page 374 gross anatomy the meninges - wordpress.comdural venous sinuses are modiﬁed veins that...

GROSS ANATOMYThe Meninges

The meninges are connective tissue coverings of the brain and spinal cord and consist of the dura mater,arachnoid mater, and pia mater. These coverings perform many functions and their structure variesslightly depending on whether they are covering the brain or the spinal cord. Exercise 15.1 exploresmeningeal structures as they apply to the brain, and exercises in chapter 16 consider meningealstructures as they apply to the spinal cord.

EXERCISE 15.1CRANIAL MENINGES

1. Obtain a model of the dura mater of the brain or a cadaveric specimen of the head with intactdural structures.

2. Identify meningeal structures listed in figure 15.1 on a model or cadaveric specimen, using table15.1 and the textbook as guides.

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Figure 15.1The Meninges.(a) Coronal section through the superior sagittal sinus. (b) Dural venous sinusesand cranial dural septa.

Table 15.1 Meninges, Dural Septa, and Dural Venous Sinuses

Structure Description and Function Word Origin

Meningeal Layer

Dura Mater Very tough, durable membrane composed of denseirregular connective tissue that protects CNS structureswithin the cranial cavity and vertebral canal; composedof two layers

durus, hard, + mater,mother

PeriostealLayer

Outer layer of dura mater that composes the innerperiosteum of the cranial bones and anchors the duramater tightly to the cranial bones (not present withinthe vertebral canal). In most places within the cranialcavity it is anchored to, or continuous with, themeningeal layer.

periosteal, relating tothe periosteum

MeningealLayer

Inner layer of dura mater that forms cranial dural septaand dural venous sinuses within the cranial cavity

meninx, membrane


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1. Observe the dura mater (figure 15.1). The dura mater (dura, hard, + mater, mother) iscomposed of two layers: the outer periosteal layer (which is simply the periosteum thatlines the internal portion of the cranial bones), and the inner meningeal layer. Themeningeal layer folds inward to form cranial dural septae and dural venous sinuses (figure15.1b). Cranial dural septae are partitions that stabilize and support the brain. Duralvenous sinuses are modified veins that transport venous blood from the brain to the internaljugular vein. The largest dural venous sinus is the superior sagittal sinus. Eventually allsinuses drain into the sigmoid (sigmoid, S-shaped) sinus, which drains into the internaljugular vein. Table 15.1 includes a list of dural venous sinuses and describes their generallocations.

2. If a cadaveric specimen is available, locate the superior sagittal sinus and note the smallgranular structures located in and around the sinus. These structures are called arachnoidvilli (granulations) and serve as sites for absorption of cerebrospinal fluid from thesubarachnoid space into the dural venous sinuses.

3. Obtain a human brain with the arachnoid mater and pia mater intact. If one is not available,this activity will be completed when performing the sheep brain dissection. On the humanbrain, note the thin, transparent covering that lies over the surface of the brain and does notfollow the sulci (shallow grooves). This is the arachnoid mater (arachnoid, shaped like aspiderweb). Deep to the arachnoid mater is the finest, thinnest meningeal layer, the piamater (pia, soft or tender). The pia mater is in direct contact with the neural tissue of thebrain and follows all of the contours (gyri and sulci) on its surface. The space between thearachnoid mater and the pia mater is the subarachnoid space. What fluid is normally foundin the subarachnoid space?

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3. Label the meningeal structures in figure 15.2.


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Figure 15.2Meningeal Structures.Use the terms listed to fill in the numbered labels in the figure. Someanswers may be used more than once.

4. Optional Activity: 7: Nervous System—Watch the “Meninges” and “Dural Sinus BloodFlow” animations to reinforce your understanding of these structures and their relationships.

arachnoid mater

arachnoid villi

dura mater

dura mater (meningeal layer)


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dura mater (periosteal layer)

falx cerebri

pia mater

subarachnoid space

superior sagittal sinus

confluence of sinuses

diaphragma sellae

falx cerebelli

falx cerebri

inferior sagittal sinus

straight sinus


tentorium cerebelli

Meninges (first link) AND Dural Sinus Blood Flow (second link)

CLINICAL VIEWMeningiomas

Meningiomas are brain tumors that originate in the meninges of the central nervous system. These slow-growing tumors develop from cells within the arachnoid villi and project into the venous sinuses withinthe brain. Incidence is higher in women than in men, although the reason is unknown. Most commonly,meningiomas develop either superior to the frontal and parietal lobes near the falx cerebri (parasagittalmeningioma) or between the cerebral hemispheres (falcine meningiomas). Some may grow directlyunder the skull (convexity meningiomas), and some develop within the ventricles (ventricularmeningiomas). Other tumors are considered skull base meningiomas due to their location. Subtypes ofskull base meningiomas include those located near the sphenoid bone (sphenoid wing meningioma),near the posterior fossa (petrous meningioma), or near the ethmoid bone (paranasal/olfactorymeningioma). Meningiomas are dome-shaped, encapsulated tumors that typically attach to the duramater. In some cases, the tumor may project into and infiltrate surrounding bone tissue. Manymeningiomas are asymptomatic and benign. As a tumor grows in size, it may cause symptoms such asheadaches, seizures, muscle weakness, sensory and visual disturbances, and increased intracranialpressure. Surgery is required to remove tumors that become symptomatic or malignant (this means thatthe tumor has spread to other tissues). Some meningiomas are associated with genetic disorders, someare linked to radiation exposure, and some have no known cause at all. Because many meningiomas areasymptomatic, they may not be discovered unless an autopsy is performed.

