For comparison, see brain dissections of a bat, a fox, and a tree shrew.

Our dissection of the heart in the previous lab revealed the great veins. We could easily identify these vessels because the animal had a colored latex injection. In a double-injected animal, the veins are injected with bright blue latex to indicate that they had held deoxygenated blood. The great vessels are ventral to the trachea and esophagus and dorsal to the sternum. When the sternum and ribs are removed, the cranial vena cava is clearly visible, as are the left and right subclavian veins and the left and right external jugular veins. The subclavian veins bring blood from the brachii and the external jugular veins descend from the head. Both sets of vessels drain into the cranial vena cava. The five-point intersection of these vessels is three to four centimeters caudal to the shoulder joints. The drainage point of the subclavian veins is just caudal to that of the external jugulars.

The cranial vena cava has a diameter of three to four millimeters and is the largest vessel cranial to the heart. The external jugular veins are much larger than the subclavians, and are nearly the same size as the cranial vena cava. We could not immediately see the internal jugular veins. The large size of the external jugulars suggests that the internal jugulars do not have a great diameter. Other carnivores with enlarged external jugular veins have reduced or nonexistent internal jugulars. The right and left vertebral veins also flow into the superior vena cava, but they run deep within the vertebral foramina in the cervical vertebrae and we could not observe them.

We removed the external jugular veins and the subclavian veins to observe the deeper arteries. The left internal jugular vein can be identified once the superficial veins are removed. It is just lateral to the left common carotid artery, and the two vessels are enclosed in a sheath with the vagus nerve. This sheath is just lateral to the trachea. The internal jugular is much smaller than all of the superficial veins. We never found the right internal jugular, probably because it was destroyed when the animal was injected with latex. The difference in size between the external and internal jugular veins suggests that most of the principal veins of the head drain into the external jugular. The external jugular veins run lateral to the internal jugulars, and they have a greater diameter just cranial to the heart. There are no communications between the two veins until they both drain into the cranial vena cava.

We observed the arterial system once the superficial veins had been removed; the arterial system is dorsal to the venous system. In a double-injected specimen, the arteries are injected with red latex to indicate that they had held oxygenated blood. We removed the aortic arch with the heart, so we could not see the branch point of the brachiocephalic and left subclavian arteries. The arteries have small diameters compared to the veins. The brachiocephalic artery has a diameter of two millimeters. The brachiocephalic artery branches into the right subclavian artery and the right and left common carotids at the thoracic inlet [1]. We had cut the left common carotid during the dissection of the heart. We found the right vagus nerve just lateral to the right common carotid. The common carotid arteries branch into the internal and external carotids just posterior to the larynx [1], but we could not locate this branch point. A possible reason is that the internal carotid artery is very small in the ferret, and we may have misinterpreted it as a smaller artery branching off the common carotid. The difference in size between the internal and external carotids suggests that most of the arteries of the head branch off the external carotid.

To view the circulation of the head and the brain, we removed the temporalis muscle and the remaining thick temporal fascia by scraping it away from the skull. The temporalis has a broad insertion into the sagittal crest, a short ridge along the midline of the skull. Once we had removed the temporalis muscle from the top of the skull, we detached the neck muscles from the nuchal crest. We then attempted to crack the skull using a chisel. The skull is very thick and this was difficult to do. After several unsuccessful attempts, we used a saw to make a preliminary notch in the parietal bone and then used the chisel to break the skull open. We continued to use the chisel to remove the parietal and frontal bones of the skull in pieces until we had removed the skull from just rostral of the eyes to the nuchal crest to expose the dorsal side of the brain.

