from
         the small intestine are absorbed into the bloodstream. The small intestine has folds to
          increase surface area, as well as microscopic fingerlike extensions called villi (internal) to
          allow for the maximum amount of digestion and absorption. The small intestine is divided
          into three segments: the duodenum, which begins at the pyloric sphincter, the jejunum,
          and the ileum. Identify the duodenum attached to the stomach. Identifying the jejunum and
          the ileum require histological (tissue) study.

                 Go back to the end of the small intestine, where it joins to the large intestine, where
                  you should see a small fingerlike projection called the cecum. The cecum is thought
                  to be an extra area for digestion. In humans, this cecum has a fingerlike projection
                  coming from it called the appendix. This appendix serves no function in us. An
                  organ that no longer serves a function in an organism, but likely served a function in
                  the distant ancestors that gave rise to the species is called a vestigial structure.

Accessory Organs
The accessory organs of the digestive system do not have food pass through them, but they are very
important to the process of digestion. Often these organs are glands that secrete enzymes or other
digestive juices through a tube or duct to one of the organs of the alimentary canal.

 8.    We will however, see the next accessory organ, the liver. It is a large brown organ with several
       lobes. The liver has many vital functions such as detoxifying drugs and poisons, making and
       storing glycogen, making cholesterol, etc… Its main digestive function is to produce bile. Bile
       is not an enzyme. It is a greenish-brown fluid that breaks fats and oils into smaller droplets,
       increasing the surface area so they will be able to be digested and absorbed more easily.
 9.    Underneath one of the lobes of the liver, is a small, thin walled sac that may appear flattened in
       your mink. It will have a green color to it.
                 Locate the gall bladder, and its function is to store the bile that the liver is
                  constantly producing, and to concentrate it by removing some of the water. The bile
                  will pass from the gall bladder to the duodenum of the small intestine through a
                  small tube or duct called the common bile duct. The gall bladder will be stimulated
                  to release the bile through the bile duct into the small intestine when food enters the
                  small intestine.
10.   The final accessory organ is one of the most important of the digestive glands, the pancreas. It
      is a light colored, many lobed organ that lies under the stomach, between the stomach and the
      intestines. The lobes are wrapped by a thin transparent mesentery. It almost looks like a thin
      layer of tan-colored cottage cheese covered by a sheet of Saran wrap. The pancreas releases the
      hormone insulin. It is pinkish (brown in some minks) and rather loose in structure. The left
      limb lies near the stomach and extends to the spleen. The products of the pancreas (digestive
      enzymes) and of the liver (bile) are carried into the small intestine by a common duct system.

11.   While the spleen is not a member of the digestive system, but rather the circulatory system, we
      will discuss it at this time, because it is most noticeable when examining the abdominal organs.
      The spleen is a reddish brown strip of tissue that is attached near the stomach and lies along the
      body wall. It is held into place by the mesentery. The spleen in humans stores and destroys
      blood cells. In the embryo, the spleen is the site of red blood cell production, but no longer
      performs this function in the adult. In adults, the spleen does not seem to be vital for life.

12.   Carefully remove the entire alimentary canal, esophagus to anus, from your mink in one piece
      after observing the accessory organs.
13.   Cut open the stomach with your scissors. Wash out the stomach under a stream of water in the
      nearest sink. The greenish debris that you see is sloughed off epithelial cells that have been
      stained by bile. What was your mink’s last meal?
14.   Examine the stomach lining under the dissecting microscope, and note its appearance. Think
      about why this structure would be allow for the stomach’s function.
15.   Where the esophagus joins the stomach is the cardiac sphincter. A sphincter is a circular
      muscle that will close off a tube or opening when the muscle contracts. The cardiac sphincter
      closes when the stomach churns, preventing the acidic stomach contents from flowing back up
      the esophagus. At the end of the stomach where it joins the small intestine is another sphincter,
      the pyloric sphincter. Turn the stomach inside out and locate the pyloric sphincter, a
      donut-shaped muscle. This sphincter closes off the stomach while it is churning, and also
      regulates how much food enters the small intestine at a time.
16.   Cut into the stomach. When doing this you will notice many folds or ridges that run the length
      of the stomach. These folds, called gastric rugae, allow the stomach to expand when it is filled
      with food.

17.   Carefully cut the small intestine loose from the stomach. Cut the mesentery holding the
      intestines in loops, and slowly unravel your intestine. Try to unravel it just one piece, and then
      measure the length of the whole small intestine. Record your value.
18.   Measure the length of the large intestine. Record your value.
19.   Cut open a small portion of the small intestine, wash it, and observe it under the dissecting
      microscope, and note its appearance. How does this structure compare with the stomach? How
      does the function of the small intestine compare to the function of the stomach?
20.   Cut open a small portion of the large intestine, wash it, and observe it under the dissecting
      microscope, and note its appearance. How does this structure compare with the stomach and
      small intestine? How does the function of the large intestine compare to the function of the
      stomach and small intestine?

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