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Ventricles of the Brain

The ventricles are fluid-filled spaces within the central nervous system that are complex in shape.Exercise 15.2 involves viewing a cast of the ventricles, which is produced by filling the ventricularspaces with plastic, allowing the plastic to harden, and then removing the brain tissue so only the cast isleft. A cast allows one to visualize the three-dimensional structure of the ventricles without the brainliterally “getting in the way.” If casts of the brain ventricles are not available, this exercise can beperformed using table 15.2 and figures in the textbook.

Table 15.2 Ventricles of the Brain

Structure Location and Description Secondary BrainVesicle Derivative

Word Origin

LateralVentricles

Horseshoe-shaped ventricles within thecerebral hemispheres containing anteriorand posterior horns whose shape followsthe developmental shape of the cerebralhemispheres

Telencephalon latus, to the side,+ ventriculus,belly

ThirdVentricle

Narrow, quadrilateral-shaped ventriclelocated in the midsagittal plane inferiorto the corpus callosum and medial to thethalamic nuclei; surrounded bystructures of the diencephalon

Diencephalon ventriculus,belly

CerebralAqueduct

Narrow channel that lies in the midbrainbetween the cerebral peduncles and thetectal plate (corpora quadrigemina)

Mesencephalon aquaeductus, acanal

FourthVentricle

Diamond-shaped ventricle locatedanterior to the cerebellum and posteriorto the pons

Metencephalon(superior part)Myelencephalon(inferior part)

ventriculus,belly

The central nervous system initially develops as a neural tube. As it grows, the neural tube begins tochange size and shape. The cephalic end develops into the brain, while the rest develops into the spinalcord. Both the brain and the spinal cord contain fluid-filled spaces inside. The pattern of growth of theneural tissue surrounding the neural tube changes the size and shape of the fluid-filled spaces within.Thus, because the spinal cord remains mostly a tubular structure as it grows, the fluid-filled space inside,the central canal, remains tubular. On the other hand, because the cephalic (brain) end of the neuraltube undergoes extensive folding as it grows, the fluid-filled spaces within develop into irregular shapes.


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These shapes tell a story about how the parts of the brain developed.

The cephalic end of the neural tube first develops into three primary vesicles (prosencephalon,mesencephalon, and rhombencephalon) and then into five secondary vesicles (telencephalon,diencephalon, mesencephalon, metencephalon, and myelencephalon). Figure 15.3 lists the secondaryvesicles of the brain and the parts of the ventricular system that develop from each of them. Forinstance, the telencephalon undergoes extensive growth as it develops into the cerebral hemispheres.This growth results in the horseshoe-shaped structure of the lateral ventricles in the adult brain. On theother hand, the mesencephalon does not undergo such extensive growth as it develops into the midbrain.Hence the fluid-filled space inside, the cerebral aqueduct, remains tubular in shape within the adultbrain.

Figure 15.3Secondary Brain Vesicles and Associated Ventricular Structures of the Brain.Page 379EXERCISE 15.2BRAIN VENTRICLES

1. Obtain a cast of the ventricles of the brain (figure 15.4).


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Figure 15.4Cast of the Ventricles of the Brain.Use the terms listed to fill in the numbered labels in the figure.

2. Identify the ventricles of the brain listed in figure 15.4 on the cast of the ventricles, using table15.2 and the textbook as guides. Then label them in figure 15.4. When identifying each of theventricles, relate each ventricle to the secondary brain vesicle from which it developed (table15.2 and figure 15.3).

LEARNING STRATEGY

When observing whole brains or brain models in the laboratory, always begin by locating the ventricularspaces and associating each ventricular space with a secondary brain vesicle (table 15.2). Next, identifythe adult brain structures that surround the ventricular spaces. Finally, correlate each adult brainstructure to the secondary brain vesicle from which it formed. For example, the lateral ventricles(ventricular space), which are part of the telencephalon (secondary brain vesicle), are surrounded by thecerebral hemispheres (adult brain structures). Thus, the cerebral hemispheres (brain structure) arederived from the telencephalon (secondary brain vesicle).

cerebral aqueduct

fourth ventricle

interventricular foramen

lateral ventricles

third ventricle

Page 380EXERCISE 15.3CIRCULATION OF CEREBROSPINAL FLUID (CSF)

Cerebrospinal fluid (CSF) is a clear, colorless fluid produced from blood plasma, which is filtered bythe choroid plexus within each brain ventricle. Approximately 500 mL of CSF is produced each day.Following its production within the lateral ventricles, CSF moves through the following structures: theinterventricular foramen, the third ventricle, the cerebral aqueduct, and the fourth ventricle. CSF exitsthe fourth ventricle through the median and lateral apertures to enter the subarachnoid space, whichsurrounds both the brain and the spinal cord. CSF is returned to the blood through arachnoid villi withinthe dural venous sinuses. CSF provides buoyancy to the brain, protecting both the brain and the spinal


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cord from sudden movements. CSF also provides environmental stability for cells in both the brain andthe spinal cord by maintaining extracellular fluid composition within narrow limits.

1. Using table 15.2, figure 15.4, and the textbook as guides, label the structures involved in CSFproduction and circulation in figure 15.5.

Figure 15.5Cerebrospinal Fluid (CSF) Production and Circulation. Note that arrows show the directional flowof CSF.Use the terms listed to fill in the numbered labels in the figure. Some terms may be usedmore than once.

arachnoid mater

arachnoid villi

arachnoid villus

central canal

cerebral aqueduct


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cerebral cortex

choroid plexus of fourth ventricle

choroid plexus of lateral ventricle

choroid plexus of third ventricle

CSF flow

dura mater

interventricular foramen

lateral aperture

median aperture

meningeal dura

periosteal dura

pia mater

subarachnoid space


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2. Trace the flow of CSF from the choroid plexus within the ventricles to the arachnoid villi. Use theterms listed in column A to complete the pathway for CSF circulation in column B, in the spacesprovided.

