Laboratory Objectives:

You will supplement your measurements of circulatory function this week with dissection and examination of the fetal pig circulatory system and some associated structures.  The following dissection guide gives you both the general procedures to follow at each step and the structures to look for. 

 
 Glands and Respiratory Structures of the Neck and Thoracic Cavity

*Wear Disposable gloves when dissecting preserved animals*

Figure 1. Ventral view of the anterior region of the pig, showing structures in the neck region and the thoracic cavity. The pericardial sac encloses the heart.

Procedure

 Open the pig's thoracic cavity by making an incision with scissors.  Make this shallow at first and
gradually go deep to get into the thoracic cavity.  You will need to cut through the ribcage to get into the thoracic cavity.  To do so, avoid the sternum by going to either side of it where you find softercartilage to cut through.  Cut through the rib cage and upwards until you are through the skin and intothe lower jaw area.

  Work your way through the muscle in the neck region (a probe or closed scissors are good for this).  Find the thymus gland overlying the heart here.  This gland is quite large in young mammals and serves an important immune function.  A second location where thymus can be found is as two longish masses flanking the thyroid gland more anteriorly in your pig. The thyroid gland is a small reddish structure lying over the trachea more anteriorly in your pig.  The thyroid gland is the source of thyroid hormones (thyroxin and triiodothyronine) and is a key endocrine gland regulating metabolism among other things.  Note the trachea underneath the thyroid gland and extending posteriorly towards the lungs and anteriorly towards the larynx.  What type of tissue gives the trachea its necessary rigidity?

   
The Arterial and Venous Systems
 

 
































Figure 2 (Left) and Figure 3 (Right) below are schematics of the arterial and venous systems respectively.

Procedure

 The major vessels in each system are labeled, but there are also smaller vessels that are not shown.  Use these diagrams in combination with the photos below to identify these structures (the diagrams should open in a separate window, allowing you to click back and forth for reference).  The major veins on your pigs should have blue latex in them while the major arteries should have red/pink latex in them to facilitate identification.

 
   The Heart and Pulmonary Blood Circuit

Figures 4 and 5 (below, left) illustrate the fetal heart, showing the four chambers and the major associated blood vessels.

Procedure 

The heart lies inside a tough connective tissue membrane - the pericardium.  The pericardium actually consists of two parts, the visceral pericardium covering the heart itself and the parietal pericardium which forms the sac enclosing the heart.  The space between these two membranes is termed the pericardial cavity.  Open the parietal pericardium to expose the heart.

You should be able to identify the four chambers of the pigs heart externally: the right and left atria(singular: atrium) and right and left ventricles.  The right and left atria are the smaller dark
structures at the top of the heart that receive blood from the vena cava and pulmonary veins
respectively (see below).  The division between the right and left ventricles can be seen externally bythe location of a prominent groove termed the coronary sulcus that is also the path the coronary
artery and vein take across the heart (the coronary artery is the first branch off the aorta).

    Next, locate the key large blood vessels near the heart.  Recall that there are two separate
circuits that the heart pumps blood into and receives blood from in adulthood: the pulmonary circuit serving the lungs and the systemic circuit serving the rest of the body (these are not completely separate during fetal life - Why not? Answer this question for yourself in both thefunctional and morphological senses).  You should be able to find the pulmonary trunk leaving the right ventricle.  This vessel soon splits into three branches: pulmonary arteries headed towards the right and left lungs and the ductus arteriosus (What is the function of theductus arteriosus and what other structure serves a similar function?). 

 
 
 
 
 
 
  Thoracic and neck vessels of the Systemic Circuit

Figure 6 and 7 (Right) show the Veins of the thoracic cavity and neck area and the Arteries of the thoracic cavity and neck area, respectively.(click on the images for to see each respective shematic diagram).
 
Procedure

  Move anterior to the heart now to locate the major veins of the thoracic and neck regions.  You should be able to see that blood returning from the anterior part of the body is collected into the cranial vena cava (also called the superior vena cava) before entering the heart at the right atrium.  The cranial vena cava splits anteriorly to form two brachiocephalic veins, which themselves split almost immediately into three veins on each side: a subclavian and the internal and external jugular veins.  The subclavian veins split to give rise to the subscapular and axillary veins (draining the shoulders and forelimbs respectively).  You may also find the cephalic vein entering the external jugular near its base.

We will focus now on the arteries of the thoracic cavity and neck region.  Move the pulmonary trunk down and back to observe the aortic arch.  As shown on the diagram above, this structure exits the left ventricle and immediately curves downward while giving off branches that run anteriorly primarily.  The first major branch is the brachiocephalic trunk, which soon gives rise to three branches: the right subclavian artery and the two common carotid arteries.  The next major branch off the aortic arch is the left subclavian artery.  This artery gives rise to two prominent branches: the left subscapular artery running deep into the shoulder and the left axillary artery serving the forelimb. 

By moving the pig's lungs aside, you should also be able to see where the aortic arch gives rise to the dorsal aorta and its connection to the pulmonary trunk via the ductus arteriosus.

 
  Arteries and Veins of the Abdominal Cavity
 

Figure 8 shows the branches of the aorta and caudal vena cava in the abdomen - branches of the aorta supply blood to the stomach (the coeliac artery), the small intestine (the cranial mesenteric artery), the kidney (renal arteries), the hind limbs (iliac arteries), and placenta (umbilical arteries). Branches of the caudal vena cava drain blood from the kidney (renal veins) and
posterior limbs (common iliac veins).

Procedure

The first major branch of the dorsal aorta going posteriorly is the coeliac artery, which is found at approximately the level of the aorta and serves the stomach and spleen.  You may have to detach parts of the diaphragm muscle to see it.  Moving more posteriorly, look for the cranial mesenteric artery that serves the small intestine.  Continuing caudally, find the pair of renal arteries going to the kidneys and the closely associated renal veins.  The dorsal aorta continues caudally until it divides to form the umbilical arteries and external iliac arteries.  If you follow an external iliac artery and probe through the muscles of the leg, you should be able to find the femoral artery (the lateral branch) and the deep femoral artery (the more medial branch).

Now examine the venous system in the abdominal cavity.  First, find the caudal vena cava (also known as the inferior vena cava), the vein which collects blood from the lower portion of the body for return to the heart.  Since this vessel actually lies under the parietal peritoneum, you will be able to see it better if you peel this epithelium back.  The vena cava splits posteriorly to form two common iliac veins, which then split to form external and internal iliac veins.  As with the external iliac artery discussed above, the external iliac veins split to give rise to the femoral veins and the deep femoral veins.


*N.B. -You will observe the posterior branches of the aorta after the dissection of the reproductive system*

Please, give your T.A.'s a break  -- Don't forget to CleanYour Station Before Leaving!!  -- oink oink