Comprehensive Sheep Brain Dissection Study Guide

Specimen: A sheep brain with the hypophysis (another term for the pituitary gland) still attached or included.
Tools for Dissection:
- Scalpel/Knife (caution: always cut away from fingers).
- Probe (primary pointing and exploring tool).
- Pickups (forceps).
- Two pairs of scissors (primarily used for initial removal of the brain from the specimen bag).
- Paper towels (used to clean up excess preservative liquid).
Initial Setup:
- Dispose of the specimen bag.
- Dry the work surface with paper towels.
- Orient the brain based on anatomical landmarks.

Surfaces:
- Superior Surface: The top surface of the brain, located directly beneath the skull.
- Inferior Surface: The underside or base of the brain.
- Lateral Surface: The sides of the brain.
Positional Terms:
- Anterior: Towards the front.
- Rostral: A specific brain-related term meaning "towards the nose."
- Posterior: Towards the back.

The brain is protected by three layers of tissue collectively known as the meninges.
Dura Mater:
- The outermost layer.
- Etymology: Latin for "tough mother" ( $Dura$ = tough; $Mater$ = mother).
- Characteristics: Very thick and durable; often removed during specimen preparation but shards may remain, especially around the pituitary gland.
Arachnoid Maternal Layer:
- The middle layer located beneath the dura.
- Appearance: A glistening, translucent surface resembling "Saran wrap."
- Etymology: Named after spiders ("arachnid") because of spider-web-like projections that extend down to the next layer.
- Vascularity: Contains blood vessels, most of which are arteries, visible as brown lines beneath the glistening film.
Pia Mater:
- The innermost layer sitting directly on the surface of the brain tissue.
- Etymology: Latin for "soft mother" ( $Pia$ = soft).
- Characteristics: Not as glistening as the arachnoid; it follows the contours of the brain's hills and valleys.

Gyri and Sulci:
- Gyrus (plural: gyri): The "hills" or ridges on the surface of the brain.
- Sulcus (plural: sulci): The "valleys" or grooves between the ridges. Blood vessels are typically found nestled within the sulci.
Fissures (Deep Cracks):
- Medial Longitudinal Fissure: The major deep crack that separates the brain into right and left cerebral hemispheres.
- Lateral Fissure (Sylvian Sulcus/Fissure): A prominent groove found on the lateral aspect of the brain.
Major Brain Divisions:
- Cerebrum: The largest part of the brain, occupying the anterior and superior portions.
- Cerebellum: Part of the hindbrain, located posterior and inferior to the cerebrum.
- Spinal Cord: Extending from the posterior portion of the brain stem.

Pituitary Gland (Hypophysis):
- Sits at the base of the brain.
- Often pulled out when the dura mater is removed because it is encased between two layers of the dura.
Cranial Nerves and Structures:
- Olfactory Tract: Located anteriorly; while olfactory nerves are in the cribriform plate, the tract is visible extending posteriorly on the inferior surface.
- Optic Nerves: Carry visual information.
- Optic Chiasm: The X-shaped structure where the optic nerves cross.
- Optic Tract: The pathway continuing from the chiasm into the brain.
- Cranial Nerve III (Oculomotor): Seen as a small wisp near the brainstem.
- Cranial Nerve IV (Trochlear): Wraps around from the posterior aspect.
- Cranial Nerve V (Trigeminal): A very large, thick nerve found laterally on the pons.
- Cranial Nerves VII through XII: Located sequentially down the brainstem (Medulla oblongata).

Frontal Lobe: The most anterior portion.
Temporal Lobes: Found laterally; described metaphorically as the "earmuffs" of the brain.
Parietal Lobe: Located superior and posterior to the frontal lobe.
Occipital Lobe: The most posterior portion of the cerebrum.
Central Sulcus: A groove that separates the frontal and parietal lobes. In the sheep brain, it is less defined than in humans but can be found running toward the medial longitudinal fissure (though it does not enter it).
- Precentral Gyrus: Located anterior to the central sulcus; associated with motor function.
- Postcentral Gyrus: Located posterior to the central sulcus; the primary area for receiving sensory information.

Sectioning Method: Coronal cuts provide an anterior and posterior view. Slices should be approximately $10\,mm$ in thickness.
Gray matter vs. White Matter:
- Gray Matter: The darker outer layer (cortex). Consists of unmyelinated structures, primarily neuron cell bodies.
- White Matter: The lighter inner layer. Consists of myelinated tracts (axons) traveling between brain regions.
Corpus Callosum:
- Meaning: "Colossal body."
- Function: A thick, white matter tract that connects the right and left hemispheres, allowing for communication between them.

Ventricles: Defined as "little bellies," these are fluid-filled spaces within the brain.
- Lateral Ventricles: Large spaces beneath the corpus callosum. By tradition, the right is Ventricle I and the left is Ventricle II.
- Septum Pellucidum: A thin membrane separating the two lateral ventricles.
- Third Ventricle: A narrow space located in the midline between the two halves of the thalamus. The lateral ventricles drain into the third ventricle.
- Cerebral Aqueduct: A narrow channel in the midbrain that drains the third ventricle into the fourth ventricle.
- Fourth Ventricle: A space located anterior to the cerebellum and posterior to the pons and upper medulla.
Deep Gray Matter Structures:
- Caudate Nucleus: Part of the basal ganglia, located near the lateral ventricles.
- Thalamus: Meaning "inner chamber." It is a large, egg-shaped structure that makes up the bulk of the floor of the lateral ventricles.
- Pineal Gland: A small structure located posterior to the thalamus.

Midbrain: The most superior portion of the brainstem.
Pons: The middle portion of the brainstem.
- Descriptor: Often described as looking "pregnant" due to its protruding, rounded anterior shape.
Medulla Oblongata: The most inferior portion of the brainstem, transitioning into the spinal cord.

Sectioning Method: A midsagittal cut divides the brain into equal right and left halves down the medial longitudinal fissure.
Cerebellum Internal Structure:
- Arbor Vitae: Meaning "tree of life."
- Appearance: A distinct, branching, tree-like pattern of white matter surrounded by gray matter cortex within the cerebellum.