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

Brain of the Beaver (Castor canadensis)

Photos and text by Jason Lavender and Wei-Chung Chen

Overview of Blood Circulation and the Brain:

In examinations of the brain, its arterial supply and venous drainage are inseparable components of the overall anatomy. In the beaver, there are three large arterial branches off the aortic arch. From right to left, these branches are the brachiocephalic trunk, left common carotid artery, and left subclavian artery. The brachiocephalic trunk splits into the right subclavian artery and the right common carotid artery, which supply the right upper limb and the brain/face, respectively. Similarly, the left common carotid artery supplies the brain and face and the left subclavian artery supplies the left upper limb. The left and right common carotid arteries further divide into internal and external carotid arteries. The external carotid artery goes on to send off several branches, including into facial, lingual, superficial temporal, maxillary, and auricular arteries. These arteries help supply the more superficial tissues of the head, the face, and the greater part of the neck. The internal carotid artery is relatively small, and only supplies a small percent of the arterial blood supply to the head. The vertebral arteries arise from the left and right subclavian arteries and they supply the majority of the blood supply to the brain.

The facial, lingual, and maxillary veins collect deoxygenated blood from the facial regions and unite to form the internal jugular vein. Likewise, the auricular and superior temporal veins (which drain other parts of the deeper facial regions and tissues on the superficial surface of the skull) unite to form the external jugular artery. The two jugulars proceed caudally, and unite to form a brachiocephalic vein, which then empties into one of the two precavals that return blood to the right atrium of the heart.

At the base of the brain is the cerebral arterial circle. The cerebral arterial circle contains all the major arteries that supply the cerebral hemisphere of the brain. Specifically, the union of the two internal carotid arteries forms the rostral portion, and the basilar artery, a union of the two vertebral arteries, forms the caudal portion.

Dissection:

We began the lab by removing the right and left temporalis muscles. This proved to be difficult because the muscles were tightly bound to the cranium, especially the more caudal portions. After we removed the majority of the muscle from the superior and superolateral aspects of the cranium, we began to cut into the cranium. Due to the thickness of parietals, the frontal, and the occipital bones, we used an electric stryker saw to cut through the cranium. We began by making a lateral cut, from the rostral portion of the cranium just above the orbit to the caudal portion just behind the ear. We made a similar cut on the opposite side. Next, we made a horizontal cut along caudal portion of the cranium, followed by another horizontal cut along the rostral portion.

With all four cuts penetrating the cranium, we removed the top of the braincase. The dura mater was relatively easy to remove, as most of it was torn during removal of the brain case. The other meninges were not readily visible. The olfactory bulbs are quite large, and when the top of the cranium was removed, a major portion of them was pulled away from the brain. The hemispheres of the brain are smooth without any convolution. We also made several incisions on the caudal portion of the cranium using the electric saw to provide a better view of the cerebellum, brainstem, and spinal cord. We next removed the brain in order to view its ventral aspect. We accomplished this by prying the brain gently from the base of the cranium, in an attempt to maintain some of the cranial nerves. There were remnants of several cranial nerves, including the paired optic nerve and the optic chiasm.

Conclusions and Comparisons:

The brain is composed of separate portions including the cortex, cerebellum, brainstem, and pons. The cerebral cortex is divided the right and left hemispheres. Caudal to the cortex is the cerebellum, which is responsible for fine movements and motor coordination. The hindbrain and brainstem (including the medulla and pons) are responsible for autonomic functions, including thermoregulation, respiration, etc.

There are many similarities between the rat brain and the beaver brain. First, and most obvious upon initial inspection, was the equally large size of the olfactory bulbs in each. Rodents rely heavily upon scent for interpretation of the environment, and the increased size of the olfactory bulbs relative to the size of the brain itself reflects this fact. In comparing the cortex in both the rat and beaver, both appeared to be relatively smooth. There were slightly more convolutions in the beaver cortex, although overall the differences were negligible. The superior sagittal fissure appeared deeper and better defined in the beaver as well.

Given the size of the beaver, it was surprising that there were so few convolutions in the cerebral hemispheres. Compared to other mammals of smaller size, the beaver cortex actually had fewer convolutions. This may be an individual peculiarity, however, or even a matter of poor preservation, due to the fact that other references to the beaver brain indicate the presence of more convolutions that were seen in this dissection [3].

Another interesting difference between the beaver brain and those of the other mammals was the inequality seen in the size of the beaver’s cerebral hemispheres. In the beaver, the right cerebral hemisphere appeared larger than the left, while this size difference was not seen in the other mammalian brains. This is most likely another case of individual difference, however, as difference in cerebral hemisphere size is not usually a mammalian trait [3].

The arterial circulation to the brain is variable among mammals, and the venous circulation is even more variable among mammals, as well as among individuals. In humans, for example, the major blood supply to the brain is obtained from the internal carotid artery, whereas the vertebral arteries are much more important for supplying the beaver brain with blood. In this matter, the rat is more similar to the human form, as its major blood source is the internal carotid artery (although the rat depends on its vertebral arteries as another source of blood for the brain more than a human does). In most mammals, the external carotid artery does not supply the brain with blood, or if it does, it is usually only a minor source. In the beaver, as well as in the rat, the external carotid branches into many smaller arteries that supply most of the tissues of the face, and do not supply the brain at all.

References:

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

2. Hebel, R., & Stromberg, M. W. 1986. Anatomy and Embryology of the Laboratory Rat. BioMed Verlag, Federal Republic of Germany.

3. Welker, W., Johnson, J. I., & Noe, A. Comparative Mammalian Brain Collections. University of Wisconsin, Michigan State University, and the National Museum of Health and Medicine. 22 Apr. 2004.  <http://brainmuseum.org/index.html>