Column A Column B

cerebral aqueduct 1. lateral ventricles

fourth ventricle 2.

interventricular foramen 3.

median and lateral apertures 4.

subarachnoid space 5.


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third ventricle 6.

7.

8. arachnoid villi

3. Optional Activity: 7: Nervous System—View the “Brain Ventricles” and “CSF Flow”animations to see how cerebrospinal fluid flows through the brain ventricles.

Brain (first link) AND CSF Flow (second link)

The Human Brain

The next series of exercises involves identifying structures that are visible in four views of the humanbrain: superior, lateral, inferior, and midsagittal. Table 15.3 lists the main brain structures, the views inwhich each is visible, and a description of its function.

Table 15.3 Brain Structures Visible in Superficial Views of Whole or SagittallySectioned Brains

Brain Structure Description Function(s) Word Origin ViewsWhereVisible

Cerebrum

Frontal Lobe Lies deep to thefrontal bone

Controls consciousmovement of skeletalmuscle; containsBroca’s area, whichcontrols motor speech.Controls conjugate eyemovement (the ability tomove the eyes together).Higher-level functionsinclude judgment andforesight (the ability tothink before acting).

frontal, in thefront, + lobos,lobe

Superior,lateral,inferior, andmidsagittal

Precentral Gyrus Fold of brain tissuelocated immediatelyanterior to the

Primary somatic motorarea of the brain.Neurons from this gyrus

pre, before, +central, relatingto the central

Superior,lateral, andmidsagittal


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central sulcus are somatic motorneurons that initiatemotor signals to controlvoluntary muscleactivity.

sulcus, + gyros,circle

Occipital Lobe Lies deep to theoccipital bone

Primary visual area (thefirst area of the cerebralcortex where visualinformation synapses,after the thalamus).Visual association area(the ability to interpretvisual information).

occiput, theback of thehead, + lobos,lobe


Parietal Lobe Lies deep to theparietal bone

Receives sensory inputfrom the skin andproprioceptors. Higher-level functions includelogical reasoning (math,problem solving).

parietal, a wall,+ lobos, lobe


Postcentral Gyrus Fold of brain tissuelocated immediatelyposterior to thecentral sulcus

Primary somatic sensoryarea of the brain.Sensory informationthat comes in from thebody travels to this areaof the cerebral cortex.

post, after, +central, relatingto the centralsulcus, + gyros,circle


SeptumPellucidum

A thin membranelocated between thecorpus callosum(above) and fornix(below)

Contains neurons andglial cells, and forms athin connection betweenthe corpus callosumabove and the fornixbelow; also forms a thinwall between the twoanterior horns of thelateral ventricles

saeptum, apartition, +pellucidus,allowing thepassage of light

Midsagittal

Temporal Lobe Lies deep to thetemporal bone

Primary auditory andauditory associationarea of the brain;conscious perception ofsmell

tempus, time Lateral


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Diencephalon

Epithalamus A small projectionextendingposteriorly from thesuperior portion ofthe third ventricle

Contains the pinealbody (pineal gland)along with otherstructures

epi, above, +thalamos, a bedor bedroom

Midsagittal

Pineal Body(Gland)

Small gland foundwithin theepithalamus. It is notpossible to establishthe differencebetween theepithalamus and thepineal gland ongross observationalone.

Secretes the hormonemelatonin from itsprecursor molecule,serotonin, in response todecreased light levels.Melatonin has an effecton circadian rhythms.May also play a role inestablishing the onset ofpuberty.

pineal, shapedlike a pine cone

Midsagittal

Hypothalamus Located deep to thewalls of the inferiorpart of the thirdventricle

Regulates bodytemperature,metabolism(hunger/thirst), sleep,sex, and emotionalcontrol (limbic systemfunctions). It is also a“master” endocrinegland, controllinghormone secretion fromthe pituitary gland.

thalamos, a bedor bedroom

Midsagittal

MammillaryBodies

Two small bump-like (“breast-shaped”) structuresof the hypothalamuslocated immediatelyposterior to theinfundibulum

Involved in short-termmemory processing;part of the limbicsystem (the emotionalbrain). Also involvedwith suckling andchewing reflexes.

mammillary,shaped like abreast

Inferior andmidsagittal

Thalamus Paired nuclei locateddeep to the lateralwalls of the thirdventricle. A pinpierced through the

Primary relay center forall sensory informationcoming into the brain(except olfaction)

thalamos, a bedor bedroom

Midsagittal


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lateral wall of thethird ventricleadjacent to theintermediate masswill pass into thethalamic nuclei.

IntermediateMass (Inter-thalamicAdhesion)

A fiber tract thatcrosses the thirdventricle. The cutend of this structureis visible in amidsagittal sectionof the brain in themiddle of the thirdventricle.

