organ systems III: neuroanatomy

this lecture covered some basic ideas in the anatomy of the brainstem and cerebellum as well as introducing a little bit of the embryology of the brain. neurulation is the process in the 4th week by which the neural plate, which is the central part of the ectoderm, folds into the neural tube. primary neurulation occurs when the top layer of the neural plate transitions to columnar epithelium, then folds inwards to form a cylinder. secondary neurulation occurs near the sacral region and involves condensation of mesenchymal cells, followed by an epithelial transition that hollows the mass of cells into a tube. the neural folding happens in 5 "waves"- starting at the brainstem, upper spinal cord, head, neck, and lower spinal cord. some conditions such as anencephaly can be due to failure of closure of a specific wave (in the case of anencephaly, failure of wave 2 closure, leading to a lack of cerebrum and skull).

the spinal cord anatomy is looked at in some detail [see diagram]. the grey matter that was introduced in previous semesters was differentiated further into laminas, which are areas within the grey matter that contain neurons that are relatively distinct functionally. in general, the ventral laminas receive sensory afferents, the intermediate laminas contain autonomic neurons, and the dorsal laminas project motor efferents to muscles. the white matter, described as the "freeway" system for axons, is made up of ascending and descending tracts of neurons and can also be divided into discrete functional groupings called funiculi. the dorsal funiculi mainly contains tactile and proprioceptive information up to the brainstem. within the lateral funiculi, there is the lateral corticospinal tract, which is the main pathway for descending motor information from the cortex, the spinocerebellar tract, which sends tactile and proprioceptive information to the cerebellum, and the anterolateral system, which sends pain and temperature information to the thalamus. finally, the ventral funiculi contains the ventral corticospinal tract, which is another pathway for descending motor information from the cortex, and the vestibulospinal and reticulospinal tracts, which receives descending motor information from the brainstem.

next is the brainstem: the midbrain, pons, and cerebellum. the midbrain includes cranial nerves, regulatory centers, sensory and motor pathways, and the reticular formation, which is a system of neurons that projects to the cortex and spinal cord and is central to states of arousal, attention, motivation, and wakefulness, among other things. the cerebellum is described as an outgrowth of the pons and cerebellar peduncles are thick axonal bundles that are the communication channel between the pons and the cerebellum. within the midbrain, there are different nuclei that are involved in different physiological functions: the substantia nigra is involved in dopamine modulation of motor control, the periaqueductal gray nucleus is involved in the stress response, the superior/inferio colliculi are involved in looking and listening, and the red nucleus is part of the descending motor pathway.

next, different areas of the cerebral cortex are looked at: frontal lobe contains the primary, premotor, and association motor cortex. parietal lobe contains the primary, secondary, and association somatosensory cortex. temporal lobe contains the primary, secondary, and association audition cortex. occipital lobe contains the primary, secondary and association visual cortex. finally, the insula is contained within the lateral sulcus and contains the gustatory and visceral cortices.

lastly, we looked at the diencephelon, basal ganglia, and limbic systems. the diencephelon is made up of the thalamus (which is involved in sending sensory and motor information to the cortex), hypothalamus (involved in autonomic and hormonal regulation, and stereotypic behavior), and epithalamus (which houses the pineal gland). the basal ganglia includes the structures: caudate nucleus, putamen, globus, pallidus, substantia nigra, subthalamus. the limbic system includes the structures: limbic cortex, anterior/medial dorsal thalamic nuclei, hippocampus, amygdala, and ventral striatum.

neurulation...
1. what is the neural plate derived from?
2. when does the neural plate begin neurulation?
3. how does the neural tube differentiate along the dorsal/ventral axis?
4. what is the difference between the mechanisms of primary vs. secondary neurulation?
5. describe primary neurulation.
6. describe secondary neurulation.
7. describe the sequence of closure of the neural tube.
8. CNS formation is complete when...
9. what is anencephaly and what is it caused by?
10. what is spina bifida and what is it caused by?
11. where is the caudal neopore?
12. describe the beginning of differentiation of different CNS regions.
13. what is the cephalic flexure and what is its purpose in humans?
14. where is the pontine flexure and what is its significance?

