LABORATORY STUDY OF THE SHEEP BRAIN
WEEK 2 Medial Structures
You should be able to work on the
cranial nerves before Wednesday. (Possible
extra credit? Know the role of each CN
– for example optic nerve = incoming vision signals)
Cranial Nerves. Human Brain 5.7 – KNOW
THE LOCATION AND NAME *NOT JUST CNX or Cranial Nerve 3* FOR EACH CRANIAL NERVE
– Color code the pairs of nerves, it will help!
a. The origins
of the olfactory (CNI) and optic (CNII) nerves have been indicated on
the week 1 sheet. *SHEEP BRAIN
b. The oculomotor
nerve (CNIII) arises from the ventral surface of the cerebral peduncles close
to its medial border.
trochlear nerve (CNIV) is found
caudal and lateral to the oculomotor nerve and rostral and medial to the
trigeminal nerve (CNV) is a very
large flat nerve extending forward from the lateral border of the pons. It arises by two roots; the larger sensory
root is placed lateral to the smaller motor root.
abducens nerve (CNVI) arises from
the trapezoid body about 1/8 inch from the ventral median fissure.
facial nerve (CNVII) arises from
the trapezoid body lateral to the abducens.
g. The vestibulocochlear
nerve (CNVIII) is seen just caudal to the facial nerve. Having arched downward
from the dorsolateral border of the medulla, it appears from under the
flocculus of the cerebellum.
The glossopharyngeal (CNIX) and vagus (CNX) nerves arise by a series of
filaments from a groove along the lateral border of the medulla. CNIX arises from this groove just caudal to
the trapezoid body and just lateral to the olive. CNX arises caudal to CNIX.
nerve (CNXI) is on the same line with CNIX and X, but caudal to them; it is
seen running forward along the side of the spinal cord and medulla, receiving
filaments from both.
nerve (CNXII) arises as a series of roots from the ventral surface of the
medulla, near the caudal end of the lateral border of the pyramid.
Medial sagittal section of brain.
- The corpus callosum, the great band of
fibers passing across the midline between the two hemispheres, has been
cut transversely in this section. It appears as an elongated mass of white
matter that slopes in a rostroventral direction. The caudal club-shaped end is the splenium. The body
runs from the splenium to the genu.
The rostral extremity bends ventrally as the genu (knee) and ends in
the caudally-turned rostrum (beak). The corpus callosum should also be
identified as the particular region such as “splenium of the corpus
callosum” or “body of the corpus callosum” – using just “splenium”,
“body”, or “corpus callosum” is not acceptable.
gyrus is the gyrus just dorsal to the corpus
callosum, bounded dorsally by the cingulate sulcus.
- The cortical area under the rostrum of the corpus
callosum, appearing as a ventrocaudally-turned continuation of the gyrus
cinguli, is a part of the rhinencephalon. It is in close relation to the medial olfactory gyrus and
anterior perforated substance.
- The fornix is the white sickle-shaped band
that lies under the caudal end of the corpus callosum. It arches in a rostroventral direction
to end behind the optic chiasm.
The fibers of the fornix arise in the hippocampus, a cortical
elevation in the floor of the inferior horn of the lateral ventricle, in
the temporal lobe. The fiber
bundles from the two sides arch upward to join under the corpus callosum,
some of the fibers crossing the midline as commissural fibers. Rostral to the thalamus, the fibers
again separate into two bundles, the anterior column of the fornix
to terminate ventrally into mammillary bodies.
- The septum pellucidum is the thin
double-layered membrane which covers the triangular space between the
genus of the corpus callosum and the anterior columns of the fornix. It separates the two lateral
ventricles toward their rostral extremities.
- The thalamus is the large rounded mass of
gray matter around whose rostral aspect the anterior columns of the fornix
arch. The thalami are bilateral
paired egg-shaped masses, joined together across the midline by the massa
intermediate, which has been cut transversely in making this
- The third ventricle, the cavity of the
diencephalon, is a narrow vertical cleft forming a ring around the
intermediate mass. Its walls are
constituted as follows:
- The lateral walls are formed mostly by the thalami.
