Walle J. H. Nauta, "The Gross Anatomy of the Human Brain” - Brain Dissection Course, MIT Department of Psychology
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NAUTA: I am Dr. Walle Nauta. We have now arrived at the third tape in our brain dissection course, in which we will be dealing with the cranial nerves.
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The same ventral aspect of the brain that gave us such a good view of the great arterial vessels supplying the brain-- that same ventral surface-- will also show almost the entire contingent of cranial nerves leaving or entering the brain. Let us try to identify some of these cranial nerves.
Here again, it is a bit of a problem to do it before the arachnoid has been removed. But if we postpone it until after the arachnoid has been removed, we risk losing the more fragile rootlets of some cranial nerves. That is why a preliminary survey of the cranial nerves is most usefully done at this stage.
Let us have a look at the cranial nerves that we can see upon inspecting the ventral aspect of the brain. It will be necessary to look for these cranial nerves in three different brains. It is quite rare to find all of them intact and optimally demonstrable in a single brain-- so you would understand if, from time to time, the specimen is going to be changed.
Starting at the caudal end of the brain, at the spinomedullary junction, we note that many cranial nerves make their exit or enter in the most posterior parts of the brainstem and the rhombencephalon. The most important orienting line is this border line here between medulla oblongata and the pontine protuberance.
Then we start at that line, the so-called pontomedullary sulcus-- we can count three great cranial nerves. The one closest to the midline is here, this is the abducens nerve. Somewhat farther out laterally, emerging through the same deep groove between the pons and the medulla oblongata, is number VII, the facial nerve.
And finally, if I were to start laterally, here is a double nerve that is actually composed of the cochlear nerve with the vestibular nerve. I have it here, and I hope it is somewhat visible. It is this double nerve here, number VIII, the nervus octavus or statoacusticus. VI, VII, and VIII, abducens, facial, and statoacusticus here.
If you now go in a straight line down from that grove here, we encounter-- close to the midline-- a cranial nerve that is characterized by appearing on the surface in quite a large series of little fascicles of rootlets here. This is cranial nerve number XII, the hypoglossal nerve.
The hypoglossal nerve, as you can rather clearly see in this specimen, emerges from just underneath this olive-shaped-- this overt bulge here, the so-called inferior olive. On its medial side here runs the pyramidal tract. It is through that groove between the olivary bulge and the pyramid, that the hypoglossal nerve makes its exit from the brainstem. It is a pure motor nerve that innervates the tongue.
A little bit farther laterally here, through the side of the medulla oblongata and taking their exit through a line quite a bit dorsal to the olivary prominence here, are two further nerves: IX, the glossopharyngeus here, and its immediate caudal neighbor-- the somewhat larger nerve-- that we call nervus vagus, IX and X here. Let me count the nerves once more here, name them by number.
Here in the pontomedullary grove from medial to lateral: the abduncens nerve here, the facial nerve-- number VII-- and this double nerve, the statoacoustic nerve-- composed of the cochlear and the vestibular nerve-- here. Just behind that VIII nerve emerges cranial nerve number VIIII, the glossopharyngeus.
And immediately in line with it and like it, emerging through the lateral surface of the medulla oblongata, is the vagus, number X. Here is number XII-- and you may now ask, where is number XI? Where is the 11th cranial nerve?
The 11th cranial nerve is a most devious one. It is actually, to the largest extent at least, a spinal cord nerve. However, in its course, it enters the cranial cavity through the foramen magnum, and it emerges in the company of the glossopharyngeus and the vagus through a common opening in the base of the cranial cavity, the so-called jugular foramen.
IX, the glossopharyngeus, X, the vagus, and XI, the accessory nerve here, emerge together from the cranial cavity through the jugular foramen. You see here, only a short stretch of the accessory nerve. I must pull the vertical artery a little bit toward the medial side to show it.
This nerve, here running alongside the spinal cord really, is not in the typical position of a ventral root of the spinal cord-- here is a ventral root of the spinal cord-- nor does it correspond in its line of exit to a dorsal root of the spinal cord. It follows an intermediate position here, emerging from the side of the upper five to six spinal cord segments. These rootlets of the accessory nerve join each other into this longitudinal strand here, that then quite anomalously-- quite atypical for a spinal cord nerve here-- enters the cranial cavity through the foramen magnum.