spinal cord...
15. spinal cord can be divided into...
16. grey matter is divided into...
17. white matter is divided into...
18. what are rexes lamina?
19. in general what are the functions of the dorsal, intermediate, and ventral lamina?
20. afferents in the dorsal horn convey...
21. what becomes of afferent neurons in the dorsal column?
22. what sort of neurons are contained within the ventral horn and where do they project?
23. what are the three divisions within white matter?
24. what type of neurons does the dorsal funiculi carry and where do they project?
25. within the lateral funiculi, which is the major descending tract from the cortex?
26. what types of neurons does the spinocerebellar tract convey and where do they project?
27. what type of information does the anterolateral system convey?
28. what is the propriospinal tract and what does it do?
29. what type of neurons does the ventral funiculus convey?

midbrain, pons, cerebellum...
30. brainstem consists of...
31. what are some major structures contained within the brain stem?
32. cerebellum is an outgrowth of...
33. what is the function of the medulla and pons?
34. where is the cerebellum located?
35. what are cerebellar peduncles?
36. what are some functions that the cerebellum is responsible for?
37. what are cerebral peduncles?

which midbrain nuclei are responsible for...
38. dopamine modulation of motor control?
39. pain and stress response?
40. looking and listening?
41. part of the descending motor pathway?

diencephelon...
42. the thalamus contains several nuclei that...
43. what is the hypothalamus responsible for?
44. epithalamus contains...

cerebral cortex: describe what is contained within the...
45. frontal lobe
46. parietal lobe
47. temporal lobe
48. occipital lobe
49. insula

other structures of the brain...
50. what is the reticular formation?
51. what are the components to the basal ganglia?
52. what are the components to the limbic system?
53. describe the interconnection between the cortical regions.

answers
1. central part of ectoderm.
2. the 4th week.
3. via the Shh and BMP growth signalling factors.
4. primary neurulation involves folding a flat surface into a column whereas secondary neurulation is the hollowing out of a solid mass.
5. primary neurulation involves the "columnarization" of existing epithelium following by the folding into a tube.
6. condensation of mesenchyme to form a rod, which then undergoes epithelial transition to form a tube.
7. the neural tube closure occurs in waves in 5 distinct regions. it begins in the brainstem and upper spinal cord, followed by the head and neck, followed by the lower spinal cord.
8. rostral and caudal neuropores close.
9. lack of cerebrum and skull formation due to dysfunctional wave 2 closing.
10. lack of development of spinal cord and overlying vertebrae due to incomplete closure of caudal neopore.
11. at the junctions of wave 1 and 5, the site at which the primary and secondary neurulations join.
12. dilations and flexures of different sections of the neural tube differentiates into different regions of the CNS.
13. the cephalic flexure is a bending forward of the neural tube such that the brain is at a right angle to the spinal cord; an adaptation for upright animals that evolved to let the vision be parallel to the ground.
14. located at the 4th ventricle and pons. significant because at this point the cerebellum is differentiated from the edge of the pons.

15. white and gray matter.
16. sensory, autonomic, motor areas.
17. ascending and descending tracts.
18. functionally distinct areas of grey matter.
19. dorsal is sensory, intermediate is autonomic, ventral is motor.
20. tactile, temperature, proprioceptive, pain sensations to neurons in lamina 2-4
21. afferent axons split and send processes rostrally and caudally.
22. efferent neurons that project to muscle groups.
23. dorsal, ventral, lateral funiculi.
24. tactile, proprioceptive to the brainstem.
25. lateral corticospinal tract.
26. tactile and proprioceptive to the cerebellum.
27. pain and temperature to thalamus.
28. surrounds grey matter and interconnects different spinal levels.
29. descending tracts from the cortex via the ventral corticospinal tract, and descending motor pathways from the brainstem via the vestibulospinal and reticulospinal tracts.

