Dissector Answers - Brain, Cranial Nerves, and Cranial Cavity

Learning Objectives:

Upon completion of this session, the student will be able to:

1. Identify the three cranial fossa and openings for the spinal cord, cranial nerves, arteries (carotid, vertebral, and middle meningeal), and the internal jugular vein.
2. Identify the following surface features of the brain: cerebrum, cerebellum, brainstem; longitudinal fissure; lateral sulcus (including opercula) and central sulcus; and the pre- and post-central gyri; frontal, parietal, temporal, occipital and insular lobes.
3. Identify the following features on an axially sliced brain: gray and white matter; cerebral cortex; lateral ventricles; third ventricle.
4. Identify each of the 12 pairs of cranial nerves, both on the brain and in the anterior, middle and posterior cranial fossae.

Learning Objectives and Explanations:

1. Identify the three cranial fossae and openings for the spinal cord, cranial nerves, arteries (internal carotid and middle meningeal), and the internal jugular vein. (N6, N7A, N7B, N9, N11, TG7-07, TG7-08)

The internal surface of the skull is divided into three fossae (depressions):

• Anterior cranial fossa - This fossa is shallow and the crista galli projects upward from its surface. The cribriform plate contains multiple foramen through which branches of the olfactory nerve pass. The lesser wing of the sphenoid marks the posterior border of the anterior cranial fossa.
• Middle cranial fossa - This fossa is of intermediate depth and is notable for containing the sella turcica, which holds the pituitary. The bulk of the middle cranial fossa is composed of the greater wings of the sphenoid and the squamous portion of the temporal bones, upon which rest the temporal lobes. The posterior border of the middle cranial fossa is a ridge of bone called the petrous portion of the temporal bone. The middle cranial fossa contains the optic canal, superior orbital fissure, foramen rotundum, foramen ovale, foramen spinosum, and foramen lacerum.
• Posterior cranial fossa - This fossa is relatively deep and contains the cerebellum. Projecting posterior to the foramen magnum is the internal occipital protuberance. The internal acoustic meatus is located on the petrous portion of the temporal bone. The jugular foramen is anterolateral to the hypoglossal canal.

2. Identify the following surface features of the brain: cerebrum, cerebellum, brainstem; longitudinal fissure; lateral sulcus (including opercula) and central sulcus; and the pre- and post-central gyri; frontal, parietal, temporal, occipital and insular lobes. (NI19A, NI19B, NI19C, NI20, TG7-53A, TG7-53B)

The lobes of the cerebral hemispheres are named for the bones that overlie them: frontal bone, parietal bone, temporal bone, and occipital bone.

The frontal lobes lie on the anterior cranial fossa, above the orbits. The temporal lobes sit in the middle cranial fossae behind the orbits. The occipital lobes sit above the cerebellum, which occupies the posterior cranial fossa.

There are 5 lobes of each cerebral hemisphere, but only 4 are visible on the surface of the brain. They are the frontal lobe, Parietal lobe, occipital lobe, and temporal lobe.

The brain features many grooves or sulci separating multiple gyri or rounded ridge-like elevations. The major sulci and fissures include the midline longitudinal fissure separating the two cerebral hemispheres, the lateral sulcus that lies above the temporal lobe, the parieto-occipital sulcus and preoccipital notch (both of which can be difficult to locate) and the central sulcus, which separates the precentral gyrus from the postcentral gyrus.

The central sulcus separates the precentral gyrus, the primary motor cortex, and the postcentral gyrus, the primary somatosensory cortex.

Down in the bottom of the lateral sulcus, overshadowed by parts of the frontal, parietal, and temporal lobes called opercula, is the insula, the hidden 5th lobe.

Some people consider the limbic system as a 6th lobe, with its cingulate gyrus lying above the corpus callosum, the big connection between the two cerebral hemispheres.

