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 Beaver (Castor canadensis)

Photos and Text by Jason Lavender & Wei-Chung Chen

Overview of Heart and Thorax:

The function of the mammalian heart is to circulate blood through the body. In the beaver, the process begins when deoxygenated blood from the caudal and cranial portions of the body returns to the heart through two precavals (right and left), one postcaval (right), and one large azygous vein (left). The deoxygenated blood flows into the right atrium and then drains into the right ventricle through the right atrioventricular (AV) valve. The AV valves prevent the backflow of blood into the atria from the ventricles, and their function and general form is the same in most mammals. The valves are composed of long, thick papillary muscles originating from the muscular walls of the heart with chordae tendinae attached at the valve. From the right ventricle, the deoxygenated blood is sent through the pulmonary arteries, which branch into multiple capillaries in the lungs. These capillaries surround the alveoli of the lungs where oxygen and carbon dioxide exchange occurs. Number of lobes, size, and shape of the lungs vary across mammalian species. The beaver lungs are divided into unequal right and left sides. The right lung has two lobes, one of them bifid. The left lung has four lobes, one of which is lies along the midline of the thorax caudal and somewhat dorsal to the heart.

After the blood is reoxygenated in the lungs, it returns to the heart through the pulmonary veins, emptying into the left atrium. This blood passes through an AV valve to enter the large, muscular left ventricle, which pumps blood into the aorta to be distributed throughout the body. This circulation pathway is the same in most mammals, but differences appear in the branching pattern of arteries from the aortic arch. In the beaver, there are three large branches from the aortic arch. From right to left, these branches are the brachiocephalic trunk, left common carotid artery, and left subclavian artery. The brachiocephalic trunk is short (~0.5 cm) and splits into the right subclavian artery and the right common carotid artery, which supply the right upper limb and the head, respectively. Similarly, the left common carotid artery supplies the brain and left side of the face and the left subclavian artery supplies the left upper limb.

In contrast to the position of the human heart, the right and left divisions of the beaver heart reflect the heart’s anatomical position. The atria are positioned cranially to the ventricles, with portions also located craniodorsally and cranioventrally. Adipose tissue is found on and around the heart, in the pericardium, attached to the aortic arch, and around the precavals and postcaval. There is a particularly large deposit of fat embedded in the caudal pericardium between the heart and the diaphragm. Due to the greater pressure on the left side of the heart, the muscular wall of the left ventricle is thicker than that of the right ventricle, another feature true of mammalian hearts in general. Other noticeable features of the beaver heart are the auricles. While the right auricle is completely continuous with the right atrium, the left auricle is attached loosely on the cranial left portion of the heart. These muscles are believed to aid the atria in driving blood into the ventricles [1]. The rest of the atria are very thin-walled and sac-like, containing no visible musculature at all.

Dissection:

Before making an incision, we located the caudal boundary of the thorax by palpating the xiphoid process. We then began the dissection of the thorax by making a long horizontal incision just caudal to the xiphoid process. We then made two lateral cuts (one left and one right) from the respective ends of the horizontal incision and up to the beaver’s neck. Finally, we made another horizontal cut just caudal to the neck region. While making this horizontal cut, we encountered a large mass of glandular tissue. Based on knowledge of the beaver’s eating habits and the location of the deposit, we determined that this was in fact an expansive fused salivary gland.

Immediately deep to the skin, we encountered a very thick layer of fat (1-2cm), which we removed for the most part before continuing with the deeper dissection. Beneath this layer of fat are the pectoral muscles. There are three layers of muscle that overly the thoracic cavity. The superficial layer is comprised mainly of pectoralis profundus. Beneath this layer lies the pectoralis sublimis*. The deepest layer is comprised of intercostal muscles situated between the ribs. After we identified the muscles overlying the thorax, we removed them, and then proceeded to remove a ventral section of the ribcage. We first used the scalpel to outline the area of the ribcage that we desired to remove. In doing so, we penetrated more deeply into the previous incision made just caudal to the xiphoid process. We were able to see an organ through the incision at this point. We determined that the organ was the liver, which meant that we had unintentionally penetrated the abdominal cavity.

The ribs are thick and robust, and thus required a larger tool to cut them. We used gardening shears to make two cranial-to-caudal cuts along the same lines as the two previous lateral cuts. Following the same path as the more cranial horizontal incision, we cut through the manubrium, and were thus able to remove a ventral portion of the ribcage and sternum, exposing the internal organs of the thorax. Due to the attachments of the diaphragm and the unique proportions of the thorax compared to the abdomen, we were forced to cut the diaphragm. Upon lifting the rib cage, the first observation we made was of the unique shape of the xiphoid process. The bony element of the xiphoid process is relatively small and fused to end of the manubrium. It is surrounded by cartilage, which forms a nearly perfect semicircle, so that in situ, this cartilaginous extension of the xiphoid process lies on the upper lobe of the liver. The cartilage is relatively thick, and the true bony portion of the xiphoid process was only visible by holding it up to a light.

After lifting the rib cage, we made our initial observations of the heart. The anatomical position of the heart corresponds almost perfectly to the left and right divisions. We hypothesized that the lungs would be large because the beaver must hold its breath underwater for long periods of time, but at first sight, the lungs appeared to be smaller than expected. When we removed the lungs, however, we discovered they are indeed large, and had been occupying deep pleural recesses. Next, we removed ribs 1-3 and the clavicles, which are situated cranially to thoracic cavity. This allowed us to better observe the esophagus, trachea, and the aortic arch. Looking carefully at the aortic arch, we uncovered the three aortic branches, mentioned above and figured below.

We completed the dissection by identifying the veins entering the heart (mentioned above and figured below). First, we removed the heart. When we cut the arteries/veins exiting and entering the heart, we left enough of a stump/root from the heart to identify these vessels with the heart removed.

* For more information on the pectoralis muscles, see the Beaver Shoulder Lab.

Conclusions and Comparisons:

The figure to the right diagrams the aortic branching pattern that is common to the beaver, the laboratory rat, and to humans. In this pattern, there are three branches from the aortic arch: the brachiocephalic trunk (which splits into the right subclavian artery and the right common carotid artery), the left common carotid artery, and the left subclavian artery.

Reference:

1. Morgan, L.H. 1868. The American Beaver and His Works. J. B. Lippincott & Company, Philadelphia.