30. medulla, pons, midbrain
31. cranial nerves, sensory and motor pathways, reticular formation, regulatory centers. (mem: brain regulates the cranium through ridiculous S+M)
32. the pons
33. they contain the regulatory centers for respiratory, CV, GI systems.
34. dorsal side of pons and medulla.
35. large axon bundles that interconnect the pons and cerebellum. contain input and output tracts between the two areas.
36. muscle coordination, motor planning, procedural memory, balance, and eye movements. (mem: muscles plan balance procedures with their eyes)
37. axonal bundles that carry sensory and motor pathways to and from the cerebral cortex and the midbrain.

38. substantia nigra.
39. periaqueductal gray
40. superior and inferior colliculi
41. red nucleus

42. process and distribute sensory and motor information to and from the cerebral cortex.
43. autonomic and hormonal regulation and stereotypic behavior.
44. the pineal gland.

45. motor cortex: primary, premotor, association. also has broca's area.
46. somatosensory cortex: primary, secondary, association, wernicke's language area.
47. auditory cortex: primary, secondary, association.
48. visual cortex: primary, secondary, association.
49. contains gustatory and visceral cortex within the lateral sulcus

50. the system in the brain that runs through the medial brainstem and projects to the cortex, limbic structures and spinal cord, and is associated with arousal, attention, motivation, wakefulness, and many other physiological states.
51. caudate nucleus, putamen, globus pallidus, substantia nigra, subthalamus.
52. limbic cortex, anterior and medial dorsal thalamic nuclei, hippocampus, amygdala, ventral striatum
53. white matter axon bundles interconnect different cortical regions: longitudinal, occipitofrontal fasciculi interconnect cortices longitudinally. arcuate fibers interconnect local gyri. corpus callosum interconnects left and right hemispheres.

ms anatomy II: neurocranium part II

[picture courtesy of erica zelfand]
this is the second lecture in the series on the "neurocranium" and dealt with a variety of topics such as structure of the brain, functions of each lobe, blood supply, spinal cord, meninges, glial cells, and the blood brain barrier.

the cerebral portion of the brain is divided up into different lobes, which have been shown to have distinct cognitive and emotional correlates. for example, the frontal lobe is the primary motor area, and is also involved in speech and behavior. the parietal lobes are involved in somatosensory input, prioprioception, sense of self. the temporal lobes are involved in audition, olfaction, and memory. the occipital lobes are involved in vision. cerebellum, midbrain, medulla, are underneath the cerebral portion and are involved more in basic physiological processes. for example, the medulla is involved in autonomic regulation of the cardiovascular system and respiration. the hypothalamus is involved in autonomic, affective, and hormonal activity. the midbrain is involved in motor control. the cerebellum is involved in motor coordination and timing.

the blood supply to the brain comes ultimately from the brachiocephalic branch of the aorta, which branches into the common carotid and the subclavian. the vertebral arteries branch off of the subclavian, travel in the transverse foramen of C1-C6, and penetrate the atlanto-occipital membrane. from there they ascend on the ventral surface of the brainstem and combine to form the basilar artery, from which other arteries branch out (see diagram) such as the pontine and cerebellar arteries. the common carotid artery, on the other hand, branches into the internal and external common carotid; the internal common carotid branches into the anterior and middle cerebral arteries, which supply blood to the lateral and medial cerebral cortex, internal capsule, basal ganglia, and cingulate gyrus. the occlusions in each of these small branches can produce different effects (see diagram).

