Hold a sheep brain and you are holding a smaller, simpler version of your own, built on the same basic plan. The differences are not random. They line up with the life of a grazing animal that relies on smell and wide vision more than on planning ahead. Comparing the two shows which brain parts all mammals share and which ones grew larger in humans.
The Parts a Sheep Brain Shares With Yours
A sheep brain has the same four basic regions as a human brain: the cerebrum, the cerebellum, the brainstem, and the diencephalon sitting between them.1Open Educational Resources. Brain and Cranial Nerves Lab The cerebrum is divided into four lobes marked by visible surface landmarks, much like the human frontal, parietal, temporal, and occipital lobes.2PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas Its surface is folded into ridges called gyri and grooves called sulci, and these are sharply defined, as in humans.3PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas
The folds matter because they pack more outer brain tissue into a skull of limited size, and that outer tissue is where much of the sensing and thinking happens. Human brains are more deeply folded, but sheep folding is still extensive.
The lobes are named for the skull bones that cover them in life, so frontal sits at the front and occipital at the back, with parietal on top and temporal low on the side. On a sheep brain the boundaries are not drawn as clear lines. They are judged from landmark grooves and position, which is why labs often show them on a diagram.
The brain is wrapped in thick, well-developed meninges, the protective membranes, in both sheep and humans.4PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas On a lab specimen, the tough outer layer may still cling to the brain and feels like thin leather. Underneath is a delicate, web-like layer that holds tiny blood vessels on the surface. Peeling the outer layer back carefully lets you see the ridges and grooves clearly. These same layers protect your brain, and they are what becomes inflamed in meningitis.
Deeper inside, the white matter that links brain regions is spread through the cerebrum in a pattern similar to ours.5PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas The caudate nucleus and putamen, movement-control structures deep in the brain, are two separate parts in sheep, just as in humans.6PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas The hippocampus, a memory structure, sits on the lower inner side of the cerebrum in both species.7PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas
How Big a Sheep Brain Is Compared With Yours
A sheep brain weighs about 130 to 140 grams, roughly the weight of a large apple. A human brain weighs about 1,300 to 1,400 grams, about ten times more.8PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas A preserved specimen is usually firmer and a little different in weight from a fresh brain, so a lab brain may not match the fresh figure exactly.
The sheep skull is 5 to 6 millimeters thick, slightly thinner than the 6.5 to 7.5 millimeters of a human skull.9PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas It is also fairly round, more like a human head than a pig’s flat, thick skull.10PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas
Among humans and several other mammals and birds compared, sheep ranked fifth for skull space for the brain, after humans, horses, gorillas, and pigs.11PubMed Central. Indices of comparative cognition: assessing animal models of human brain function They ranked third for how folded the brain surface is, behind only humans and horses.12PubMed Central. Indices of comparative cognition: assessing animal models of human brain function But they ranked sixth for the number of nerve cells in the outer brain, after humans, gorillas, horses, rhesus monkeys, and pigs.13PubMed Central. Indices of comparative cognition: assessing animal models of human brain function For brain size relative to body size, sheep came seventh, behind humans, monkeys, marmosets, dogs, jays, and horses.14PubMed Central. Indices of comparative cognition: assessing animal models of human brain function
Nerve cell counts say more than raw size, because bigger animals often have bigger brains simply to run bigger bodies. Counting outer-brain nerve cells tracks problem-solving better, which is why monkeys and crows can be clever despite small brains.
About 43% of a sheep brain is grey matter, the cell-rich tissue, and about 24% is white matter, the wiring.15PubMed. Volumetric study on sheep brain using stereology technique When you cut a sheep brain, you will see a darker outer rim over paler inner tissue.
Why the Smell Bulbs Are So Large
On the underside at the front, a sheep brain has two obvious swellings called olfactory bulbs, which process smell.16Open Educational Resources. Brain and Cranial Nerves Lab In sheep they are large and well developed, while in humans they are small.17Frontiers in Veterinary Science. Mapping sheep to human brain: The need for a sheep brain atlas Grazing animals rely heavily on smell to judge food, detect predators, and recognize other sheep. Ewes, for example, use smell to recognize their own lambs soon after birth. Human bulbs are thin and tucked under the frontal lobes, so they are easy to miss in a human brain specimen.
Smell molecules in the air dissolve in the lining of the nose, where sensory cells send signals through tiny nerve fibers straight into the bulbs. From the bulbs, signals travel back to deeper parts of the brain linked to emotion and memory, which helps explain why smells can stir strong memories in people. Most other senses are relayed through a middle station first, so smell reaches the brain unusually directly.
A common mistake in the lab is to think the bulbs are damaged tissue or nerves. They are often torn off when the brain is removed. If they seem to be missing, look for stubs at the front of the underside.
How Sheep Vision Shapes the Brain
Sheep eye sockets face the sides, while human eye sockets face forward.18PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas The optic chiasm, the X-shaped spot where the optic nerves cross on the underside of the brain, is more pronounced in sheep.19PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas The visual area at the back of the brain also sits more to the side in sheep and closer to the midline in humans.20PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas
Side-facing eyes let a sheep see almost all the way around without turning its head, which helps a prey animal spot predators while grazing. The trade-off is a narrow zone where both eyes overlap for depth.