A fiber tract thatconnects the twothalamic nuclei to eachother. It is absent inabout 20% of humanbrains.

intermediate, inthe middle, +mass, a mass

Midsagittal

Brainstem

MedullaOblongata

The most inferioraspect of thebrainstem, formingthe transition zonebetween the brainand spinal cord

Contains the centers forregulation of respirationand cardiac function,and contains nuclei ofthe reticular activatingsystem, which is a groupof nuclei that areimportant in regulatingwakefulness andselective attention

medius, middle,+ oblongus,rather long

Lateral,inferior, andmidsagittal

Midbrain(Mesencephalon)

Found superior tothe pons

Contains externalstructures, inferior andsuperior colliculi, and isresponsible for auditoryand visual processing.Internal structuresinclude substantia nigra(movement andemotional responses)and tegmentum(integrate motorinformation fromcerebrum andcerebellum).

mesos, middle+ enkephalos,brain



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Tectal Plate(CorporaQuadrigemina)

Consists of four twinbodies, the superiorand inferior colliculi

Control center for visualand auditory reflexes

corpus, body, +quad, four, +geminus, twin

Midsagittal

InferiorColliculus

A pair of ovalprojections thatmake up the inferiorpart of the tectalplate (corporaquadrigemina)

Controls audtoryreflexes, such as thesudden turning of thehead toward the sourceof a very loud sound

inferior, lower,+ colliculus, amound or hill

Midsagittal

SuperiorColliculus

A pair of roundedprojections thatmake up superiorpart of the corporaquadrigemina (tectalplate)

Controls visual reflexes,such as the suddenturning of the headtoward the source of aflashing light

superus, above,+ colliculus, amound or hill

Midsagittal

Pons Appears as a largemass just superior tothe medullaoblongata

A “bridge” of nervetracts that connect thecerebral hemispheres tothe cerebellarhemispheres. Containscenters for control ofrespiration.

pons, bridge Lateral,inferior, andmidsagittal

Cerebellum

Cerebellum The second largestpart of the brain

Regulation of muscletone (a low-level musclecontraction),coordination of motoractivity, andmaintenance of balanceand equilibrium

cerebellum,little brain


Limbic System

Cingulate GyrusA gyrus located justsuperior to thecorpus callosum

This area of the brain isnot well understood. Itis predominantly motorand may play a role in

cingo, tosurround, +gyros, circle

Midsagittal


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the limbic system (suchas controlling motorfunctions with a strongemotional component).

Fornix An arching fibertract located inferiorto the septumpellucidum

Connects limbic systemstructures to each other

fornix, arch Midsagittal

Olfactory Bulbs Swellings connectedto the anterior end ofthe olfactory tractsthat lie on theinferior surface ofthe frontal lobes ofthe brain lateral tothe longitudinalfissure

Location where cranialnerve I (CN I), theolfactory nerves, firstsynapse after passingthrough the cribriformplate of the ethmoidbone

olfactus, tosmell, + bulbus,a globularstructure

Inferior

Olfactory Tracts Nerve fibers thatextend from theolfactory bulbsposteriorly to thejunction where thefrontal lobes meetthe optic chiasm

Carry the axons ofneurons from theolfactory bulbs towardstructures in other areasof the brain involvedwith olfaction

olfactus, tosmell, + tractus,a drawing out

Inferior

Fissures/Sulci

Central Sulcus A deep groove thatextends along thecoronal plane

Separates the frontallobe from the parietallobe

central, in thecenter, + sulcus,a furrow


Lateral Sulcus A horizontal groovebetween the frontal/parietal lobes andthe temporal lobe

Separates the frontaland parietal lobes fromthe temporal lobe

latus, the side,+ sulcus, afurrow

Lateral

LongitudinalFissure

A deep fissurebetween the twocerebral hemispheres

Separates the twocerebral hemispheres;the falx cerebri occupiesthis fissure in a living

longus, long, +a deep furrow

Superiorand inferior


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human

Parieto-occipitalSulcus

Small groove thatruns along thecoronal plane

Separates the parietallobe from the occipitallobe

parieto-occipital,between theparietal andoccipital lobes,+ sulcus, afurrow


TransverseFissure

A deep fissurebetween thecerebrum and thecerebellum

Separates the cerebralhemispheres from thecerebellar hemispheres.The tentorium cerebellilies in this fissure.

transversus,across, +fissure, a deepfurrow

Lateral

Fibers/Tracts

CerebralPeduncles

Tracts locatedbetween themidbrain and thepons

The fibers connect theforebrain (cerebralhemispheres anddiencephalon) to thehindbrain (medullaoblongata, pons, andcerebellum)

cerebrum,brain, +pedunculus, alittle foot

Inferior,midsagittal

Corpus Callosum A fiber tract locatedsuperior to thelateral ventricles

Contains axons thatconnect the two cerebralhemispheres

corpus, body, +callosus,thickskinned

Midsagittal

Infundibulum A funnel-shapedinferior extension ofthe brain locatedimmedatelyposterior to the opticchiasm

Consists of tracts thatconnect thehypothalamus to theposterior pituitary (parsnervosa)

infundibulum, afunnel


Optic Chiasm The X-shapedstructure formedwhere the two opticnerves join, withmost fibers crossingto the opposite side;

Location where fibersfrom both optic nervescross over and travel inthe optic tract on theopposite side. Not allfibers from the optic

optikos, relatingto the eye orvision, +chiasma, twocrossing lines



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located just anteriorto the infundibulum

nerves cross over.

Optic Nerves Anterior to the opticchiasm

Sensory neuronscarrying visualinformation from theretina to the opticchiasm, where mostfibers cross to theopposite side

optikos, relatingto the eye orvision, + nevus,a white, cord-like structure

Inferior

Optic Tracts Posterior to the opticchiasm

Sensory neuronscarrying visualinformation from theoptic chiasm to thelateral geniculatenucleus of the thalamus

optikos, relatingto the eye orvision, +tractus, adrawing out

Inferior

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When identifying parts of the brain, follow these steps to make things easier:

Name the structures in a logical order, such as the order in which the structures appear fromanterior to posterior. Learning the structures in an orderly fashion will allow for improvedinformation recall later on.

