Sheep Brain Anatomy Overview

Overview

This lecture covers the key anatomical structures and differences of the sheep brain, comparing it to the human brain, and details the process and findings of a mid-sagittal dissection.

External Anatomy & Key Differences

• Sheep brains are smaller and more elongated than human brains, with a more horizontal orientation due to quadrupedal posture.
• The brain's four main regions are the cerebrum, cerebellum, brainstem, and diencephalon.
• Major external features include gyri (ridges), sulci (shallow grooves), and fissures (deep grooves).
• The longitudinal fissure divides the cerebrum into left and right hemispheres.
• The cerebrum is divided into frontal, parietal, occipital, and temporal lobes on each side.

Major Structures Visible Without Dissection

• The cerebellum ("small brain") also has left and right hemispheres, connected by the vermis.
• The brainstem consists of the medulla oblongata (lowest), pons (middle), and mesencephalon (highest, including cerebral peduncles).
• The corpora quadrigemina contains four protrusions: two superior and two inferior colliculi.
• Other visible features: optic chiasm (crossing of optic nerves), olfactory bulbs (larger in sheep for enhanced smell), and sites of infundibulum and pituitary gland attachment.

Mid-Sagittal Dissection & Internal Structures

• Dissection reveals the corpus callosum, the largest white matter bundle connecting hemispheres.
• The fornix, below the corpus callosum, helps form the ceiling and floor of the lateral ventricle.
• The septum pellucidum is a thin membrane separating the two lateral ventricles.
• The thalamus (part of the diencephalon) appears as a round structure under the fornix.
• The epithalamus houses the pineal gland, which produces melatonin.
• The hypothalamus, located below the thalamus, regulates the pituitary gland and autonomic functions.
• The mammillary body is involved in memory and spatial orientation.

Ventricles & Cerebrospinal Fluid Pathway

• Lateral ventricles are separated by septum pellucidum and contain cerebrospinal fluid (CSF).
• The third ventricle surrounds the thalamus; choroid plexus within produces CSF.
• The cerebral aqueduct connects the third and fourth ventricles.
• The fourth ventricle continues into the central canal of the spinal cord.

Cerebellum & Arbor Vitae

• The cerebellum’s white matter pattern is called "arbor vitae" (“tree of life”).
• Gray matter is visible along the outer regions of the cerebellum.

Key Terms & Definitions

• Gyri — Elevated ridges on the brain surface.
• Sulci — Shallow grooves between gyri.
• Fissures — Deep grooves in the brain.
• Corpus callosum — White matter tract connecting cerebral hemispheres.
• Fornix — White matter tract below the corpus callosum, part of the limbic system.
• Septum pellucidum — Thin membrane separating lateral ventricles.
• Thalamus — Diencephalon structure relaying sensory/motor signals.
• Epithalamus — Region housing pineal gland.
• Hypothalamus — Diencephalon region controlling endocrine/autonomic activities.
• Mammillary body — Diencephalon structure for memory processing.
• Arbor vitae — Tree-like white matter in the cerebellum.
• Olfactory bulb — Structure for sense of smell.

Action Items / Next Steps

• Review labeled diagrams of sheep brain anatomy.
• Practice identifying structures on brain specimens or models.