Name ___________________________________
Mrs. Dasher 9___
____/____/2012 – ____/____/2012
Dissection of the Sheep’s Brain
The purpose of dissecting the sheep brain is to become acquainted with the three-dimensional
structure, or anatomy, of the brain. This exercise will expose you to one of the best methods for
studying organs – actually examining their structure. The following days devoted to the sheeps’ brains
will allow you to look closely at anatomy; I will ask you to think carefully about what this means about
the operation of the brain.
There are many similarities between the sheep brain and the human brain. You will see throughout
dissection that even differences can be instructive and help us to learn more. Being able to locate
important structures in the sheep brain will help along your understanding of how structures are related
to each other in the human brain. If the same structure exists in both brains, and most structures are
the same, they are in the same relative location.
Please follow the succeeding steps in order. You are responsible for the terms in bold.
Materials & Preparation
1.
Before beginning inspection and dissection of the brain, you should collect these materials:
Dissection pan
Scalpel
Sheep brain
Probe
Scissors
Gloves
2.
Each brain is stored in a preservative solution. You may rinse the brain under a slow stream of
running water before beginning the dissection – you should begin the slow stream of water
FIRST before carefully holding the organ under the water.
3.
Steps 1 and 2 should be repeated prior to each lab session.
Procedure
Before beginning any dissection, you will need to know the terms used to specify the locations and
relative locations of various structures. Next, you will find illustrations specific to the sheep brain.
Directions
All terms are both absolute and relative. For example, think of the term lateral. It both means at the
side of the brain, and closer to the side. A structure that is very close to the middle can be ‘lateral’ to
another structure that is even closer to the middle of the brain. To summarize, anterior or rostral
means in front or towards the front. Posterial or caudal is at or towards the back. Lateral means on the
side of or towards the side. Medial is at or towards the middle. Dorsal means on top, in the brain and
head only, and ventral means on the bottom, in the brain and head only. The positions and directions
are illustrated in the next to images.
LATERAL
POSTERIOR
OR
CAUDAL
MEDIAL
ANTERIOR
OR
ROSTRAL
VENTRAL
ANTERIOR
OR
ROSTRAL
POSTERIOR
OR
CAUDAL
DORSAL
Planes of Orientation
In addition to the direction, the brain can be divided into three planes. First, the frontal or coronal
planes which divide front from back. It can divide the brain and any location as long as it divides the
brain from front to back. Next are the saggital planes which divide the left from the right of the brain.
In the figure below, the most important saggital plane is illustrated below, the mid-saggital plane.
However, as with the frontal planes, any plane that is parallel to the mid-saggital plane is also a saggital
plane. The last planes are horizontal planes that divide the brain in to top and bottom portions. These
planes can be seen in the following images.
FRONTAL /
CORONAL
HORIZONTAL
FRONTAL /
CORONAL
SAGITTAL
MID-SAGITTAL
The procedure is divided into three main sections:
Examination of the Exterior Brain
Examination of the Mid-Saggital Plane
Examination of Two Frontal Cuts
Examination of the Exterior of the Brain
The first portion of the dissection will be a detailed examination of the external brain. No cutting is
necessary for this portion of the dissection. As you proceed, you should identify the parts of the brain
listed below. Note the structures you observe and how they are related to other parts of the brain. BE
VERY CAREFUL DURING THIS EXAMINATION – THE BRAIN IS VERY FRAGILE.
1.
2.
First examine the exterior of the entire brain. Locate the following major brain divisions:
Spinal cord, brainstem, midbrain, cerebellum, and cerebrum.
The cerebrum is shaped like a sheet that is folded into bumps called gyri (singular: gyrus) and
grooves called sulci (singular: sulcus). The deepest grooves are sometimes called fissures.
Fissures form boundaries for the various brain lobes. Don’t worry if you can’t find distinct
boundaries; they are often difficult to find in the sheep’s brain. Locate the following: Frontal
lobe, Parietal lobe, Temporal lobe, Occipital lobe.
PARIETAL LOBE
CEREBRUM
OCCIPITAL LOBE
CEREBELLUM
SPINAL
CORD
BRAINSTEM
FRONTAL
LOBE
MIDBRAIN
GYRUS
3.
4.
SULCUS
TEMPORAL
LOBE
Reveal a few more occluded structures with some GENTLE prying.
a.
Grasp the cerebral hemispheres and gently pull them apart. You might need to tear any
pia mater (very thing membrane) but do not tear any other structures. The white
matter that can be visualized is the corpus callosum, which contains the axon fibers
connecting the two hemispheres.
b.
Grasp the cerebrum and cerebellum and gently pry them apart. Again, you may need to
tear any pia mater, but do NOT rip off the cerebellum. You should be able to see the
superior and inferior colliculus.
Next turn the brain so it rests on its dorsal surface (ventral side up). Locate the brainstem. This
area is made up of the pons, medulla, and cerebellum. Find also the root where the pituitary
gland was attached to your brain. The pituitary gland itself may no longer be attached.
CEREBELLUM
MEDULLA
OBLONGATA
PITUITARY GLAND
PONS
5.
6.
Identify any intact cranial nerves which are axon bundles coming from and going to the sensory
organs and muscles in the head. In addition to their descriptive names, they are numbered
based on where they enter the brain along the rostral to caudal axis. Most are visible from the
ventral surface of the brain. Most are removed during the cleaning process. See if you can find
these: Optic nerve and oculomotor nerve.
Identify any additional structures visible from the various external view of the brain.
OCULOMOTOR
NERVE
OPTIC TRACT
OPTIC
CHIASM
OPTIC
NERVE
HIPPOCAMPAL
GYRUS
PYRIFORM LOBE
7.
OLFACTORY
BULB
Notice how blood vessels are present nearly everywhere in the brain. The brain is a
metabolically expensive organ!
Examination of the Mid-Saggital Plane
DO NOT PROCEED TO THE NEXT STEP BEFORE CHECKING WITH MRS. DASHER.
8.
Now you will make the mid-saggital cut. Hold the brain level and flat. Cut along the longitudinal
fissure. Here you can find the largest of the commisures, which is a band of fibers that connects
the two sides of the central nervous system. This is the corpus callosum. It is big enough to be
divided into three separate areas – the genu, the body, and the splenium. In addition you
should be able to locate the pineal body and the hypothalamus.
9.
While looking at this cut, please find the cerebellum. Notice the pattern of grey and white
matter. To some it appears like a tree or bush. As a result it is known as the arbor vitae.
GENU OF CORPUS
CALLOSUM
PINEAL
BODY
CEREBELLUM
SPLENIUM
OF CORPUS
CALLOSUM
HYPOTHALAMUS
Examination of the Frontal Cuts
BODY OF
CORPUS
CALLOSUM
DO NOT PROCEED TO THE NEXT STEP BEFORE CHECKING WITH MRS. DASHER.
You will examine cut A below.
10.
From this cut you will have a view of the white matter, the gray matter, the thalamus, the
pineal gland, and the hippocampus.
GRAY
MATTER
HIPPOCAMPUS
WHITE
MATTER
THALAMUS
PINEAL
GLAND
You have completed the dissection of the sheep brain. All marked areas are testable for the lab
practicum – DO NOT save your studying for this until the last minute.
CLEAN UP – With a paper towel, scrape all specimen litter out of dissection tray into designated garbage
container. Rinse tray thoroughly and stack. Rinse gloves and store. Wash your hands thoroughly!!!