Now I'll just quickly review these cranial nerves again. Through the pontomedullary sulcus emerge cranial nerve VI-- the abducens-- VII-- the facial-- VIII-- the statoacoustic nerve here. Going straight back from the nerve number VIII-- the statoacoustic-- emerge the cranial nerves, IX, the glossopharyngeus here, and immediately associated with it on the caudal side, the somewhat larger cranial nerve number X, the vagus.
Cranial nerve number XI originates from the spinal cord and joins IX and X from the caudal side to leave the cranium through the jugular foramen. Here, as the only nerve to emerge from below the olive, the hypoglossal nerve with its many rootlets here, that come together to form the quite substantial motor nerve innervating the tongue musculature.
This small territory here of the upper medulla oblongata and to lower border of the pons, really is the site of exit of no less than 1, 2, 3, 4, 5, 6, 7 cranial nerves. By comparison, the other parts of the brain surface are really much more sparsely supplied by cranial nerves. When I say supplied, I should have said sparsely used as exit or entrance by cranial nerves.
However, the largest cranial nerve-- number V, the trigeminus-- is seen here in a slightly more forward position entering the brain through the brachium pontis, or the middle cerrebellar peduncle here.
The trigeminus is very largely a sensory nerve, but it has a small motor component that sometimes forms a separate little rootlet here on the ventral and medial side. In this particular case, I can not really say that I could identify it with any certainty. It could be this one, but I am not sure at all that that is true.
The fifth cranial nerve, a most remarkable nerve that breaks really all the rules of nerve exit. Here is the cranial nerve number IV, the trochlear nerve. The trochlear nerve originates from the caudal part of the midbrain. It emerges immediately behind the inferior colliculus. Through a very anomalous site-- namely the dorsal surface-- there are no other motor nerves known to leave either the brainstem or the spinal cord through the dorsal surface. It is really quite unique to the trochlear nerve.
And then to add an other peculiarity, at its exit, it immediately crosses over to the opposite side, so that the trochlear nerve of the left side innervates the superior oblique muscle of the right eye. Let us see if we can find the trochlear nerve.
It is deeply hidden here in the depths of the transverse fissure. I can show it already, where it emerges from around the cerebellum. It is here. I might even turn this back to its original position, and here is the trochlear nerve. But its position at the moment is quite deceptive. It does not emerge through the ventral surface of the brainstem, it comes from the dorsal surface. And let us see if we can follow it there and identify its point of origin.
It's a bit risky to pull off the arachnoid under which it is covered here, but there it is. And here we see this remarkable cranial nerve as it comes from the dorsal surface of the midbrain here, runs circumferentially over the lateral surface of the midbrain. And here appears, on the ventral side, the trochlear nerve, number IV.
The trochlear nerve innervates one of the external eye muscles. Another one, the lateral rectus muscle, is innervated by the abducens nerve. The trochlear nerve innervates a single external eye muscle, and that is the superior oblique muscle.
Then the remainder of the external eye muscles-- the rectus superior, the rectus medialis, the rectus inferior, the obliquus inferior, and the levator palpebrae superioris-- are all innervated by a single cranial nerve, the oculomotor.
The oculomotor nerve, unfortunately in this specimen, is lacking. It undoubtedly must have [INAUDIBLE] when the brain was lifted up from inside the cranial cavity. And we will have to identify it in another specimen. If I may ask for another specimen please. Thank you.
It is here. The oculomotor nerve is really, as you can see here, an unexpectedly large nerve. For so little volume of striated musculature as it has to innervate, its size is truly remarkable. It all indicates that the innervation ratio-- nerve fiber versus number of striated muscle fibers-- is really quite high here.
Here, once more, is the trochlear nerve emerging from around the midbrain here. Here is the fifth cranial nerve. Here is the oculomotor nerve emerging just rostral to the pons from the depths of the interpeduncular fossa here, and making its exit characteristically between the superior cerebellar artery immediately behind it and the posterior cerebral artery immediately in front of it.
This is really the typical position of the oculomotor nerve at its exit, almost pinched between the two end branches of the basilar artery-- the superior cerebellar artery and the posterior cerebral artery here.
What now remains in the way of cranial nerves are two cranial nerves that were counted as number I and II, although later it became obvious that they are not really nerves in the strict sense of the word. The two nerves in question are cranial nerves I and II. We find here the structure that was described in original anatomical dissection, as the first cranial nerve.