The brain is covered in a tough membrane called the dura mater. This is the outermost meninx of the brain. This membrane reaches into only the deepest sulci of the brain, mainly the median sagittal fissure that divides the two cerebral hemispheres. A large section of the dura mater remained attached to the skullcap when we removed it. We detached the remaining dura mater. The next meninx is the arachnoid mater, which is separated from the dura mater by the subdural space. The arachnoid mater is a thin semi-transparent sheath that is made of a loose network of fibers [2]. We easily removed this layer. The innermost meninx is the pia mater, which adheres closely to the brain and follows every crevice. This thin membrane adhered so closely that we could not have removed it without damaging the brain. However, we were able to see the entire external surface of the brain without removing it.

After observing the brain in situ, we began to remove it from the skull starting from the rostral end. We first cut the optic tract, which contains the optic nerve (cranial nerve II), just rostral to the optic chiasma. Next we cut the olfactory nerves (cranial nerve I) to lift the rostral end from the bottom of the skull. This allowed us to cut the other cranial nerves and blood vessels that connect the brain to the floor of the braincase. We also cut the infundibulum, the stalk of the brain that connects the pituitary gland to the brain. The infundibulum remained attached to the brain while the pituitary gland or hypophysis remained in the floor of the braincase within the sella turcica, its bony enclosure [2]. The sella turcica lies within the Circle of Willis, the cerebral arterial circle, which remained attached to the brain when we removed it. The Circle of Willis is comprised of the internal carotid arteries and the vertebral arteries. The vertebral arteries pass through the foramen magnum from the transverse foramena of the cervical vertebrae and then join to form the medial basilar artery [2]. This splits into two branches that encircle the infundibulum. At this point, the internal carotid arteries, which enter the skull via the posterior carotid foramen, meet the vertebral arterial branches to form the cerebral arterial circle [2].

The brain was difficult to remove from the braincase after the infundibulum was cut. We realized that the ferret has an ossified tentorium cerebelli, which is an ossification of the dura mater between the cerebrum and cerebellum. This is a characteristic of Order Carnivora [2]. The ossified tentorium has two sections, one of each side of the brain. We used sharp scissors to cut the ossified tentorium and severed the spinal cord to remove the brain from the braincase. We easily lifted the brain out once these connections were cut.

The brain is about five centimeters long and three centimeters wide. The largest features of the brain were the cerebral hemispheres, which are separated by a prominent sagittal fissure. The cerebral hemispheres have numerous deep sulci. The sulci on the ferret brain are similar in number and relative size to those found on the cat brain. In contrast, the bat's brain has few sulci and is nearly smooth. The number of sulci on the brain determines its surface area and it is hypothesized that this indicates the intelligence of the animal [3]. We identified the pons and cerebellum on the external surface of the brain. The cerebellum is on the caudal end of the brain and can be distinguished from the cerebral hemispheres by its numerous parallel ridges. These ridges are shallower and smaller than those found on the cerebral hemispheres. The pons has a smooth texture and is also on the caudal end of the brain, rostral and ventral to the cerebellum. We were also able to identify several cranial nerves that we had cut to remove the brain from the braincase. These include the optic nerves (cranial nerve II), which meet and cross at the optic chiasma, the trigeminal nerve (cranial nerve V), which is the sensory nerve for the face and provides motor innervation to the muscles of mastication, and the oculomotor nerves (cranial nerve III), which innervate the muscles of the eye. We also observed the piriform lobes on the ventral side of the brain. These serve to integrate information received from the olfactory sensors [2].

With the brain removed, we could see several structures remaining on the floor of the brain case, including the pituitary gland, the cavernous and sigmoid venous sinuses, and the remnants of the ossified tentorium.

Adapted from Klingener 1979.

References

1. Fox, J.G. 1998. Biology and Diseases of the Ferret (2nd Ed.). Williams & Wilkins, Baltimore.

2. Klingener, D. 1979. Laboratory Anatomy of the Mink (2nd Ed.). William C. Brown, Dubuque, Iowa.

3. Purves, D. et al. 2001. Neuroscience (2nd Ed.). Sinauer Associates.

Links

to Comparative Mammalian Anatomy home

to mammalian brains

to ferret face and head muscles