a few details about the spinal cord: it extends down to L1, beyond which the dura extends until S2. the spinal cord ends at the conus medullaris, a tapering down of the cord which ends in the caudus equinus, which is a splaying out of a horse tail-like arrangement of nerve rootlets. around the spinal cord, there are the three meninge layers: pia, arachnoid, dura mater. denticulate ligaments are pial "projections" into the arachnoid and dura mater which serve to anchor the spinal cord. the filum terminale is the pial strand that connects from the end of the spinal cord (L1) to the end of the dural sac (S2).

a few notes about glial cells and the blood brain barrier. glial cells were covered briefly in histology as the cells that "support" the neurons. in the central nervous system these cells are astrocytes, oligodendrocytes. oligodendrocytes are the cells that produce the myelin sheath that increases the rate of neuronal conduction. whereas schwann cells can only wrap around 1 axon, a single oligodendrocytes can wrap up to 50 different axons. astrocytes provide electrical insulation between neurons, secrete neuronal growth factors and cytokines, and absorb neurotransmitters. they can be further divided into protoplasmic and fibrous- fibrous astrocytes are involved in repairing damaged neuronal tissue. astrocytes also aid in the maintenance of the blood brain barrier, which is made up of astrocyte foot processes, basal lamina, pericytes, and endothelium.

questions
general anatomy and fissures...
1. cerebral hemispheres include...
2. the cerebral cortex is the site for...
3. what are gyri and sulci/fissures?
4. what are the three main fissures in the brain?
5. what does the longitudinal fissure separate?
6. what does the lateral fissure separate?
7. what does the central fissure separate?

functions of...
8. frontal lobe
9. parietal lobe
10. occipital lobe
11. temporal lobe
12. medulla
13. cerebellum
14. pons
15. midbrain
16. thalamus
17. hypothalamus

cerebral arteries...
18. where does the common carotid artery branch off from?
19. where does the common carotid artery split?
20. what does the internal carotid artery split into?
21. what does the internal carotid artery supply blood to?
22. what is the difference between an ischemic and hemorrhagic stroke?
23. where does the middle cerebral artery run?
24. what do the cortical branches supply blood to and what occurs during a stroke of these arteries?
25. what do the lateral striate branches supply blood to and what occurs during a stroke of these arteries?
26. what does the anterior cerebral artery supply blood to?
27. what happens after stroke in the anterior cerebral artery?

basivertebral arteries...
28. describe the passage of the vertebral artery.
29. vertebral arteries unite to form...
30. where is the path of the basilar artery? what does it branch into?
31. what does occlusion in the anterior and posterior spinal branch lead to?
32. what does occlusion in the posterior inferior cerebellar branch lead to?
33. what does occlusion in the anterior inferior and superior cerebellar branch lead to?
34. what does occlusion in the pontine arteries lead to?
35. what does occlusion in the labyrinthine branch lead to?
36. how do occlusions of the vertebral basilar arteries result in deficits in vision?
37. how do occlusions of the vertebral basilar arteries result in problems with balance?

posterior cerebral and circle of willis...
38. where does the posterior cerebral artery project to?
39. what happens when the posterior cerebral artery is occluded?
40. what is the circle of willis?
41. what does the anterior communicating artery connect?
42. what does the posterior communicating artery connect?

spinal cord...
43. how far down does the spinal cord extend?
44. how far down does the spinal dura extend?
45. what is the conus medullaris?
46. what is the cauda equina?
47. where is the junctional zone between the central and peripheral nervous systems?
48. what does the dura turn into at this point?

meninges and spinal veins...
49. how far down does the dura mater extend?
50. what is in the epidural space?
51. what is arachnoid mater?
52. what are denticulate ligaments?
53. what is the filum terminale?
54. where would one extract CSF from the spinal cord?
55. what are the two ways of administering anesthetic to the spinal cord?
56. what does the basivertebral vein do?
57. what is the connection between the basivertebral veins and prostate cancer?
58. describe the vertebral vein's use as a shunt.