This is the olfactory bulb. This is the olfactory tract. And here is its attachment to the ventral surface of the brain, a region known as the olfactory trigone here. You can see the triangular shape of the area there. The olfactory tract inserts itself to the base of the brain.
This then used to be called cranial nerve number I. We now know that it is not a nerve in the sense that it develops as part of the central nervous system, and not in the form of nerve fibers growing out or growing toward the central nervous system.
Cranial nerve number II is the optic nerve that we see here, going through its hemidecussation-- immediately in front of the pituitary stalk here there's a hemidecussation between the right and the left optic nerve-- to continue into the optic tract here. This too was counted as a nerve, although since that early time, it has come to be recognized that this is not really a cranial nerve, but part of the central nervous system.
The retina and the optic nerve are really outpouchings. They are everted portions of the central nervous system itself-- forwarded outposts of the brain itself, rather than being a true nerve.
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It may be useful at this juncture-- now that we have seen the external appearance of some of the cranial nerves as they make their exit from the brain or enter into the brain-- it might be useful to review briefly the appearance of these same cranial nerves and their nuclei-- their motor and sensory nuclei-- in microscopic slides. We'll do this very briefly. It can of course not substitute for an accurate examination of the slides in total.
This first level that you see here is typical for the medulla oblongata-- the caudal half or so of the rhombencephalon. We see here at the very base of the brain, the pyramidal tract, close to the midline. Lateral to it, the remarkable configuration of the inferior olivary nucleus that is responsible for the bulge that we saw in inspecting the ventral surface of the brain.
And we see, as we proceed along the lateral surface, we see that we come to a point that there appears to be a deep valley. That deep valley is actually the floor of the fourth ventricle. The choroid membrane that forms the roof of the fourth ventricle cannot be seen here, because the staining technique is unable to bring it out.
Our main point at this level is to show the relationship of the hypoglossal nerve that emerges below the olive. You see it here, emerging through the grove between the pyramidal tract on the medial side and the olivary prominence on the lateral side. We see it's not a nucleus-- the nucleus from which the hypoglossal nerve originates, close to the midline, immediately below the floor of the fourth ventricle.
If we compare that medial exit, that is really a very typical position for a pure motor nerve to take its exit-- close to the ventral midline, in the same manner that characterizes the motor or ventral root in the spinal cord-- if you compare it with the exit point of a mixed nerve, in this particular case, the vagus nerve.
The vagus nerve, as you can see here, takes a far more lateral exit. It is almost in the position within the spinal cord we would call dorsal root. And it's cell groups of origin are several. Instead of one single motor nucleus, the vagus nerve has no less than two motor nuclei. The main one is lateral to the hypoglossal nucleus in the floor of the fourth ventricle. It is known as the dorsal motor nucleus of the vagus.
A second motor nucleus is situated more deeply hidden in the substance of the brainstem tegmentum dorsal to the olive, approximately at this position. And from these two sources, axons unite with each other and together take their exit through the lateral dorsal aspect of the medulla oblongata, in that characteristic position in which mixed cranial nerves exit from the brain.
The vagus nerve-- and much like the glossopharyngeus, its immediate anterior neighbor-- in addition, contains sensory fibers. These come in at the same point, and then they proceed inside the brainstem to the so-called solitary tract, which is shown here. And from here on, the distribution of impulses to the nucleus of the solitary tract-- and hence to central stations higher up in the brainstem-- they'll begin.
We see here, in other words-- very clearly illustrated at this level-- the differential exit zone of a pure somatic motor nerve-- the hypoglossus-- with a mixed nerve that is partly motor, partly sensory-- the nervus vagus.
On the next slide, we see the characteristic entrance into the brain of a pure sensory nerve. The sensory nerve that is shown here is the VIII nerve, in particular, that component that we call the cochlear nerve, because it collects its impulses from the spinal ganglion, which in turn picks up its impulses from the hair cells in Corti's organ.
We see the nerve here coming in far lateral into a lateral and dorsal zone of the cross section. We see its two recipient secondary sensory nuclei: the ventral cochlear nucleus and the dorsal cochlear nucleus. And from here on then, the path will continue as a lemniscal pathway that will ascend, ultimately, to the inferior colliculus.
This level marks exactly the pontomedullary groove that we saw a moment ago in inspecting the ventral surface of the gross brain. If you see here, the characteristic sequence of nerves taking their exit from the ventral surface closest to the midline is the abducens nerve-- a pure motor nerve. And hence, it should not surprise us to see it take its exit so close to the ventral midline. It too could be compared to a ventral root in the spinal cord.
