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 Rat (Rattus norvegicus)

Photos and Text by Ana Panduro and Yu Jin Kim

Part I: Thoracic Cavity

The thoracic cavity is separated from the abdominal cavity, which contains the gastrointestinal tract, by the thoracic diaphragm. The thoracic cavity contains the great vessels, the heart and the lungs, and some smaller structures.

To expose the contents of the thoracic cavity, we first used a scalpel and made a transverse incision anterior to the xiphoid process of the sternum in order to avoid cutting into the abdominal cavity. The incision was caudal to the ribs that attached to the sternum and ran from the right side of the ventral thorax to the left side of the ventral thorax. Then, two caudal-to-cranial incisions were then made from each side (right and left) until the clavicles were reached. Finally, we cut across the area posterior to the clavicles and removed the cut ribcage. The opening we made was about 5 cm wide.

The rat has right and left lungs, each covered in parietal and visceral pleura.  The right lung has four lobes and is positioned lateral and dorsi-lateral to the right side of the heart.  A small lobe of the right lung extends caudally to the heart.  The left lung has one lobe and is positioned lateral and dorsi-lateral to the left side of the heart.  The lobes of the lungs branch out laterally and are not as close together as the lobes of the cat or human. Because of this effect, the lungs do not seem to provide as much protection or cushioning to the heart as they do in the cat, whose lobes are bunched together. However, the rat lungs encase the heart more than those of the rabbit because the rat lungs are larger relative to the heart.

The mediastinum is the space between the lungs. In this area we identified the two phrenic nerves, which run cranial-caudally from each side of the pericardium to the diaphragm, which they innervate.  In humans, the diaphragm cradles the human thoracic viscera. However, the same phenomenon does not occur in rats.  There is a lot of space between the diaphragm and the posterior ends of the heart and lungs, indicating that the rat lungs expand a lot. 

Located deep to the sternum and anterio-superior to the heart is the thymus gland.  The thymus plays an important role in an animal’s immune system by producing antibodies and lymphocytes [1].  The younger the animal is, the larger the thymus is. The thymus of our rat was about 1/5 of the size of the heart.  In humans as in rats, it degenerates after puberty and is gradually replaced by fat [1]. Not surprisingly, we discovered a large fat deposit deep to the thymus, indicating that degeneration had already begun.  We were nonetheless astonished by the amount of fat present since our specimen has had little fat everywhere else.

After carefully removing the left lung, we were able to see the dorsal aorta, esophagus, vagus nerve, and azygous vein. The most medial of the four is the descending aorta. Lateral (left side) to it is the left vagus nerve, which is located superior to the esophagus. The esophagus is situated deep to the vagus and medial to the azygous. The azygous vein, which flows posterior-anteriorly into the left vena cava, starts out being medial to the esophagus, but proximally, it becomes superior to the vagus and esophagus.

As in most mammals, the apex of the rat heart is oriented ventrally. Before cutting the heart out in order to examine its anatomy, we had to first remove the fat from the pericardium. Though there was less fat surrounding the heart than deep to the thymus, a significant amount of fat is necessary to cushion and protect the heart.

Rattus norvegicus has two precavals, a cranial (superior) vena cava and a left cranial vena cava. The left cranial vena cava receives veins of the thorax and the azygous vein, and travels ventral to the aortic arch and the root of the lung to reach the right atrium [2].  Except for the additional precaval (left cranial vena cava), the circulation through the rat heart is similar to that of any other mammal. Deoxygenated blood enters the right atrium through the cranial vena cava, the inferior vena cava, and the left cranial vena cava. Once in the right atrium, deoxygenated blood flows to the right ventricle, then through the pulmonary arteries and into the lungs to be oxygenated. The newly oxygenated blood flows out of the lungs and into the left atrium through the pulmonary veins. From the left atrium, the blood flows into the left ventricle and into the aortic arch to be distributed to the rest of the body.

The right and left auricles of the heart are proportionally large compared to those of the other animals in the class. The right atrium is much larger and darker than the left atrium. The opossum and the rabbit also have proportionally large auricles. Like the rat, they have two precavals, a right (anterior) vena cava and a left vena cava. The rest of the mammals in lab have small auricles and only one precaval (anterior vena cava).  Based on this sample, our hypothesis is that mammals with two precaval veins have proportionally larger auricles.

The rat has three arterial branches from the aortic arch. The most lateral branch on the left side is the left subclavian artery. Medial to it is the left common carotid artery, which runs cranially. Medial to the left common carotid is the brachiocephalic trunk, which splits into two: the right common carotid and the right subclavian. The right common carotid runs parallel to the left common carotid and flows cranially. The right subclavian inserts deep into the right arm. The arterial branching pattern is identical to those of humans and beavers. Although the number and order of these branches vary within the Class Mammalia, the beaver and the rat may have identical branches because they belong in the same order Rodentia.

References:

1. Cartmill, Matt et al. Human Structure. Cambridge: Harvard University Press, 1987.

2. Hebel, Rudolf and Melvin W. Anatomy and Embryology of the Laboratory Rat. Stromberg. 1986. BioMed Verlag, Germany.

3. Greene, Eunice C. 1935.  Anatomy of the Rat. American Philosophical Society, Philadelphia.