glial cells and blood brain barrier...
59. what are oligodendrocytes and what do they do?
60. "unlike schwann cells, oligodendrocytes do not..."
61. what do astrocytes do?
62. what is the difference between protoplasmic and fibrous astrocytes?
63. what are microglia and what do they do?
64. what cell types line the ventricles?
65. what makes up the blood brain barrier?
66. what is the blood brain barrier maintained and induced by?
67. describe the transport of glucose, amino acids, and gases through the blood brain barrier.
68. which brain regions is there no blood brain barrier?

answers
1. white matter, basal ganglia, cerebral cortex.
2. sensorimotor integration, perceptive quality of our experiences
3. gyri are convolutions of the cortex and sulci are divisions or gaps between the gyri.
4. longitudinal, lateral, central.
5. the left and right hemispheres.
6. the frontal and temporal lobes.
7. the frontal and parietal lobes.

8. primary motor area, speech, behavior
9. sensorimotor, prioprioception, association of sensorimotor-audition-vision, formation of egocentric space, sense of self
10. vision
11. audition, olfaction, memory
12. autonomic control over respiration, cardiovascular systems
13. motor coordination and timing
14. cerebellar connection
15. motor control
16. sensorimotor information to cerebral cortex
17. autonomic, hormonal, affective activity

18. the brachiocephalic branch of the aortic arch.
19. at the carotid sinus into the internal and external carotid arteries.
20. anterior and middle cerebral arteries
21. most of the cerebral hemispheres.
22. ischemic is blockage of the cerebral artery via a thrombus or embolus which leads to necrosis. hemorrhagic is rupture of the artery which causes a hematoma, which leads to necrosis.
23. in the lateral fissure; along the lateral surface of the cerebral cortex.
24. lateral surface of cortex. stroke causes sensory, motor, language deficits.
25. internal capsule and basal ganglia. stroke causes hemiplegia.
26. medial surface of cerebral cortex, including cingulated gyrus.
27. sensory, motor, emotional deficits.

28. branches off the subclavian artery, passes through transverse foramina of C1-C6, and penetrates the atlanto occipital membrane.
29. basilar artery on ventral medulla.
30. the ventral surface of the brainstem; branches into cerebellar, pontine, posterior cerebral arteries.
31. loss of spinal cord function
32. Wallenberg syndrome: loss of sensation of pain, heat, muscle coordination.
33. loss of muscle coordination.
34. cranial nerve dysfunction.
35. deafness and vertigo.
36. torsion/compression of vertebral basilar arteries can reduce blood flow to brain stem, cerebellum, occipital lobe- anoxia in the occipital lobe causes loss of vision.
37. anoxia in cerebellum or inner ear can cause problems with balance.

38. temporal and occipital lobes.
39. visual deficits.
40. the anterior and posterior communicating arteries.
41. anterior cerebral arteries.
42. middle and posterior cerebral arteries.

43. down to L1.
44. down to S2.
45. tapered end of the spinal cord.
46. the “horse’s tail”, the end of the spinal cord which branches into nerve roots that extend to lumbar and sacral foramina.
47. the intervertebral foramina
48. the epineurium that covers the dorsal and ventral rami and ganglia.

49. S2
50. veins and fat.
51. the meninge layer in between the dura and pia mater, with trabeculae inside the subarachnoid space.
52. pial connective tissue that suspends spinal cord to the inside of arachnoid / dura mater.
53. a pial strand that connects down to the end of the dural sac.
54. from the subarachnoid space.
55. to the epidural and subdural spaces.
56. drains vertebral bodies.
57. prostate cancer can metastasize into vertebrae through the basivertebral veins.
58. blood shunts from caval veins into vertebral veins if IVC constricted (while coughing, for example)