As we proceed laterally from here, the next nerve to be encountered is number VII. It's clearly shown in this preparation, taking a more lateral exit from the brain. The seventh nerve is partly motor-- is largely motor even-- but it contains sensory fibers. As a mixed nerve it-- just like the vagus nerve and the glossopharyngeus-- takes its exit or entrance in a more lateral part of the brainstem circumference.
The most lateral nerve to be seen here is the vestibular nerve. It is part of the eighth cranial nerve. On the interval between the facial nerve and the purely sensory vestibular nerve, you see an extra little nerve. This extra little nerve is known as the nervus intermedius of Wrisberg.
This nerve actually belongs to the complex of the facial nerve. It is composed of its autonomic motor fibers and its sensory fibers. Together, these fibers will-- once they have left the confines of the cranium-- they will appear as the chorda tympani in the periphery.
Recapitulating, at this level we find-- as the medial element to emerge-- a pure somatic motor nerve-- the nervus abducens-- a mixed nerve-- the nervus facialis-- with its sensory component here as separately shown, and then in the farthest lateral position, a purely sensory nerve-- the vestibular.
This level is included in this survey because it shows the peculiar mode of exit of the trochlear nerve. We are here at the level, just at the caudal border of the midbrain. This region of the brainstem is sometimes known as the isthmus-- the narrowest part of the brainstem. One sees here only one cranial nerve emerging, and quite anomalously, it emerges from the dorsal midline of the first crossing through a thin plate that forms the roof of the isthmus of the cerebral aqueduct.
This thin roof is known as the anterior medullary velum. We see the fiber material of the left and right trochlear nerves crossing through it before the nerve emerges as a peripheral nerve from the dorsal surface of this level of the brainstem.
This level of the midbrain corresponds to the inferior colliculus. One sees the inferior colliculus as a prominent structure forming the roof of the midbrain. Below it, and somewhat between the left and right inferior colliculus, appears the cerebral aqueduct. Surrounding it is a ring of gray matter, the central gray substance.
Immediately below it, close to the midline, appears the medial longitudinal fasciculus. And nestled in its dorsal surface, is the motor nucleus of the trochlear nerve. The motor nucleus in other words, as you can now understand, lies a little rostral to the point of exit of the trochlear nerve.
At this highest level of the brainstem, we are in the rostral part of the midbrain, and we see much the same configuration that we saw in the preceding slide, except that the prominent structure on the dorsal surface is no longer the inferior colliculus. We are now at the level of the superior colliculus.
Surrounding the aqueduct, here again, we see the central gray substance. Close to the midline, along the ventral circumference of the central gray substance-- the medial longitudinal fasciculus. And the somewhat light staining zone, immediately to the medial side of it, is the motor nucleus of the oculomotor nerve.
We see emerging from that motor nucleus, numerous rootlets. We can see them almost fanning out through the midline portion-- the medial portion-- of the midbrain tegmentum. And throughout the ventral surface, we can see how many such little rootlets combine to form the exit rootlets of the oculomotor nerve that emerges between the cerebral peduncles through the interpeduncular fossa.
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Hello, I'm Dr. Walle Nauta, and I will be-- you might say-- your tour guide through this series on the gross anatomy of the human brain. The series consists of 15 parts of rather different lengths. In a very quick survey, these various parts or chapters can be listed as follows. You will see from this listing that the first six parts or chapters of the series will not include any actual dissection in the literal sense of cutting asunder.
Instead, the first two parts deal with the bony and membranous envelopes of the brain, parts III and IV on tape 2, with the brain's blood vessels, and parts V and VI on tape 3, with the cranial nerves entering and leaving the brain.
It is only in part VII on tape 4 that we will start removing such surface structures on our way into the brain itself, beginning at the cerebral cortex and proceeding from there-- in tape 5, part VIII-- into the deeper reaches of the cerebral hemisphere.
The separate tape 6, part IX, is focused on the temporal lobe. In parts X on tape 7, and parts XI and XII on tape 8, we will deal with the white matter of the cerebral hemisphere, the complexity of which will make it necessary to include in part XII, the survey of some histological sections stained for myelin.
