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 Rabbit (Oryctolagus cuniculus)

Photos and Text by Janelle Cooper and Alexandra Sardi

Dissection Notes

Thoracic Cavity:

In order to get to the thoracic cavity we made two sagittal incisions from the neckline down to the second most caudal rib, at the level of the diaphragm.  We then made two lateral incisions and lifted off the “breastplate” to expose the thoracic cavity.  This procedure was easily performed with a scalpel since the ribs are not very strong. We noticed an extremely large thymus gland wrapped around the cranial half of the heart.  We removed the thymus and then cut through the pericardium. The pericardium fit loosely around the heart and was easy to remove.  We discovered what looked like three lobes of the lungs, covered by parietal pleura on each side.  Once we removed the lungs, we discovered there were actually three right lobes and two left lobes of the lung. The caudal lobe on the left had two parts.  Part of the right caudal lobe was situated caudal to the heart.  We found the diaphragm caudal to the left lung.  The diaphragm was very thin and easy to accidentally cut through.  The heart, lungs and thymus were all easy to remove.   Once removed and cleaned with water the atria of the heart tended to collapse.  Their lack of rigidity and the presence of blood and latex made it hard to dissect and distinguish structures within the heart.

Circulation To the Head: 

It was easy to reach the veins and arteries that supply and drain the head.  After dissecting the neck muscles and removing most of them, the arteries and veins were visible, yet covered in fascia.  Once we removed the fascia, most of them were clearly visible.   Our only surprise was discovering what seemed to be fused internal jugular veins. These will be discussed below.

Discussion

Thoracic Cavity: 

The first thing we noticed upon opening the thoracic cavity was the extremely large thymus gland.  The thymus gland is known to be active only during fetal and juvenile life so it is possible that our rabbit was a subadult (1).  The lungs were also plainly visible and had three lobes on the right and two on the left, an asymmetry that is common to most mammals.  Exceptions to this pattern are the horse, which has no lobes, and monotremes, and rats, which have lobes on the right side only (1).   The heart is large and found slightly left of the center of the thoracic cavity.  It rests against the sternum and is protected by fat and the lungs.   The apex of the heart is oriented posteriorly.  We also identified the aorta, superior vena cava, inferior vena cava, pulmonary arch, and pulmonary veins.  Once we removed the heart, the relative positions of the great vessels became much clearer.  It was possible to see the aortic arch, as it first branches into the brachiocephalic trunk and then further cranially where it branches into the left subclavian and left common carotid arteries.  On the dorsal side, we could see the point at which the caudal vena cava enters the heart. We could also see the cranial vena cava and the pulmonary trunk.   There are two cranial venae cavae: one on each side. These vessels drain the brain and head.  The azygous vein that enters the right cranial vena cava is small, and we did not see it.  Both pair of the internal and external jugular veins were identified, but as stated before, the internal jugulars were fused into one vein that was positioned ventral to the esophagus.  The possibility of this unpaired structure being a varicosity of the esophageal vein was ruled out after ascertaining the presence of a separate esophageal vein.  

The relative positions of the common carotid arteries and subclavian arteries are extremely variable.  Cats, dogs, pigs, humans and rabbits usually have three arteries that branch off the aortic arch. The right side brachiocephalic artery then splits into the right subclavian artery and right common carotid artery.  Sometimes there will be only two branches from the aortic arch and both will split to give the four typical arteries (1).  This last pattern is less common but may happen in some mammals.  Our rabbit displayed the more common pattern.  There is great variation in the pattern of arteries of the heart. There is not only great variation among different species, as described above, there is also great variation between individuals of the same species.  If individuals can survive and reproduce with so many different patterns, it is unlikely that there is much functional significance to the order in which the arteries branch (2).   Since embryonic mammals display up to six aortic arches, of which three develop into major arteries, it is probable that these differences arise during development (2,1).

Our dissection of the heart consisted of a coronal section through the heart to expose the atria and ventricles.  We were able to identify the chordae tendinae and the papillary muscles as well as the right and left atrioventricular valves.  The lungs of the rabbit seemed to wrap around the heart.  The functional significance of this specialization is that when the rabbit is running, the lungs act as a cushion for the heart to keep it in place and protect it from trauma (4).  Since rabbits must make quick, rapid movements, it is imperative that their hearts are protected during locomotion (4).  None of the other specimens in class had this specialization.

Circulation To The Head:

We dissected cranially in the neck to follow the common carotid artery into the head.  We discovered that it bifurcates into internal and external branches at the joint of the head and neck.  The external carotid artery then splits into the maxillary artery and further cranially, into the auricular and superficial temporal arteries.  The external carotid artery also splits into the facial and lingual arteries.  The transverse capular vein connects the two external jugular veins.   The lingual, auricular, superficial, and temporal veins drain into the maxillary vein, which drain into the external jugular vein. The maxillary and facial veins drain directly into the external jugular (3).   We also followed the internal carotid artery and found that it leads into the occipital artery cranially. 

References:

1. Kardong, Kenneth V., Vertebrates: Comparative Anatomy, Function, Evolution. McGraw Hill, New York: 2002.

2. McLaughlin C.A., Chiasson R.B., Laboratory Anatomy of the Rabbit, McGraw-Hill, New York: 1990.

3. Popesko, P., Rajtova V., Horak J., A Colour Atlas of Anatomy of Small Laboratory Animals, V1.  Wolfe Publishing Ltd. London, England: 1992.

4. Simons, R., “Lung morphology of cursorial and non-cursorial mammals: lagomorphs as a case study for a pneumatic stabilization hypothesis,” Journal of Morphology, 230:299-316.