Do not think of brain regions as isolated structures. The structures are easier to identify within thecontext of their surroundings. Think about this the next time you are traveling to your anatomy &physiology class—would you know how to get to the classroom if all of the buildings on campus(with the exception of the one to which you are traveling) were suddenly moved around?

Always associate a function with each structure identified. Use table 15.3 as a reference.

LEARNING STRATEGY

Because structure/function relationships are not easily visualized when it comes to the brain, considermaking flashcards using table 15.3 as a reference. List the brain structure on one side of the card and itsfunction on the other side. Once the names and locations of the listed structures have been mastered,quiz others on the structure and function of each item.

Page 384CONCEPT CONNECTION

The hypothalamus is the homeostatic center of the brain, as it regulates such fundamental processes inthe body as metabolism (thirst/hunger), body temperature, sleep, and sex. The hypothalamus serves asthe master control center for the autonomic nervous system and the endocrine system. That is, there areconnections between the hypothalamus and nuclei in the brainstem that regulate heart rate, respirationrate, and digestion. Although the hypothalamus is composed of nervous tissue, it is also considered an


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endocrine organ, as it plays a critical role in the release of hormones. It has connections with thepituitary gland, which lies inferior to the hypothalamus. The pituitary gland is composed of two lobes,called the posterior pituitary gland and the anterior pituitary gland. The hypothalamus containsneurosecretory cells, which are neurons that secrete hormones. A hormone is a chemical messenger thattravels to a distant target via the bloodstream. The axons of the neurosecretory cells located in theparaventricular or supraoptic nuclei extend inferiorly through the infundibulum to the posterior pituitary.Upon stimulation, these neurosecretory cells release a hormone by exocytosis from their axon terminalsin the posterior pituitary gland. This occurs much like acetylcholine being released from somatic motorneurons at a neuromuscular junction. Two hormones are released from the posterior pituitary gland:antidiuretic hormone (ADH) and oxytocin.

Neurosecretory cells arising from other nuclei within the hypothalamus release hormones into capillariesin the median eminence. These hormones enter a portal vein and are transported to the anterior pituitarygland. There, the hypothalamic hormones stimulate cells in the anterior pituitary to release theirhormones. As was the case with the posterior pituitary gland, these hormones then enter the systemiccirculation and are carried to distant targets. The specific hormones released and the cells responsible forhormone production will be covered in detail in chapter 19.

The hypothalamus also plays a role in regulating emotional behavior due to its involvement with thelimbic system. The hypothalamus is located at the center of this system, connected to surroundinglimbic system structures, such as the hippocampus and amygdala, via the fornix. Therefore, thehypothalamus influences aggression, fear, and pleasure. There are nuclei within the hypothalamus thatregulate body temperature (preoptic area), circadian rhythms (suprachiasmatic nucleus), and hunger andsatiety (ventromedial nucleus). The central location of the hypothalamus and its proximity tosurrounding brain structures will be covered later in this chapter.

EXERCISE 15.4SUPERIOR VIEW OF THE HUMAN BRAIN

1. Obtain a human brain or models of a human brain and observe the superior surface.

Numerous grooves are associated with the brain surface. A large groove is a fissure,whereas a small groove is a sulcus. The most prominent feature in this view is thelongitudinal fissure, which is a deep groove that separates the two cerebral hemispheresfrom each other. Observe the many sulci on the brain surface. One major sulcus to identifyis the central sulcus. Identification of the central sulcus is difficult, though not impossible,on a real human brain. The following are two features to look for:

The precentral gyrus and the postcentral gyrus (two raised areas approximately in themiddle of the brain’s superior surface) should become continuous with each other on thelateral aspect of the central sulcus just above the lateral sulcus (a groove that separates thetemporal lobe from the frontal and parietal lobes). This means the central sulcus will notenter the lateral sulcus.

The central sulcus will dip down into the longitudinal fissure.

Three of the five lobes of the cerebrum are visible on the superior surface view.

2. Identify the structures listed in figure 15.6 on the superior view of the brain, using table 15.3 and


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the textbook as guides. Then label them in figure 15.6.

Figure 15.6Superior View of the Brain.(a) Classroom model of the brain. (b) Preserved human brain. Use the termslisted to fill in the numbered labels in the figure.

central sulcus

frontal lobe

longitudinal fissure

occipital lobe

parietal lobe

postcentral gyrus

precentral gyrus

Page 385EXERCISE 15.5LATERAL VIEW OF THE HUMAN BRAIN

1. Obtain a human brain or models of a human brain and observe the lateral surface.

As with the superior view, the central sulcus should be visible by identifying the locationwhere the pre-and postcentral gyri become continuous with each other just above thelateral sulcus.

2. Identify the structures listed in figure 15.7 on the lateral view of the brain, using table 15.3 andthe textbook as guides. Then label them in figure 15.7.


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Figure 15.7Lateral View of the Brain.Use the terms listed to fill in the numbered labels in the figure.

central sulcus

cerebellum

frontal lobe

lateral sulcus

medulla oblongata

occipital lobe

parietal lobe

pons

postcentral gyrus

precentral gyrus

temporal lobe

transverse fissure

3. Optional Activity: 7: Nervous System—Watch the “Divisions of Brain” animation for anoverview of the regions of the brain and their general functions.


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Divisions of Brain

Page 386EXERCISE 15.6INFERIOR VIEW OF THE HUMAN BRAIN

1. Obtain a human brain or models of a human brain and observe the inferior surface. This view isconsiderably more complicated than the superior or lateral views because of the cranial nervesthat arise from the brain. Cranial nerve identification is covered later in this chapter.

Both the brainstem and the cerebellum are visible from this view.

Prominent features associated with the cerebrum are the optic chiasm and the optic tracts,which extend from it into the brain.