Continuing on tape 8, part XIII will revisit the arterial circle of Willis at the base of the brain and expose the various subdivisions of the olfactory system. In part XIV on tape 9, we will begin our descent from the forebrain to the midbrain, a descent to be continued in the 15th, and last part, on tape 10 with the dissection of the hindbrain and cerebellum.
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The dura has, in a word, its own arterial blood supply. These blood vessels are understandably called the meningeal arteries. The most prominent among them is the middle meningeal artery, which we see here with several branches distributed over the various parts of the dural cap.
We can see that this midline duplicature almost subdivides the cranial cavity into a left and a right half. Obviously, it has to have a relatively wide opening here to permit the passage of connections between the left and the right hemisphere.
We see that here, this very deep incisure between the spinal cord and brainstem on the one hand, and the lower surface of the cerebellum on the other hand, the arachnoid stretches over, and thereby comes to form the cover for a very deep and large cistern that is of great practical importance. This is the so-called cerebellomedullary cistern, also called the cisterna magna.
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Now this vein is often known as the posterior anastomosing vein of Labbé. And one sees that the way in which the vein's laid out on the surface of the cortex virtually ensures that there are multiple possibilities for directional change of the blood flow. Then a vein would become obstructed somewhere in this area, let us say-- the blood flow would have no difficulty finding an alternate route into the middle superficial vein here and through Labbé's vein into the transverse sinus.
We can see here the curve taken by the anterior cerebral artery coming from this point here-- the curve around the genu of the corpus callosum-- and from there on moving caudally over the surface of the corpus callosum. We'll open this now. We see here in the depth, the corpus callosum here. And with a little bit of inconvenience, we can see here one of the two anterior cerebral arteries lying on the dorsal surface of the callosum.
And here laterally, the middle cerebral artery lying on the surface of the insular-- quite far away from the brain surface and communicating with it only through secondary vessels coming off of the lips of the opercula here. And finally, the posterior cerebral artery here, that is quite well-hidden in the groove between the medial surface of the occipital and temporal lobes here on the one hand and the midbrain on the other hand.
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Starting at the caudal end of the brain, at the spinomedullary junction, we note that many cranial nerves make their exit or enter in the most posterior parts of the brainstem and the rhombencephalon. The largest cranial nerve-- number V, the trigeminus-- is seen here in a slightly more forward position, entering the brain through the brachium pontis or the middle cerebellar peduncle here.
We are in the rostral part of the midbrain, and we see much the same configuration that we saw in the preceding slide, except that the prominent structure on the dorsal surface is no longer the inferior colliculus-- we are now at the level of the superior colliculus.
Surrounding the aqueduct, here again, we see the central gray substance. Close to the midline, along the ventral circumference of the central gray substance-- the medial longitudinal fasciculus. And the somewhat light staining zone, immediately to the medial side of it, is the motor nucleus of the oculomotor nerve.
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Here, this gyrus is called the supramarginal gyrus. And another one, riding over the top of this long sulcus here, that runs the length of the temporal lobe and is known as the superior temporal sulcus. We see a similar convolution riding over the top of that superior temporal sulcus, and this is often called the angular gyrus-- gyrus angularis.
This fusion of the two sulci, of the two fissures, is deep enough to cause a marked prominence in the medial wall of the occipital horn of the lateral ventricle, as we will see a bit later on. This prominence is known as the calcar avis-- the bird's spur.
We see that the hippocampal gyrus is here, deeply incised by the fusion of the parieto-occipital and the calcarine fissures. And This leads to a marked narrowing of the hippocampal gyrus, known as the isthmus of the gyrus fornicatus-- isthmus gyri-fornicati. We see it here in the depths.
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I will take two of these slices of cortical tissue in sequence, and then we'll stop for a moment to look at the contrast between the gray matter of the cortex and the white matter.
And at this point, you will see how the cingulate fasciculus reaches the anterior border of the corpus callosum, which is exactly here. This is the so-called genu, and you can see here how the fascicular cinguli bends over the genu to disappear from view. It distributes itself there in the subcallosal component of the medial cortex.
These chordate veins-- you see here how you can follow them into the collecting channel. The name of this vein is the thalamostriate vein. It used to be called the terminal vein. At this point, immediately behind Monro's foramen, it makes a sharp curve medialward here and disappears.
This curve here, immediately behind the interventricular foramen-- this curving vein is known as the venous angle, and it serves the neuroradiologist as an orientation for the most likely position of the interventricular foramen.