To find the crossing of the optic nerves on a specimen, look on the underside, behind the smell bulbs. It looks like a small X of white nerve tissue. The pituitary gland may sit just behind it if still attached.
At the crossing, some nerve fibers from each eye move to the opposite side of the brain, so each side of the brain receives information from both eyes. In sheep, whose eyes face sideways, most of the fibers cross over, since each eye sees mostly its own side of the world. In humans, forward-facing eyes see a shared view, so a large share of fibers stay on the same side while the rest cross.
The Front of the Brain and Its Layers
The front lower part of the brain, which in humans helps with decisions and reward, is thinner in sheep, about 1.7 millimeters versus about 2.5 in chimpanzees.21PubMed Central. The orbitofrontal cortex of the sheep. Topography, organization, neurochemistry, digital tensor imaging and comparison with the chimpanzee and human In sheep this area also lacks layer IV, one of the six standard cell layers, which chimpanzees do have.22PubMed Central. The orbitofrontal cortex of the sheep. Topography, organization, neurochemistry, digital tensor imaging and comparison with the chimpanzee and human
The outer brain is stacked like sheets of different cell types, and each layer sends or receives signals in a typical way. Layer IV usually receives incoming signals from other brain regions, so lacking it suggests a different way of handling information in that area. You cannot see layers by eye; they need a microscope.
The sheep hearing area still has all six cell layers, and it is about 1.6 millimeters thick.23PubMed Central. Cytoarchitecture and myeloarchitecture of the sheep auditory cortex The missing layer is a feature of the front region, not the whole sheep brain.
A Different Blood Supply
Humans get much of their brain blood from the internal carotid arteries, which feed a ring of vessels at the brain’s base. In sheep these internal carotid arteries are tiny or missing.24Journal of the Anatomical Society of India. Comparative study of formation of circle of Willis in human and sheep brain Instead, sheep have a four-sided net of small arteries at the brain’s base called the rete mirabile, “wonderful net.”25Journal of the Anatomical Society of India. Comparative study of formation of circle of Willis in human and sheep brain Two arteries leave this pooled blood supply to feed the brain.26Journal of the Anatomical Society of India. Comparative study of formation of circle of Willis in human and sheep brain In a net, many small arteries lie side by side, so blood spreads through a wide set of thin channels before moving on.
On a lab specimen, the blood-vessel net usually stays in the skull and is lost when the brain is removed. Visible surface vessels on your specimen are smaller branches. Many hoofed grazers share this kind of arterial net, and scientists think nets like this may help cool blood reaching the brain.
Finding the Main Structures During a Dissection
Start by orienting the brain. The smell bulbs mark the front underside, and the cerebellum is the small, finely ridged part at the back. The brainstem extends down from the underside toward the spinal cord. On top, a deep groove separates the two halves. The top surface is smooth and rounded with folds, while the underside is uneven because it holds the smell bulbs, nerve stubs, and brainstem.
On the underside, stubs of nerves come off the brainstem. The optic nerves are the easiest to spot. Many are cut short, so match them to a lab diagram.
To see the inside, cut lengthwise through the groove between the two halves. Splitting the brain this way shows the corpus callosum, a curved white band that connects the two halves.
Find the bridge between the two halves curving above the open fluid spaces. Its underside, which forms the roof of those spaces, is flatter in sheep than in dogs and cats but more curved than in humans and horses.27Eurasian Journal of Veterinary Sciences. Sexual dimorphism in the sheep corpus callosum using 3 tesla MRI Look below it for the thalamus area and brainstem. The cut face of the cerebellum shows a tree-like white pattern.
The two halves mirror each other, so left and right are hard to tell apart on an isolated brain. Labs usually only need you to name them as hemispheres. If you need a side, mark it on your diagram before you cut.
Why Schools and Scientists Use Sheep Brains
Sheep brains are widely used in classes because they are easy to get, inexpensive, and have many of the same visible parts as a human brain.28PubMed Central. Resources for teaching Mammalian neuroanatomy using sheep brains: a review. They are a by-product of the meat industry, so they can be supplied to schools without raising extra animals. Their size is big enough to see features by eye but small enough to handle on a lab tray.
Visible lobes, folds, and bulbs let students practice finding parts that exist in human brains too. Orienting a brain and cutting along a plane are hands-on skills that transfer to human anatomy later. Seeing differences such as the large smell bulbs also shows how structure follows lifestyle.
In research, the folds, protective layers, and white matter pattern make sheep a useful model for studying human brain disease.29PubMed Central. Mapping sheep to human brain: The need for a sheep brain atlas Researchers also use sheep to study head injuries, including mild concussion.30PubMed Central. Histological Characterisation of a Sheep Model of Mild Traumatic Brain Injury: A Pilot Study In developing lambs, the brain matures from front to back, as in humans.31PubMed Central. The Ovine Brain as a Model for Human Neurodevelopment: Immunohistochemical Profiling of Brain Maturation Markers in Preterm Lambs Early concussion studies in sheep have used very few animals, so the results are preliminary.32PubMed Central. Histological Characterisation of a Sheep Model of Mild Traumatic Brain Injury: A Pilot Study
The comparison has limits. A sheep’s front region is simpler, so it is weak for studying higher thinking, and its blood supply differs, which matters for stroke research.