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Biology LibreTexts

Part 1: External Anatomy

  • Page ID
    20791
  • Determine the sex of your pig

    1. Before you start dissecting, examine the outside of the pig and determine its sex.
      • Look for these features:
        • Males: The urogenital opening is located near the umbilicus; the penis is hidden inside. The scrotal sac may be visible as a swelling just ventral to the anus, depending on the age of the fetus. The testes are still deep inside the body cavity; they don't descend into the scrotal sac until later.
        • Females: Look for the urogenital papilla (a nipple-looking structure), located just below the anus
      • Also, both males and females have nipples, just as in humans.
      • What sex is your pig?External Anatomy 1.PNG
    2. Make sure you are familiar with anatomical terms of reference:  anterior (front), posterior (back), dorsal (above), ventral (below).  In addition, you’ll need to know the following terms:
      • Medial:  toward the midline or middle of the body 
      • Lateral:  toward the outside of the body
      • Proximal:  close to a point of reference 
      • Distal:  farther from a point of reference  

    label the sides on the pig picture on the right 

    1. Open the pig’s mouth and locate the hard and soft palate on the roof of the mouth.  Can you feel your own hard and soft palates with your tongue?
      • Note the taste buds (also known as sensory papillae) on the side of the tongue.
      • Locate the esophagus at the back of the mouth.   Feel the edge of the mouth for teeth.  Does the fetal pig have teeth?  
      • To access the following structures, you will have to cut down either side of the jaw and pry the jaw down. This can be difficult and requires some force. You, essentially, must break the jaw, and it will make a cracking sound. Once you do this locate the epiglottis, a cone-shaped structure at the back of the mouth, a flap of skin helps to cover the trachea when a pig swallows.  The pharynx (throat) is the cavity in the back of the mouth – it is the junction for food (esophagus) and air (trachea).
    2. Observe the toes of the pig.
      • How many toes are on the feet?
      • Do they have an odd or even number of toes?

    **Make sure you know the locations of all the bold words on this handout**

    You will now work on opening the abdominal and thoracic cavities of the pig and identify structures. Remember, that to dissect means to "cut into pieces" from Latin dessicare - a careful dissection will make it easier for you to find the organs and structures. Be sure to follow all directions.

    The Incision

    External Anatomy 2.PNGPlace your fetal pig in the dissecting pan ventral side up. Use string to tie the legs behind the back of the pan. Use scissors to cut through the skin and muscles according to the diagram. Do not remove the umbilical cord.

    After completing the cuts, locate the umbilical vein that leads from the umbilical cord to the liver. You will need to cut this vein in order to open up the abdominal cavity.

    Your pig may be filled with water and preservative, drain over the sink if necessary. 

    Neck region

    Using the diagram to the right begin your dissection in the neck region. Try to cut as little as possible. Once you open the body cavity, you will generally be able to separate the different organs by simply pulling them apart with your fingers, forceps, or a probe. The more you cut things up, the harder it will be to figure out what you’re looking at. Cut midline on the ventral surface of the neck to expose the underlying muscles. Carefully separate the muscles to observe the underlying structures. Locate and understand the functions of the following structures:

    • Larynx: an enlarged structure on the trachea. If you cut it open, you can see the vocal cords inside.
    • Thymus gland: an endocrine (hormone-secreting) gland that helps regulate the immune system. It’s a large, spongy structure covering the ventral surface of the trachea and extends up along either side and also resides over the heart. It is easy to cut so be extra careful.
    • Thyroid gland: another endocrine gland; it’s a small bilobed (two parts) structure just posterior to the larynx. The thyroid secretes hormones that help regulate metabolism.
    • Trachea: the airway; it's reinforced with rings of cartilage so it does not collapse.
    • Esophagus: carries food from mouth to stomach; soft and muscular so it can move a food bolus by peristalsis. It is located dorsal to the trachea (but appears behind it because the specimen is upside down).

    Thoracic cavity

    Vertebrates have true coeloms (a body cavity). In mammals, the coelom is divided into two main cavities: the thoracic cavity, which contains the lungs, and the abdominal cavity, which contains the digestive system. The thoracic cavity and the abdominal cavity are separated by the diaphragm. Note the many membranes lining the coelom and holding the organs in place.

    Look for these structures in the thoracic cavity:

    • Lungs: they have several lobes. Note how spongy the tissue is.
    • Heart: muscular and easy to find. The heart is surrounded by a pericardial sac. Note the aorta, where high-pressure blood leaves the heart on its way to the systemic circulation. You may also see the right and left carotid arteries, which supply blood to the head. For now, don’t spend too much time on the various lobes of the heart and the many blood vessels. Come back to these later.
    • Diaphragm: a sheet of muscle and connective tissue that helps in breathing and divides the two cavities described here.

    Abdominal cavity

    digestion & absorption:

    Locate and understand the functions of the following structures:

    • Liver: very large and dark. It has several lobes. You’ll need to lift it out of the way to see the organs beneath. The liver produces bile that is stored in the gall bladder. The gall bladder is a small organ attached to the underside of the liver; it's usually greenish due to the bile. It connects to the small intestine by the bile duct.
    • Stomach: a muscular, sac-like organ that sits posterior to and to the left of the liver. Here is where the gastric juices released by glands continue enzymatic digestion started in the mouth. In particular,  proteins are hydrolyzed via pepsin. At each end of the stomach are valves that regulate food entering and leaving the stomach. At the esophagus is the cardiac sphincter valve, and at the duodenum is the pyloric sphincter valve.  View the inside of the stomach by slicing it open lengthwise.
    • Small & large intestine: tube-like structures that continue the movement of food (now called chyme). The small intestine is first. The initial part of the small intestine (duodenum) is responsible for the final steps of enzymatic digestion and then eventually absorption of the degraded molecules. The large intestine primarily functions to compact the remaining waste material by absorbing water invested in the digestion and lubrication process. Also absorption of vitamins completes here.
    • Rectum: the final portion of the large intestine where wastes are stored before elimination through the anus.
    • Mesenteries: thin, transparent sheets of connective tissue containing blood vessels connecting the intestine and other organs.
    • Pancreas: white and looks a little bit like cauliflower and located along the underside of the stomach, a pancreatic duct leads to the duodenum – first part of the small intestine. The pancreas also makes insulin, which is necessary for the proper uptake of sugars from the blood. It secretes digestive enzymes and buffers as well that contribute to the digestion of material within the small intestine.
    • Spleen: The spleen is a flat organ located near the stomach. It performs several functions related to producing and maturing new blood cells and eliminating old ones. Blood passes through open sinuses in the spleen, rather than being confined to narrow blood vessels.

    Review

    Identify the organ (or structure):

    1. Opening (valve) between stomach and small intestine.
    2. Stores bile, lies underneath the liver.
    3. Separates the thoracic and abdominal cavity; aids breathing.
    4. Membrane that holds the coils of the small intestine.
    5. The part of the small intestine just after the stomach.
    6. Empties bile into the duodenum from the gall bladder.
    7. The last part of the large intestine before it exits at the anus.
    8. Bumpy structure under the stomach; makes insulin.