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At this level, we are approaching the anterior limit of the pineal gland-- cella media left and right. Here is the cistern of the velum interpositum and two internal cerebral veins would be positioned about here, in the process of coming together to form the great vein of Galen.
You remember that the temporal lobe had been detached by a series of cuts. One such cut involving the corpus callosum and the fornix bundle-- this is the fornix bundle here-- and the other one, a very deep incision into the lateral side of the temporal lobe-- deep enough to penetrate into the temporal horn. The gyrus dentatus is really the ultimate end of the cortical mantle.
Between Ammon's horn and the dentate gyrus, a very complicated topography exists. We could perhaps best sum it up by saying that the hippocampus essentially consists of two cortical gyri. It is not the same type of cortex that we would find on most of-- the complexity of the brain for example here, this cortex we would call neocortex. This cortex at the extreme medial edge-- at the medial margin of the cortical mantle-- is of a different composition, and it is often referred to as archicortex-- the arch cortex.
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At this point, one hopes that the split-brain will be a favorable one. It is a rather forcible pulling one must do here. And as you see, here the insular appears, now freed-- at least over its dorsal part here-- now freed from the overhanging wall of the parietal and frontal operculum here. One sees that the insular is composed of three or four gyri that are arranged in a fan shape.
But it is not too difficult to imagine that this more posterior part of the optic radiation here would find its ultimate distribution in the cortex surrounding the calcarine fissure, which can easily be recognized here. Part of this radiation, in other words, goes to the cuneus here-- this area dorsal to the calcarine fissure. And the other part, the more ventral part, goes to the lingual gyrus, which we see appearing here. This is the lingual gyrus.
Here is the fan here. In this part, it's called the internal capsule. In that part, where it lies now free off the lentiform nucleus, here is the corona radiata with these different radiations, including the optic radiation here in the most caudal part.
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I will make a somewhat random cut here, trying to stay as much as I can parallel to the fibers of the optic radiation. When we do that and now open up here, it is quite clear that we are looking into a lateral ventricle. We see choroid plexus appear here. I can easily guess that what I am looking at here is the calcar avis.
We see here then, once more, the carotid artery-- this of the left side, that one of the right side. Let us quickly get oriented in the basal aspect of the diencephalon. Here are the optic nerves undergoing their hemidecussation. This is the tuber cinereum with the pituitary stock invariably here disconnected from the pituitary that remained inside the skull underneath the diaphragm of the sella turcica.
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Here is the so-called massa intermedia, a bridge of gray matter interconnecting the thalamus of the left side with the thalamus of the right side. All this is the third ventricle. Here is its communication with the lateral ventricle and here its communication with the fourth ventricle here. This is the Sylvian aqueduct.
When we look into the markedly dilated third ventricle here, it is clear that in this case, there is a well-developed massa intermedia. The massa intermedia is not always present. It appears to be missing in approximately 10% of humans. But you see it here, it is nothing very voluminous, but it is an interesting detail of thalamic anatomy.
I now turn the brain on its side, and at this point we are beginning to see the lateral surface of the midbrain. We see here, looking at the thalamus once more from behind, here is the inferior colliculus here, the anterior colliculus here, the thalamus, the lateral geniculate body--
This overhanging large triangular area of the thalamus, that lies in the vertex of the curve of the thalamus, is the so-called pulvinar. The name pulvinar meant pillow, and apparently those who saw the structure for the first time were impressed with it. It reminded them of a pillow slung over the back of a chair. And here we see it, the pulvinar.
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I will cut the midbrain here at the level between the superior and inferior colliculi. Obviously, this cut would have to involve the basilar artery. Needless to say, what we will find here just lateral to the thalamus is the tail of the caudate nucleus here. And quite in general, we see and are reminded rather clearly of the fact that this whole region here marks the roof of the temporal horn of the ventricle.
We will now turn our attention to the posterior part of the brainstem-- the rhombencephalon with the cerebellum. You can clearly see here the typical configuration of the dentate nucleus. It is composed of a strongly folded membrane of-- it's a sheet of gray matter here.
And out from its hilus emerge the myelinated fibers of the brachium conjunctivum. Here, the brachium conjunctivum has left the cerebellum over this stretch. Here, it disappears under the roof of the midbrain tectum here, and at this point we see once more the complete decussation of the cistern. The brachium conjunctivum, or anterior cerebellar peduncle, is a completely closed cistern.