Dissector Answers - Scalp, Cranial Cavity, Meninges & Brain
Upon completion of this session, the student will be able to:
- Define the scalp, its structural layers, muscles, nerves, and vessels.
- Identify the prominent landmarks on the internal surface of the skull base.
- Identify the major blood vessels of the brain, the specializations of cranial meninges, and cranial dural modifications.
- Identify the cranial nerves on the brain and their courses through the skull base.
- Identify the parts of the ventricular system and trace the flow of cerebrospinal fluid from production to reabsorption.
Learning Objectives and Explanations:1. Define the scalp, its layers, muscles, nerves, and vessels. (N98, N99A, N99B, N101, N102, TG7-30)The scalp consists of five layers of tissue. The first three (scalp proper) are connected intimately and move as a unit in wrinkling the scalp.2. Identify the prominent landmarks on the internal surface of the skull base. (N6, N7A, N7B, N9, N11, TG7-07, TG7-08)
For a detailed view of the distribution of the nerves and vessels of the scalp please see 255 WB.
- Skin - thin except in the occipital region where it is thick. It has an abundant arterial supply, good lymphatic and venous drainage, as well as numerous sweat and sebaceous glands.
- Connective tissue - thick, richly vascularized subcutaneous layer that is well supplied with cutaneous (sensory) nerves. This second layer contains the arteries, veins, and cutaneous nerves, which are held tightly in place by collagenous bundles.
- Aponeurosis (galea aponeurotica) - a strong tendinous sheath that covers the calvaria as well as the frontalis and occipitalis portions of the epicranius muscle. Laterally, the anterior and superior auricular muscles (moving the ear) also attach to this aponeurosis.
- Loose connective tissue - layer over the calvaria that allows for movement of the first three layers of the scalp.
- Pericranium (also called Periosteum) - the external bone of the calvaria. (WB 254).The internal surface of the skull is divided into three fossae (depressions):
3. Identify the major blood vessels of the brain, the specializations of cranial meninges, and cranial dural modifications. (N98A, N99B, N101, N102, N103, N104A, N104B, N137, N138, N139, TG7-46, TG7-48, TG7-49, TG7-56A, TG7-56B, TG7-72, TG7-73)
- 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.Arterial supply4. Identify the cranial nerves on the brain and their courses through the skull base. (N11, N104, N114, TG7-07, TG7-51, TG7-52)
- Internal carotid artery - gives rise to ophthalmic (which you will be responsible for in a later lab), posterior communicating, anterior cerebral, and middle cerebral arteries.
- Vertebral arteries
- Anterior spinal arteries - paired branches that unite in the midline.
- Posterior inferior cerebellar arteries, from which arise the two posterior spinal arteries
- Basilar artery - formed by union of the vertebral arteries, it gives rise to anterior inferior cerebellar, superior cerebellar arteries, and bifurcates into the posterior cerebral arteries.
- Circle of Willis or Cerebral arterial circle - forms an important means of collateral circulation in case of obstruction. The circle itself has good collateral circulation, but branches of the circle are end arteries and there is little collateral circulation in the brain itself. Formed by the union of the anterior cerebral, anterior communicating, posterior communicating, and posterior cerebral arteries.
Here is a diagram of the direction of venous return in the brain.
Meninges of the brain
- Pia mater ("delicate mother")
- is a delicate investment that is closely applied to the brain and dips into fissures and sulci.
- enmeshes blood vessels on the surface of the brain.
- Arachnoid mater ("spidery mother")
- is a filmy, transparent, spidery layer that is connected to the pia mater by trabeculation.
- is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid (CSF). It may contain blood after hemorrhage of a cerebral artery (site of formation of subarachnoid hematoma).
- projects into the superior sagittal sinus to form arachnoid villi, which serve as sites where CSF diffuses into the blood.
- Cerebrospinal fluid
- is formed by vascular choroid plexuses in the ventricles of the brain and is contained in the subarachnoid space.
- circulates through the ventricles, enters the subarachnoid space, and eventually is returned to the venous system through the arachnoid granulations.
- Arachnoid granulations
- are tuft-like collections of highly folded arachnoid that project into the superior sagittal sinus and its lateral lacunae (lateral extensions of the superior sagittal sinus).
- release CSF into the superior sagittal sinus and often produce erosion or pitting of the inner surface of the calvaria (granular foveolae).
- Dura mater ("tough mother")
- outermost covering
- two layers
- periosteal dura - lines the cranial bones
- meningeal layer - inner layer that is sometimes separated from the periosteal layer, forming dural venous sinuses and partitions
- Projections of the dura mater
- Falx cerebri is the sickle-shaped double layer of the dura mater, lying between the cerebral hemispheres. It is attached anteriorly to the crista galli and posteriorly to the tentorium cerebelli. Its inferior concave border is free and contains the inferior sagittal sinus, and its attachment to the frontal and parietal bones at the sagittal suture encloses the superior sagittal sinus.
- Falx cerebelli is a small sickle-shaped projection between the cerebellar hemispheres. It is attached to the posterior and inferior parts of the tentorium and contains the occipital sinus in its attachment to the occipital bone.