59. cells in the CNS that myelinate up to 50 axons.
60. "cover unmyelinated axons, which lay bare in the CNS"
61. electrically insulate neurons from each other, uptake neurotransmitters and ions, and secrete neuronal growth factors and cytokines.
62. protoplasmic interconnect neurons, induce early growth and development of the blood/brain barrier, whereas fibrous form astrocytic scars after brain tissue destruction.
63. phagocytic cells related to monocyte/macrophages which consume debris and secrete cytokines during inflammation.
64. ependymal cells.
65. endothelium, pericytes, basal lamina, astrocyte foot processes.
66. maintained by astrocytes.
67. glucose and amino acids pass through BBB via transport proteins, and gases diffuse through lipid membrane.
68. hypothalamus, area postrema, other periventricular regions.

ms anatomy II: neurocranium pt I

this lecture covered the bones, embryology, meninges, and blood supply to the head. the neurocranium is made up of the frontal, parietal, occipital, temporal, and sphenoid bones. the temporal bone is made up of the squamous portion and the petrous portion, the latter of which makes up part of the cranial base. the sphenoid bone is a winged shaped bone that has lesser and greater "wings" and houses the pituitary in the sellae turcica. between the bones of the skull are sutures, which are fusions of the bones that ossify with age. the coronoid sutures fuse the frontal and parietal bones, the sagittal suture fuses the left and right parietal bones, the lamboidal sutures fuse the occiptal and parietal bones. the intersection of the coronoid and sagittal sutures is the bregma junction, the intersection of the sagittal and lamboidal is the lambda junction, and the intersection of the temporal, parietal, frontal, and sphenoid bones on the lateral sides is called the pteryion junction.

the spine develops from sclerotome cells which migrate medially, surround the notochord, and begin to form cartilaginous "models", from which bones form in the process of endochondrial ossification. the individual vertebrae are formed when loose and dense portions of adjacent somites fuse, and the notocord eventually becomes the nucleus pulposus. primary ossification centers in the vertebrae form the bone that ossify from 3-5 years of age, while secondary ossification centers are at the edge of the vertebrae and are active during bone growth during puberty.

the skull bones are developed from lateral plate mesoderm, neural crest cells, somitomeres, and somites. in contrast to the spine, skull bones are formed by intramembranous ossification, which is direct formation of bone without a cartilage model. fontanelles are the spaces between the developing bones that eventually develop into the sutures. the growth of the skull bones is halted in the mid teens when the spheno-occipital joint and the spheno-ethmoidal joint close. finally, the spheno-occipital synchondrosis is the junction between the sphenoid and the basioccipital bone, and can determine the jaw morphology; a large angle at the synchondrosis can cause overbite and a small angle can cause underbite.

below the skull bones lie the meninges, the connective tissue coverings of the brain- dura mater, arachnoid mater, and pia mater. dura mater is the outermost, and includes a falx cerebri septa that separates the two brain hemispheres, as well as a tentorium cerebelli septa that separates the occipital lobes from the cerebrum. the arachnoid mater is the next layer down, and has a subdural layer right underneath the dura mater which is made up of thick connective tissue, and a "subarachnoid space" which is filled with trabeculae. these connect to the innermost layer, pia mater, which is loose connective tissue that aids in anchoring the blood vessels of the brain.

the epidural space is the space between the dura mater and the skull bones and is the site of epidural hematomas, where the middle meningeal artery (the artery that branches off the external carotid and supplies blood to the bones and dura) ruptures and fills the epidural space with blood. the subdural space is the space between the dura and the arachnoid mater; this can be the site of an subdural hematoma, where blood returning from veins in the brain to dural sinuses rupture and fill the subdural space. epidural hematomas are much more severe and occur faster because of the higher arterial blood pressure.

questions
bones of neurocranium...
1. what are the bones of the neurocranium?
2. what are the two portions of the temporal bone?
3. what are four anatomical features of the temporal bone?
4. what are some anatomical features of the body of the sphenoid bone?
5. what are the bones that make up the cranial base?

sutures and junctions...
6. what are sutures?
7. where is the coronal suture?
8. where is the sagittal suture?
9. where is the lamboidal suture?
10. where is the squamosal suture?
11. where is the bregma junction?
12. where is the lambda junction?
13. where is the pteryion junction?