The mammillary bodies are two small projections posterior to the optic chiasm.

One of the more problematic structures to identify in this view on a real brain is theinfundibulum. When a brain is removed from the cranium, the pituitary gland almostalways gets removed from the brain. The only structure left connected to the brain is thestalk of tissue that connects the pituitary gland to the hypothalamus, which is theinfundibulum, or pituitary stalk. Observation of a model of the brain shows the pituitarygland to be intact. The infundibulum can be identified as a small strand of tissue that islocated directly posterior to the optic chiasm and directly anterior to the mammillary bodies.

2. Identify the structures listed in figure 15.8 on the inferior view of the brain, using table 15.3 andthe textbook as guides. Then label them in figure 15.8.

Figure 15.8Inferior View of the Brain.Use the terms listed to fill in the numbered labels in the figure.


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cerebellum

frontal lobe

infundibulum

mammillary bodies

medulla oblongata

midbrain

occipital lobe

olfactory bulb

olfactory tract

optic chiasm

optic nerve

optic tract

pons

temporal lobe

Page 387EXERCISE 15.7MIDSAGITTAL VIEW OF THE HUMAN BRAIN

1. Obtain a human brain or brain model that has been sectioned along the midsagittal plane andobserve its medial surface (figure 15.9).


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Figure 15.9Midsagittal View of the Brain. Use the terms listed to fill in the numbered labels in the figure.

central sulcus

cerebellum

cerebral aqueduct

cerebral peduncle

cingulate gyrus

corpus callosum

fourth ventricle

frontal lobe

hypothalamus

infundibulum

interthalamic adhesion

mammillary body

medulla oblongata


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midbrain

occipital lobe

parietal lobe

parieto-occipital sulcus

pineal body (gland)

pons

septum pellucidum

tectal plate (corpora quadrigemina)

temporal lobe

thalamus

third ventricle

In the very center of view, notice the third ventricle. The third ventricle is the centraldepressed area that appears to have a cut nerve in the center. The “cut nerve” isn’t really anerve, but it is similar. It is a fiber tract called the interthalamic adhesion (or intermediatemass), which connects the two thalamic nuclei to each other. Use the interthalamicadhesion, the thalamus, and the third ventricle as reference points for identification of otherstructures in this view.

Many of the structures that are located around the third ventricle of the brain belong to asystem called the limbic system (limbus, border), so named because structures of the limbicsystem are located at the border of the third ventricle and the brainstem. The limbic systemis referred to as the “emotional brain” because its structures play a role in our emotions.

2. Identify the structures listed in figure 15.9 on the midsagittal view of the brain, using table 15.3and the textbook as guides. Then label them in figure 15.9.

Page 388Cranial Nerves

An inferior view of the brain allows for visualization of the cranial nerves at the location where theyarise from the brain. Cranial nerves are numbered, starting from the anterior (rostral) part of the brainand moving posterior (caudal), using Roman numerals I through XII. Figure 15.10 shows the inferiorsurface of the brain and the cranial nerves. The olfactory bulbs (where the olfactory nerves (CN I)synapse) and the optic nerves (CN II) are very large, easily identifiable structures located on the inferiorsurface of the frontal lobes of the cerebrum. The remainder of the cranial nerves (CN III through CNXII) are generally smaller and are located closer together in the region of the midbrain, pons, andmedulla that form the brainstem, thus making their identification a little more challenging. Exercise 15.7will involve identifying the cranial nerves on a brain or on a model of the brain.


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EXERCISE 15.8IDENTIFICATION OF CRANIAL NERVES ON A BRAIN OR BRAINSTEM MODEL

1. Obtain a human brain or a model of a human brain or brainstem.

2. Turn the brain over and observe its inferior surface (figure 15.10). Note all of the small nervesexiting the brain from various locations. These are the cranial nerves (table 15.4).

Figure 15.10Cranial Nerves on the Inferior Surface of the Brain.

Table 15.4Names of Cranial Nerves

NerveNumber

Name Foramina of Exit Word Origin

I Olfactory Olfactory foramina in the cribiform plateof the ethmoid bone

olfacio, to smell


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II Optic Optic canal optikos, relating to theeye or vision

III Oculomotor Superior orbital fissure oculo-, the eye, +motorius, moving

IV Trochlear Superior orbital fissure trochileia, a pulley

V Trigeminal Superior orbital fissure (V1—ophthalmic)Foramen rotundum (V2—maxillary)Foramen ovale (V3—mandibular)

tri-, three, + geminus,twins

VI Abducens Superior orbital fissure abductio-, to moveaway from the medianplane

VII Facial Internal acoustic meatus (exits via thestylomastoid foramen)

facialis, relating to theface

VIII VestibulocochlearInternal acoustic meatus vestibulum, entrance,+ cochlea, snail shell

IX GlossopharyngealJugular foramen glossus, tongue, +pharyngeus, pharynx

X Vagus Jugular foramen vagus, wanderer

XI Accessory Jugular foramen (accessory division)Foramen magnum (spinal division)

spina, spine, +accessory, an extrastructure

XII Hypoglossal Hypoglossal canal hypo, beneath, +glossus, tongue

3. Identify the twelve cranial nerves on the inferior surface of the brain, using table 15.4 and figure15.10 as guides.

4. Complete the chart for cranial nerves III through XII by listing the specific nerves that extendfrom each of the three areas of the brainstem. Use figure 15.10 as a guide. Keep in mind thatknowledge of the general area where a specific nerve emerges can be helpful in identifying the


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nerves, even if by process of elimination.