- The floor and ventral part of the lateral walls are
formed by the hypothalamus, whose ventrally-seen structures have already
- The rostral wall is formed by:
the lamina terminalis, a thin plate joining the two
hemispheres. It is the original rostral boundary of the telencephalon. In this section it extends from the optic
chiasm upward to the anterior commissure, a fiber bundle connecting olfactory
regions of the two hemispheres, which appears here in cross section as a round
a small caudally-running portion of the rostrum of the corpus
a small part of the fornix just above the anterior commissure.
Just caudal to this part of the fornix are located, on either side, the
interventricular formina (sing. foramen), the opening by which the third
ventricle communicates with the lateral ventricles of hemispheres.
- The roof or epithalamus is formed by:
rostrally, a very thin ependymal layer which is thrust
downward into the ventricle by a fold of richly vascular pia mater. Thus is formed the choroid plexus of the third
ventricle, which extends rostrally and laterally through the interventricular
foramina to form the choroid plexus of the lateral ventricles.
caudally, the pineal body or epiphysis, a small
pinecone-shaped body whose caudal surface rests upon the midbrain.
The habenular trigone, a small triangular area just rostral to
the pineal body. It contains a nucleus
concerned with olfactory reflexes.
the posterior commissure appears as a round white
spot in the roof just caudal to the pineal body. Some of its fibers connect the two superior colliculi; the
connections of the rest are not certain.
The posterior commissure is dorsal to the junction of the third
ventricle and the cerebral aqueduct (aqueduct of Sylvius), the
cavity of the mesencephalon.
- The mesencephalon or midbrain consists of the
- Dorsal to the cerebral aqueduct, a thick roof plate
forms two pairs of rounded eminences, the superior and inferior
colliculi. The superior
colliculi, which are visual reflex centers, are much
larger and more caudal than the inferior colliculi, which are reflex
auditory centers. The pineal gland is a singular midline
structure that lies rostrally in between the two superior colliculi.
- Ventral to the cerebral aqueduct are the thick cerebral
peduncles connecting the brainstem to the cerebral
hemispheres. The dorsal part,
under the aqueduct, is the tegmentum, a reticular formation of mingled
fibers and gray masses which extends caudally into the
rhombencephalon. The ventral part
of the peduncle is the basis pedunculi, composed of longitudinal fiber
tracts connecting the forebrain with lower levels.
- The rhombencephalon.
- View the cerebellum in sagittal section through the
vermis. Notice how the folia of
gray cortex dip inward between the tree-like branchings of the medullary
white matter. Locate the fissure
prima, the prepyramidal sulcus, and the nodule. The nodule is the lobule
of the vermis
located in the most ventrocaudal position, just caudal to a recess of the
ventricle that vertically divides the ventral part of the
- The anterior medullary velum is the thin
sheet forming the roof of the pontile (rostral) part of the fourth
- The roof of the caudal part of the fourth ventricle
is very thin and is invaginated by the vascular pia mater to form the
choroid plexus of the fourth ventricle.
This has been removed in most of the sagittal sections.
- By tipping the cerebellum very slightly away from
the brain stem, the rhomboid form of the fourth ventricle will
- At its rostral end, the fourth ventricle
connects with the cerebral aqueduct.
- At its caudal end, the fourth ventricle
connects with the central canal of the closed part of the
medulla, and this canal is in turn continuous with the central canal of
the spinal cord. The caudal
portion of the fourth ventricle is termed the obex.
of the above bold and underlined structures are fair game in the human
Neuroanatomy book but it particular pay attention to the following plates:
MIDLINE STURCTURES: 10.4 10.5
NOTES ON TAKING THE LAB PRACTICALS:
that the lab practicals are CUMULATIVE.
If the pin is in the
vermis, you will not get credit for answering cerebellum.
Gyri, Sulci, Fissures, and Nerves will be so marked on your lab practical
help prepare for the lab practical, you should quiz your lab partner and have
them quiz you on more than 1 brain (they don’t all look alike!).