- Tentorium cerebelli is a crescentic fold of dura mater that supports the occipital lobes of the cerebral hemispheres and covers the cerebellum. Its internal concave border is free and defines the tentorial notch, whereas its attachment to the occipital and temporal bones encloses the transverse sinus posteriorly and the superior petrosal sinus anteriorly.
- Diaphragma sellae is a circular, horizontal fold of dura that forms the roof of the sella turcica, covering the pituitary gland. It has a central aperture for the hypophyseal stalk or infundibulum.
5. Identify the parts of the ventricular system and trace the flow of cerebrospinal fluid from production to reabsorption. (N108, TG7-46, TG7-49, TG7-50A, TG7-50B)
Nerve Cranial Exit I Olfactory Cribriform plate II Optic Optic canal III Oculomotor Superior orbital fissure IV Trochlear Superior orbital fissure V Trigeminal Superior orbital fissure (V1); f. rotundum (V2); f. ovale (V3) VI Abducens Superior orbital fissure VII Facial Enters internal auditory meatus, travels through petrous temporal bone, leaves via stylomastoid f. VIII Vestibulocochlear Enters internal auditory meatus, remains within petrous temporal bone IX Glossopharyngeal Jugular f. X Vagus Jugular f. XI Accessory Jugular f. XII Hypoglossal Hypoglossal canal
Summary: CSF is formed in the brain in the choroid plexus of ventricles, and drains via arachnoid granulations projecting into the superior sagittal sinus.
- 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. (WB 29)
- 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.
- 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.
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.2. The spread of infection is mainly facilitated through what layer of the scalp? (N98A, N101, N102, TG7-46, TG7-49)The subaponeurotic connective tissue layer (areolar tissue) of the scalp facilitates the spread of infection because of its loose character.3. Note dural attachment to the calvaria and the base of the skull. Is there any difference? (N98A, N101, N102, TG7-46, TG7-47, TG7-48, TG7-49)The spinal dura consists of one layer and is a tube with lateral extensions covering nerve rootlets. The cranial dura, on the other hand, splits to form two layers.4. Examine falx cerebri, falx cerebelli, tentorium cerebelli, and diaphragma sellae. Are these infoldings periosteal or meningeal? Define attachments and relationship of each and the compartmentalization of the cranial cavity produced by these infoldings. (N103, N104A, N104B, TG7-47, TG7-48, TG7-49A, TG7-49B)
The periosteal layer adheres to the internal surface of the skull, and its attachment is tenacious along the suture lines and in the cranial base. The external periosteal layer is continuous at the cranial foramina with the periosteum on the external surface of the calvaria; it is NOT continuous with the dura mater of the spinal cord. In most areas, the meningeal layer is intimately fused with the periosteal layer and cannot be separated from it. The fused external and internal layers of dura over the calvaria can be easily stripped from the cranial bones. A blow to the head can detach the periosteal layer from the calvaria without fracturing the cranial bones. In the cranial base the two dural layers are firmly attached and difficult to separate from the bones. Consequently, a fracture of the cranial base tears the dura and results in leakage of CSF.
- An external periosteal layer is the periosteum covering the internal surface of the calvaria.
- An internal meningeal layer, a strong fibrous membrane that is continuous at the foramen magnum with the spinal dura mater covering the spinal cord.The internal meningeal layer of dura draws away from the external periosteal layer of dura to form dural infoldings, which separate the regions of the brain from each other and form dural venous sinuses. These separations in the dural layers form the dural venous sinuses (compartments). See the objective question above for attachments and relationships of these dural infoldings.5. What does each compartment contain? (N103, N104A, N104B, TG7-47, TG7-49)The falx cerebri helps form the superior and inferior sagittal sinuses. The tentorium cerebelli separates the cerebellum from the cerebral hemispheres. In the line of the junction between the falx cerebri and the tentorium cerebelli lies the straight sinus. The diaphragma sellae is a horizontal duplication of the meningeal dura that roofs the sella turcica. The falx cerebelli is a partitioning of the dura which separates the cerebellar hemispheres. It contains the occipital sinus. (WB 323-4)6. What is the tentorial notch? (N104A, N104B, TG7-47)The tentorial notch is the opening in the tentorium cerebelli for the brainstem (specifically the midbrain).7. Observe meningeal arteries in all cranial fossae. Which is the largest? How is it held within the dura? Relation to greater wing of sphenoid? (Significance?) (N99B, N104A, N104B, TG7-51)The largest of the meningeal arteries is the middle meningeal artery. It is a branch of the maxillary artery. It goes through the foramen spinosum and supplies most of the dura mater except for the floors of the anterior and posterior cranial fossae. It runs forward for a short distance in a groove on the greater wing of the sphenoid bone, lying between bone and dura, and then divides into anterior and posterior branches. (WB 326, 269) Significance: The dura is sensitive to pain, especially where it is related to the dural venous sinuses and meningeal arteries. Consequently, piercing the dura where the meningeal arteries enter the base of the skull or near the vertex causes pain and is a major source of headaches. In addition, the rupture of the middle meningeal artery by fracture of the greater wing of the sphenoid bone causes an epidural hematoma.8. What is the innervation of dura? (N104A, N104B, TG7-51)The dura is innervated by all three divisions of the trigeminal nerves, the vagus nerves, and the hypoglossal nerves.9. What cranial nerves exit through the jugular foramen? (N11, N104A, N104B, TG7-07, TG7-51)
A. Anterior and posterior ethmoidal branches of the ophthalmic division of the trigeminal nerve in the anterior cranial fossa.