Point of Exit Cranial Nerve

Midbrain

Pons

Medulla Oblongata

Page 390

5. Mnemonic devices, simple phrases or plays on words that aid in memory recall, makeremembering long strings of anatomical names easier. One popular technique is to associate aword with the first letter of the corresponding word you want to remember. A mnemonic that willhelp you recall the names of the cranial nerves in numerical order is

Oh Once One Takes The Anatomy Final, Very Good Vacations Are Heavenly.

Mnemonics work best when they mean something to you. Develop your own mnemonic forremembering the cranial nerves.

6. Optional Activity: 7: Nervous System—Visit the quiz area for cranial nerve location,composition, and related foramina.

LEARNING STRATEGY

When observing a real brain instead of a model, the trochlear nerve (CN IV) may not be visible. It isvery small, and often detaches from the brain when the brain is removed from the skull. If this is thecase, make sure to identify the trochlear nerve on a model of the human brain. This nerve is uniquebecause it is the only cranial nerve that originates from the posterior part of the brainstem, rather thanthe anterior or medial part.

The Sheep Brain

The following exercises involve identifying on a sheep brain many of the same structures identified on ahuman brain. Sheep brains share many similarities with human brains, and are readily available forlaboratory studies. The experience of dissecting a real brain (as opposed to observing a plastic model ofa brain) will give an appreciation for the true appearance and texture of the brain and its associatedstructures. Note that the brain tissue itself is relatively delicate. Even so, the tissue is much more solidthan it would be if observing a fresh brain because it has been fixed with chemicals. Living brain tissues


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are extremely delicate, and have the consistency of firm gelatin. These dissection exercises involveidentifying both brain structures and cranial nerves on the sheep brain. Observation of the structure andfunction of cranial nerves and identification of them on a human brain comes later in this chapter.

EXERCISE 15.9SHEEP BRAIN DISSECTIONEXERCISE 15.9A Dura Mater

1. Obtain a sheep brain, dissecting tray, and dissecting tools (forceps, scissors, a scalpel,and gloves). Place the sheep brain in the dissecting tray and take turns with a laboratory partner(s)observing its gross structure. This exercise requires a sheep brain with dura mater intact. If thedura mater is missing, proceed to the section “Inferior View of the Sheep Brain” (p. 392).Otherwise begin here.

2. Using hands or blunt forceps, feel the toughness of the dura mater. Notice how it surrounds theentire brain. What type of tissue is the dura mater composed of?

3. Observe the dura mater on the superior surface of the brain (figure 15.11b), and locate thefollowing structures:

Figure 15.11Anatomical Landmarks of the Sheep Brain.

confluence of sinuses


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transverse sinuses

Page 391

4. Rotate the brain so it is resting on its superior surface and the inferior surface is visible (figure15.11a). Notice the relatively large pituitary gland projecting inferior to the dura mater. The goalin this part of the dissection will be to cut away the dura mater without disconnecting the pituitaryfrom the rest of the brain. Notice the capillary tufts found just posterior and lateral to the pituitarygland. Just lateral to these capillaries on both sides are the large trigeminal nerves (CN V). Usinga blunt probe, feel the dura mater surrounding the base of the pituitary gland. This duralmembrane is the diaphragma sellae, which lies between the pituitary and the rest of the brain(except where the pituitary stalk exits the sella turcica of the sphenoid bone).

5. To free the connections between the dura and the rest of the brain without breaking off thepituitary gland, first cut around the trigeminal nerves and capillary tufts. Figure 15.12a showswhere to make the initial incision. Cut around (lateral to) the optic chiasm, diaphragma sellae,pituitary gland, and trigeminal nerves to make a complete circle, which will free the dura materfrom its attachments.

6. Next, make an anterior cut in the dura mater along the midsagittal plane between the olfactorybulbs and olfactory tracts (figure 15.12b). Once the dura mater has been freed from itsconnections to the diaphragma sellae, gently pull the dura away from the brain. Pull in a posterior,superior direction so the falx cerebri and tentorium cerebelli slip out of their respective fissureswithout damaging the delicate brain tissues. When pulling the dura mater away from the brain,gently tease away any remaining connections. Figure 15.12b shows what the dura mater shouldlook like after it has been cut away and removed from the brain.

7. Once the dura mater is completely freed from the brain, observe it closely and compare the duralsepta and sinuses in the sheep brain to those identified in the human brain. What dural septa ismissing in a sheep brain that is present in a human brain?


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Figure 15.12Incisions to Remove the Dura Mater from the Sheep Brain.(a) Initial incision in the dura mater.(b) Appearance of the dura mater after it has been cut away from the brain.

Page 392EXERCISE 15.9B Inferior View of the Sheep Brain

1. Obtain a sheep brain without the dura mater intact, or use the brain from which the dura mater hasbeen removed. Place it in the dissecting pan on its superior surface so the inferior surface isvisible (figure 15.13).


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Figure 15.13Inferior View of the Sheep Brain.The dura mater surrounding the pituitary gland has not yet beenremoved.

2. When sheep brains are collected by a commercial vendor for use in the laboratory, the dura materis first separated from the cranial bones. In such specimens, most of the dura mater has beendissected away from the cranium and the only part remaining is the diaphragma sellae, amembrane between the sella turcica and the rest of the brain (see figure 15.12a). Surrounding thediaphragma sellae are some capillary tufts and large cranial nerves, the trigeminal nerves (CN V).