3. Identify the following features on an axially sliced brain: gray and white matter; cerebral cortex; lateral ventricles; third ventricle. (NI27A, NI27B, NI48B, NII.87.3, NII.87.4, NII.87.5, NII.87.6, NII.87.7, NII.87.8, NII.87.9, N108, TG7-46, TG7-49, TG7-50A, TG7-50B)

The gray matter in the brain has another layer than is found in the spinal cord, where gray matter is found in a butterfly-shape inside the cord, wrapped with white matter making the fiber tracts up and down the spinal cord. In the brain, the cerebral cortex is an additional layer of gray matter on the surface of the brain. White matter in the brain forms fiber tracts connecting the various parts of the brain and communicating up and down with the spinal cord. The corpus callosum is the huge white matter connection between the cerebral hemispheres, above the lateral ventricles, while the septum pellucidum lies between the lateral ventricles.

1. Parts of the ventricular system
   • The cerebral hemispheres are hollow, each containing a lateral ventricle. The ventricles contain a tuft of blood vessels called the choroid plexus, which secretes CSF. The lateral ventricles communicate with the midline third ventricle by way of the interventricular foramina. A thin membrane and attached choroid plexus roofs the third ventricle. In the midbrain, the narrow cerebral aqueduct connects the third and fourth ventricles.
   • The fourth ventricle lies between the pons, cerebellum, and the medulla. It communicates with the cerebral aqueduct, the central canal of the spinal cord, and the subarachnoid space. The roof of the fourth ventricle caudal to the cerebellum, the tela choroidea, is thin like that of the third ventricle and has a choroid plexus. It is perforated by a small median aperture and two lateral apertures that allow cerebrospinal fluid to exit the ventricular system and bathe the brain and spinal cord.
2. The flow of CSF from production to reabsorption
   • CSF is secreted (produced) by the choroidal epithelial cells of the choroid plexuses in the lateral, third, and fourth ventricles.
   • CSF leaves the lateral ventricles through the interventricular foramina and enters the third ventricle. From there CSF passes through the cerebral aqueduct into the fourth ventricle. It leaves this ventricle through its median and lateral apertures and enters the subarachnoid space, which is continuous around the spinal cord and brain. The arachnoid forms various spaces around the brain called cisterns, filled with CSF, such as the interpeduncular and quadrigeminal cisterns. CSF passes into the extensions of the subarachnoid space around the optic nerves.
3. Reabsorption of CSF (reabsorption into the venous system) - the main site of CSF absorption (reabsorption) into the venous system is through arachnoid granulations. The subarachnoid space containing CSF extends into the arachnoid granulations, which in turn project upward through the dura into the superior sagittal sinus and lateral projections from it called lateral lacunae.

Summary: CSF is formed in the brain in the choroid plexus of ventricles, and drains via arachnoid granulations projecting into the superior sagittal sinus.

4. Identify each of the 12 pairs of cranial nerves, both on the brain and in the anterior, middle and posterior cranial fossae. (N11, N104, N114, TG7-07, TG7-51, TG7-52)

	Nerve	Origin	Cranial Exit
I	Olfactory	Forebrain/Prosencephalon	Cribriform plate
II	Optic	Forebrain/Prosencephalon	Optic canal
III	Oculomotor	Midbrain/Mesencephalon	Superior orbital fissure
IV	Trochlear	Midbrain/Mesencephalon	Superior orbital fissure
V	Trigeminal	Pons/Metencephalon	Superior orbital fissure (V1); f. rotundum (V2); f. ovale (V3)
VI	Abducens	Pons/Metencephalon	Superior orbital fissure
VII	Facial	Pons/Metencephalon	Enters internal auditory meatus, travels through petrous temporal bone, leaves via stylomastoid f.
VIII	Vestibulocochlear	Pons/Metencephalon	Enters internal auditory meatus, remains within petrous temporal bone
IX	Glossopharyngeal	Medulla oblongata/Myelencephalon	Jugular foramen
X	Vagus	Medulla oblongata/Myelencephalon	Jugular foramen
XI	Accessory	Medulla oblongata/Myelencephalon and Cervical spinal cord	Jugular foramen
XII	Hypoglossal	Medulla oblongata/Myelencephalon	Hypoglossal canal

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Questions and Answers:

1. Note the choroid plexus; where is it found and what is its function? (TG7-50A, TG7-50B)

Choroid plexuses are relatively large, tuft-like carpets of capillaries. They lie in the floors of the lateral ventricles and the roofs of the third and fourth ventricles. They give off cerebrospinal fluid, filling the ventricles.