spine development...
14. describe the development of the spine from the somites.
15. how are intersegmental vertebrae formed?
16. notocord becomes the...
17. what is the explanation for the fact that we have 8 cervical nerves with only 7 cervical vertebrae?
18. describe endochondral ossification of the vertebrae.
19. what is the difference between primary and secondary ossification centers?

skull development...
20. what are the skull bones and cartilage derived from?
21. vault bones are formed by...
22. skull form determined by...
23. what are fontanelles?
24. describe the development of the cranial base.
25. how does the longitudinal growth of the skull end during the mid teens?
26. what is the spheno-occipital synchondrosis?
27. what is a large angle of the spheno-occipital synchondrosis associated with?
28. what is a small angle of the spheno-occipital synchondrosis associated with?

meninges...
29. what are the three layers in the meninges?
30. what are the dura mater septa and what do they separate?
31. what are the two portions to the arachnoid mater?
32. describe the pia mater layer.
33. what is the epidural space?
34. what is the subdural space?
35. what is the subarachnoid space?

blood supply...
36. where is the middle meningeal artery? what does it supply?
37. where is the middle meningeal artery most susceptible to damage?
38. what is a subdural hematoma?
39. what is an epidural hematoma?
40. which type of hematoma develops faster and why?

answers
1. frontal, parietal, occipital, temporal, sphenoid.
2. the squamous portion, which forms part of the neurocranium, and the petrous portion.
3. internal/external meatus, styloid process, mastoid process, zygomatic process.
4. the sellae turcica, dorsum sellae, sphenoid sinuses
5. frontal, sphenoid, temporal (petrous portion), occipital, ethmoid

6. fibrous joints between skull bones that ossify with age.
7. between frontal and parietal bones.
8. between parietal bones.
9. between parietal and occipital bones.
10. between the temporal and parietal bones.
11. between the coronal and sagittal sutures.
12. between the lamboidal sutures and sagittal suture.
13. the junction between the frontal, temporal, parietal, and sphenoid bones.

14. sclerotome cells migrate medially, surround neural tube, and form spinal column.
15. fusion of dense and loose tissues of adjacent somites.
16. nucleus pulposus
17. cranial nerve I is above C1- would have been below the "8th" cervical vertebrae, which divided into the dens of C2 and part of the basioccipital bone early in development.
18. the mesoderm begins to differentiate into a cartilage "model" around the notocord, and eventually is replaced by bone.
19. primary ossification centers form bone that fuse between 3-5 years. secondary ossification centers lie on the periphery of vertebrae and are active in puberty.

20. lateral plate mesoderm, neural crest, somitomeres, somites
21. intramembranous ossification.
22. soft tissues.
23. the cartilage between the vault bones that eventually forms the sutures.
24. cranial base is formed by the intramembranous cartilage that joins to form the chondrocranium.
25. the sphenooccipital and sphenoethmoidal joints close.
26. the fusion of the sphenoid and the basioccipital bone that closes around 12-16 years.
27. square jaws or overbite.
28. wide angled mandibles, protrusion of mandibles.

29. dura mater, arachnoid mater, pia mater.
30. the falx cerebri separates the hemispheres and the tentorium cerebelli separates the cerebellum from the occipital lobes.
31. the thick subdural portion and the subarachnoid space, which contains arachnoid trabeculae, which connect to the pia mater.
32. thin CT layer that supports blood vessels on the surface of the brain.
33. the potential space between the dura and the bone.
34. potential space between dura and arachnoid mater.
35. CSF between arachnoid and pia mater.

36. it branches off the external carotid, comes through the foramen spinosum, and supplies blood to the bone and dura.
37. near pterion.
38. cerebral veins that drain into dural sinuses rupture and blood fills the subdural space.
39. meningeal arteries rupture and blood fills the epidural space.
40. external hematomas, because of the higher arterial pressure.