Page 393

3. Identify the following on the sheep brain, using figure 15.13 as a guide:

capillary tufts

diaphragma sellae

olfactory bulb

olfactory tract


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optic chiasm

pituitary gland

trigeminal nerves (CN V)

4. Next, dissect the diaphragma sellae and the capillary tufts away from the pituitary gland withoutdamaging the cranial nerves, without detaching the pituitary from the infundibulum, and withoutdetaching the trigeminal nerves from the brain. Dissect carefully, because it is very easy toaccidentally detach these structures from the brain if there is too much tension on the diaphragmasellae while attempting its removal.

5. Gently lift the dura mater posterior to the pituitary gland to see the small nerves that enter the duramater on its deep surface (figure 15.14).

Figure 15.14Cranial Nerves Entering the Dura Mater.The abducens and trigeminal nerves can be seen exitingthe inferior surface of the sheep brain and piercing the dura mater.

6. Using scissors or a scalpel, detach the nerves where they enter the dura mater. Cut the nerveswhere they attach to the dura (not where they attach to the brain!) and then cut the dura and bonymaterial away, removing as much of it as possible while keeping the pituitary intact. Be careful,because the connection between the pituitary and the rest of the brain is delicate.

Page 394

7. Identify the following structures in the inferior view of the sheep brain, using figure 15.15a as aguide.


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Figure 15.15Inferior Views of the Sheep Brain.

cerebellum

cerebral peduncle

frontal lobe


medulla oblongata

olfactory bulb

olfactory tract

optic chiasm


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optic nerve (CN II)

pituitary gland

pons

spinal cord

temporal lobe

transverse fissure

Page 395

8. Gently lift the pituitary to observe the mammillary body (figure 15.15b). Note that the sheepbrain has only a single mammillary body, whereas the human brain has two.

9. Finally, attempt to identify the cranial nerves listed in figure 15.15b. Identification of cranialnerves IX (glossopharyngeal nerve) through XII (hypoglossal nerve) may not be possible becausethese nerves are very small and may have been damaged or torn off the brain as the dura materwas removed from the brain, or as the brain was removed from the cranium.

Page 396EXERCISE 15.9C Superior View of the Sheep Brain

1. Place the brain in the dissecting tray with the inferior side facing down (figure 15.16a). Note thethin, transparent arachnoid mater that covers the entire surface of the brain without dipping intothe sulci (grooves) between the gyri (folds) of the brain. Note the numerous blood vessels that liebetween the arachnoid mater and the pia mater. The space occupied by the blood vessels is also aspace where cerebrospinal fluid flows in the living animal. What is the name of this space?


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Figure 15.16Superior and Posterior Views of the Sheep Brain.(a) Superior view of the sheep brain. (b)Posterior view; cerebral hemispheres are pulled away from the cerebellum to reveal deeperstructures.


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2. Identify the following structures in the superior view of the sheep brain, using figure 15.16a as aguide.

arachnoid mater

blood vessels

cerebellum

cerebrum

gyrus


spinal cord

sulcus

transverse fissure

3. Pick up the brain and gently pull the cerebellum away from the cerebrum so the transverse fissureis visible. Identify the following structures, using figure 15.16b as a guide:

cerebellum

cerebrum

inferior colliculus

pineal gland

superior colliculus

EXERCISE 15.9D Midsagittal and Coronal Sections of the Sheep Brain

1. Some lab members may perform a midsagittal section of the sheep brain; others will perform acoronal section. Ask the instructor which section to make before initiating a cut. Be sure toobserve a brain that has been cut along a midsagittal plane and a brain cut along a coronal plane.

2. Midsagittal Section: Place the sheep brain in a dissecting tray with its superior surface facing up.Using a scalpel, cut the brain in half along the midsagittal plane. Start a cut on the anterior end ofthe brain by placing the scalpel blade within the longitudinal fissure. What is the first structure thescalpel blade will cut through?

3. Once the brain has been cut in half, observe its medial surface. Identify the following structureson the sheep brain, using figure 15.17 as a guide.


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Figure 15.17Midsagittal View of the Sheep Brain.

central canal (of spinal cord)

cerebellum

cerebral aqueduct

cerebral peduncle

cerebrum

corpus callosum

fornix

fourth ventricle

mammillary body

medulla oblongata

optic chiasm

pineal gland

pituitary gland

pons

spinal cord

superior colliculus

thalamus


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4. Coronal Section: Place the sheep brain in a dissecting tray with the superior surface down, andidentify the pituitary gland (or the pituitary stalk if the pituitary gland has been removed). Using ascalpel, cut the brain in half along a coronal plane that travels through the pituitary gland andcontinues toward the cerebral hemispheres.

5. Once the brain has been cut in half, observe the cut surface. Identify the following structures onthe sheep brain, using figure 15.18 as a guide:

Figure 15.18Coronal Section Through the Sheep Brain.

cerebral cortex

cerebral peduncle

choroid plexus

corona radiata

corpus callosum

fornix

hypothalamus

internal capsule

lateral ventricle


pons

thalamus

third ventricle


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6. The corpus callosum, fornix, internal capsule, corona radiata, and cerebral peduncles are allfiber tracts. Recall from chapter 14 that a tract is a bundle of myelinated axons. What is thefunction of these fiber tracts?

7. Using forceps, open the lateral ventricles a bit to find the choroid plexus in the wall of theventricle. Upon gross observation, the choroid plexus kind of looks like “junk” inside theventricle, but it is really a tuft of capillaries covered by ependymal cells.

What two structures make up the choroid plexus?

What is the function of the choroid plexus?

8. When the dissection has been completed, collect all the organic material from the dissecting panand dispose of it in the proper containers (ask the laboratory instructor what these are). Dispose ofthe scalpel blades in the sharps container, and throw used paper towels and gloves into thegarbage. Clean all dissecting tools and the dissecting pan and return them to the proper storagearea, and disinfect the laboratory workstation.


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