Fetal Pig Dissection Guide: External Anatomy and Digestive System

 

External Anatomy: Identify the following orientations and features of the external anatomy of your pig

  • dorsal, ventral, anterior, posterior
  • nares, pinna, nictitating membrane (Close up view here), umbilical cord (with arteries, veins, and allantoic stalk - close up view below)
       
   
   
 
                 

 

Examine the Oral Cavity of your pig

  1. Make a cut to allow the oral cavity to be opened and examined.You should do this with heavy scissors cutting back at the level of the tongue to where your cut exposes the back of the mouth.
  2. Identify the following structures in the oral cavity:
    • teeth
    • hard palate
    • soft palate
    • Opening to nasal chamber
    • tongue and papillae
    • Esophagus
    • Glottis and Epiglottis (what is the function of the epiglottis? Would you find this structure on a fish?)
         
                 
 

 

Examination of Abdominal Structures

  1. Tie the pig down in the dissecting tray as shown below (A).
  2. Make the incisions shown below to begin exposing the abdominal cavity. These should be first made shallowly with a scalpel, then a deeper cut with your scissors should open the cavity (B, dark lines show locations of additional cuts that should be made here)

Once you opened the cavity, take a moment to inspect the epithelial coverings of the cavity and the organs within. The covering of the walls of the cavity is referred to as parietal peritoneum, while that covering the organs is termed visceral peritoneum. These membranes slide easily against each other. Why might this be important?

       
     
                         

 

The diaphragm is a large, flat muscle that forms the anterior boundary of the abdominal cavity and is found above the liver. By contracting, it expands the thoracic cavity that lies above (why during contraction?) and the lungs within this cavity. The pressure differential created between the lungs and external environment causes air to move into the lungs (inspiration).

Next, find the liver (the large, multilobed reddish organ at the top of the peritoneal cavity). Note the placement of the liver in between the gut and the heart. Why is this important in a functional sense?

Underneath the liver, find the gall bladder. This structure stores bile, an emulsification agent important in the digestion of fats (Why is the emulsification important in this process? Think in general terms here.). The gall bladder empties into the small intestine at the duodenum first via the cystic duct. The cystic duct meets the hepatic duct coming from the liver to form the common bile duct. The common bile duct carries bile to the small intestine.

   
                 

 

Next, find the stomach. This muscular organ is primarily responsible for mechanical digestion of food, although some chemical digestion begins here as well. The stomach is a tough environment to be an epithelium in. Why is this and how is it adapted to this 'life of rigor'? (What is the medical condition that results if it does not withstand the difficult conditions?)

The stomach curves towards the midline from the anterior portion meeting the esophagus to the posterior portion meeting the small intestine. Two sphincter muscles (rings of muscle that can open or close the opening of the tube) control movement of materials into and out of the stomach - the cardiac sphincter at the anterior end and pyloric sphincter at the posterior end (click here for a view of the pyloric sphincter).

Next, find the small intestine. This organ is the major site of nutrient absorbtion and chemically-mediated breakdown of food. It has three segments:1) the duodenum meets the stomach at the pyloric sphincter (a circular band of muscle tissue that serves as a valve), 2) the jejeunum is the middle segment and 3) the ileum is the segment. The latter two segments are sometimes referred to together as the jejuno-ileum.

If you cut open a part of the jejuno-ileum and examine this with a dissecting scope, you should be able to see the finger-like villi.

Quick recall test: In what ways is the small intestine specialized for absorbtion? (hint: One part of the answer should be obvious when looking at the peritoneal cavity of your pig)

Follow the figure below in a clockwise pattern to go from macroscopic to microscopic scales. A) Intestinal Structure, B) Structure of Intestinal villi, C) Electron micrograph of the membrane of a intestinal epithelium showing microvilli projections into lumen, and D) a micrograph of villi in cross-section.
   

 

The next two structures to find are the spleen and pancreas.

(Click here for spleen and pancreas pictures)

The spleen is the reddish organ lying along the greater curvature of the stomach. It functions to remove abnormal blood cells and other blood elements by phagocytosis, store iron from recycled red blood cells (where does the iron come from?), and a role in the initiation of immune reponses.

The pancreas has both endocrine and exocrine functions (i.e., it secretes materials both into and out of the body). The bulk of the pancreas in a human (~99%) serves an exocrine function and plays a key role in digestion through the secretion of digestive enzymes important for the digestion of proteins, carbohydrates, lipids, and nucleic acids.

The endocrine function of the pancreas is equally critical. The pancreas is the source of two hormones that regulate glucose metabolism: insulin and glucagon.