B. Meningeal branches of the maxillary and mandibular divisions of the trigeminal nerve in the middle cranial fossa.
C. Meningeal branches of the vagus and hypoglossal nerves in the posterior cranial fossa.
Cranial nerves IX, X, and XI.10. Sinus rectus (is a portion of the great cerebral vein attached to it?) (N103, N104A, N104B, TG7-47, TG7-49)The sinus rectus, a.k.a. the straight sinus, is formed by the union of the inferior sagittal sinus with the great cerebral vein.11. Confluens of sinuses (significance, location, pattern, and variations). (N103, N104A, N104B, TG7-47, TG7-49)The confluens of sinuses is a meeting place of the superior sagittal, straight, occipital, and the right and left transverse sinuses. This junction is a dilitation at one side of the internal occipital protuberance. Sometimes it is a region of actual confluence, and sometimes the superior sagittal and straight sinuses (either or both) bifurcate here to form the right and the left transverse sinuses. (WB 325).12. Superior petrosal sinus (connects what?) (N103, N104A, N104B, TG7-47)The superior petrosal sinus connects the posterior end of the cavernous sinus to the bend marking the transition between the transverse and sigmoid sinuses. It receives cerebellar and inferior cerebral veins and veins from the tympanic cavity. (WB 326) .13. Define emissary veins and the mastoid, condyloid, parietal, and ophthalmic emissary veins. (N98A, N101, N102, TG7-73)The emissary veins are small veins connecting the dural venous sinuses with the veins of the scalp. They are valveless and, as a result, may conduct blood inward or outward in accordance with the pressure existing in the sinuses and in the external veins. Some are constant; others occur occasionally. The superior ophthalmic vein is the largest vein of this type. It connects the angular vein of the face with the cavernous sinus. The mastoid emissary vein unites the posterior auricular vein with the sigmoid sinus. The parietal emissary vein occupies the parietal foramen and connects the veins of the scalp with the superior sagittal sinus. The emissary vein of the foramen cecum connects the veins of the nasal cavity with the superior sagittal sinus. The condyloid canal, when present, transmits an emissary vein which passes between the lower end of the sigmoid sinus and veins of the suboccipital triangle of the neck. (WB 326)14. Remove the blood from the cavernous sinus and note trabeculae. Do the two sides communicate? (N104A, N104B, TG7-47, TG7-60)The cavernous sinuses usually communicate (WB 325). They are found on each side of the sella turcica and the body of the sphenoid bone and lie between the meningeal and periosteal layers of the dura mater.15. Expose the internal carotid artery (course?) (N104A, N104B, N138, N139, TG7-47, TG7-60, TG7-72)The internal carotid artery:16. Look for arachnoid granulations (villi). What is their function? (N98A, N101, N102, N108, TG7-46, TG7-49, TG7-50)
- has no branches in the neck
- ascends within the carotid sheath in company with the vagus nerve and the internal jugular vein.
- enters the cranium through the carotid canal in the petrous part of the temporal bone.
- in the middle cranial fossa, gives rise to the ophthalmic artery and the anterior and middle cerebral arteries and the posterior communicating artery.The arachnoid granulations are tuftlike collections of highly folded arachnoid that project through the dura mater into lateral lacunae of the superior sagittal sinus and into other dural sinuses. Through their thin membranes, the cerebrospinal fluid is passed into the blood stream. (WB 323)17. Examine the pia mater on the brain. How does it differ from the arachnoid mater in covering the brain? (N98A, N101, N102, TG7-46, TG7-49)The pia mater on the brain is a delicate, intimate, areolar investment of brain and spinal cord that enmeshes the blood vessels on their surfaces. It is a vascular membrane. On the other hand, the arachnoid is a delicate transparent membrane composed of a blend of collagenous and elastic fibers and squamous mesenchymal epithelial cells. It is NOT vascular and is NOT attached directly to the surface of the brain or spinal cord. Arachnoid trabeculae are thin strands that conect the arachnoid to the pia mater. (WB 322-23)18. What is the arterial circle of Willis? (N139, TG7-56A, TG7-56B)The circle of Willis, a.k.a. cerebral arterial circle, is an important anastomosis at the base of the brain between the following arteries:
The various components of the cerebral arterial circle give many small branches to the brain.
- Anterior cerebral arteries
- Anterior communicating arteries
- Internal carotid arteries
- Posterior communicating arteries
- Posterior cerebral arteries
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