Shoulder

Brachium

Antebrachium

Facial and Masticatory Muscles

Thorax and Heart

Brain

Body Wall

Hip and Thigh

Shank and Foot

Digestive System

Urinary and Reproductive Systems

Hip and Tail

Thorax and Heart of the Opossum (Monodelphis domestica)

Photo and Text by Sarah Ogburn and Linda Evonne Brogdon

Initial Dissection:

To open the thoracic cavity, we located the most caudal point of the sternum and selected two points about a centimeter lateral to this point to make our incision.  We then used the scalpel to cut away the muscle over the ribs, and made two small slits into which we could insert the small scissors.  We then began cutting cranially from the slits through the ribs until we reached the neck.  We were then able to remove the rectangular section we had created, thus exposing the heart and lungs.  The opening was a bit too small, so we trimmed away at the ribs by making transverse cuts just cranial to the diaphragm and just cranial to the first rib.  We were careful not to perforate the diaphragm and open the abdominal cavity.  Having done this, we were able to pry back the remaining ribs and pin them so the entire thoracic cavity was exposed. 

The heart lies slightly to the left of center between the right and left lungs. The right lung consists of four lobes: the cranial, middle, caudal, and accessory lobes.  It is very large compared to the left lung. The left lung had unseparated cranial and caudal lobes [1].

The trachea is large in proportion to lungs and when compared to that of other mammals.  It is supported by 25 c-shaped tracheal cartilages [1]. 

The very small, white phrenic nerves are visible running lateral to the pericardium and along approximately the same paths as the right and left precaval veins.  The phrenic nerves innervate the thoracic diaphragm.

The heart has right and left auricles, confluent with the atria.  The right is larger than the left.  The right auricle has sometimes been described as bifurcated, with one lobe lying ventral to the ascending arch of the aorta and the other lying dorsal to it [2]. 

The heart is divided into four sections: the right and left atria and the right and left ventricles.  The ventricles are larger and lie caudal to the atria.  Deoxygenated blood from the body enters the right atrium via the left and right precaval veins and the postcaval vein.  It then passes to the right ventricle where it is pumped through the lungs via the pulmonary arteries.  The pulmonary veins carry the oxygenated blood into the left atrium, where it passes into the left ventricle.   The left ventricle pumps the blood into the ascending aorta, where various arteries carry the oxygenated blood throughout the body.

The various arteries and veins were difficult to differentiate and locate because the thoracic cavity is so small.  The precavals were the most readily observable feature.  Marsupials have two precaval veins--a right and left--and one postcaval vein.  All drain into the right auricle.  The left precaval and postcaval enter in close proximity to one another at a point more caudal and to the left of the right precaval.  The right precaval travels cranially along the right side of the thoracic cavity where it becomes the right brachiocephalic vein. The right internal and external jugular veins drain into the brachiocephalic vein.  The internal jugular veins are substantially smaller than the external jugular veins, and drain only the deep neck area and a very small portion of the brain [3].  It was so small in our specimen that we could not locate it.

Using the microscope at 10x magnification, we traced the left external jugular cranially.  The left and right external jugular veins reach a branching point in the neck region just caudal to the mandible. At this point we could see the facial and lingual veins branching rostro-ventrally along the ventral surface of the jaw.  The maxillary vein branched deep to the rest of the veins and was difficult to see as it entered the skull.  The superficial temporal vein branched dorsally toward the ear.  The veins became increasingly smaller as we tried to trace them further, and eventually it was impossible to follow them further without breaking them and losing their paths. 

The left precaval wraps around the dorsal aspect of the heart and travels cranially along the left side of the thoracic cavity where it branches in the same manner as the right precaval.  The postcaval vein travels caudally on the right side of the thoracic cavity and perforates the diaphragm before traveling into the abdominal cavity [3].

The azygous vein branches from the postcaval just cranial to the heart.  It then continues caudally on the left side of the thoracic cavity.  Its caudal end joins the postcaval vein near the lower thoracic vertebrae.  The azygous vein gives rise to the intercostal veins, which drain the intercostal spaces [4].

The arch of the aorta gives rise to the brachiocephalic artery, which branches into the right subclavian artery and the common carotid artery.  The common carotid artery then branches into a left and right carotid artery.  The left subclavian artery arises slightly to the left (toward the left atrium, the viewer’s right) of the brachiocephalic artery. 

The descending aorta continues caudally along the dorsal surface of the heart and perforates the diaphragm before passing into the abdominal cavity. 

There are several features of the marsupial heart that distinguish it from other mammals. Some of these are considered to be primitive—that is closer to the condition seen in monotremes, birds, or reptiles.  These include:

1. The right ventricle is crescent shaped in transverse section and wraps around the left ventricle.  The degree to which it wraps is a feature shared by monotremes and birds [3].

2. There is only one atrioventricular flap of the atrioventricular valve as opposed to the three flaps found in other mammals.  This is similar to the condition found in monotremes [3].

3. The fossa ovalis, annulis ovalis and ductus arteriosus are absent in the marsupial heart.  These are present as traces of fetal structures in the hearts of placental mammals.  In placental mammals, they allow for the flow of blood between the right and left atria, bypassing the lungs.  Their absence in marsupial hearts has been attributed to the presence of small openings instead of these fossae which are present for only a short period of time and close up quickly around the time of birth.  Both monotremes and birds have these small openings [2].

4. The right auricle is sometimes bifurcated as previously mentioned [5].  We were not able to locate a complete dorsal portion of the right auricle, but it appeared that one might have been damaged and removed in the course of the dissection.

 5. The cardiac veins pursue a course similar to that in birds; that is, the cordis magna opens into the right atrium instead of the left precaval vein [4,5].

6. The cardiac arteries also pursue a course more similar to that in birds than in placental mammals [5]

Vessels of Monodelphis domestica (adapted from sources [2-6] and our observations)

References:

1. Cope, L. 2001. Comparative anatomy of the lower respiratory tract of the gray short-tailed opossum (Monodelphis domestica) and North American opossum (Didelphis virginiana). Accessed 2004 at http://etd.utk.edu/2001/CopeLeeAnne.pdf

2. Dowd, D. 1969. Gross features of the heart of a marsupial Trichosurus vulpecula.  Acta Anatomica 74: 454-471.

3. Wade, O.; Neely, P. 1949. The heart and attached vessels of the opossum, a marsupial.  Journal of Mammology. 30:111-116.

4. McClure, C. 1903. A contribution to the anatomy and development of the venous system of Didelphis marsupialis. Part I. Anatomy. American Journal of Anatomy. 2: 371-404.

5. Dowd, D. 1974.  The coronary vessels in the heart of a marsupial Trichosurus vulpecula.  American Journal of Anatomy. 140:47-56.

6. Ellsworth, A. 1976. The North American Opossum: An Anatomical Atlas. Robert E. Krieger Publishing Company, New York.