Section VII—Brain and Cranial Nerves
Meninges
In comparative anatomy courses, the brain and cranial nerves of a sheep, rather than of a cat, are usually studied. There are two main reasons for this. First, the cat brain is small and many of its structures, particularly the nerves, are extremely difficult to find.
Second, the brain of the cat would have to be carefully removed from the skull, which requires patience and the proper tools. To avoid these problems, the mammalian brain should be studied using a prepared sheep brain. Illustrations of the sheep (Figures 7.73-7.77) are provided here, as well of the cat (Figures 7.78 and 7.79). Compare the structures in the cat in these illustrations with those you observe in the sheep brain.The brain is surrounded by three membranes, or meninges (sing., meninx), which protect and isolate it (Figure 7.73). The thickest is the dura mater. It fuses with the periosteum of the bones enclosing the cranial cavity and is usually left on the skull when the brain is removed, although a ventral portion of the dura mater, covering the hypophysis, is present in most specimens. If your specimen still has the dura mater adhering to it, carefully remove it, but keep its ventral and postero- ventral portions. Some specimens that preserve the dura mater also preserve the branching of the trigeminal nerve (see below). In lateral view, this is a large, Y- shaped, horizontal structure lateral to the hypophysis. If present, sever the nerve just before it branches.
The other meningeal layers are the pia mater, a thin layer adhering to the surface of the brain, and the arachnoid mater, lying between the dura and pia. The arachnoid largely remains with the dura mater in the cranial cavity of the skull.
Telencephalon
The telencephalon (Figure 7.77) is greatly expanded in mammals, due mainly to the enormous size of the cerebrum. Indeed, the largest parts of the brain are the cerebral hemispheres, which lie dorsally and together
Figure 7.73 Right lateral view (anterior to right of page) of the sheep brain showing meninges.
The dura mater has been partially cut and reflected to reveal deeper meninges. Blowup shows the pin passing deep to the arachnoid mater.comprise the cerebrum. The cerebellum (part of the metencephalon; see below) is another prominent part of the brain, and lies posterior to the cerebrum (Figure 7.74). The surface of the cerebrum bears numerous folds, the gyri (sing., gyrus), separated by grooves, the sulci (sing., sulcus). Right and left cerebral hemispheres are separted by the deep longitudinal cerebral fissure (Figure 7.75). Gently spread the hemispheres to reveal the corpus callosum, a white structure consisting of fibers that connect the hemispheres.
The elongated, flattened olfactory bulbs lie anteroven- trally on the telencephalon (Figures 7.74 and 7.76). A
FIGURE 7.74 Left lateral view of the sheep brain and cranial nerves, with dura mater and arachnoid mater removed.
FIGURE 7.75 Dorsal view of the sheep brain and cranial nerves, with dura mater and arachnoid mater removed.
FIGURE 7.76 Ventral view of the sheep brain, with dura mater and arachnoid mater removed.
band of fibers, the olfactory tract, continues posteriorly from the olfactory bulb and almost immediately separates into lateral and medial olfactory striae. The medial olfactory stria extends posteromedially, whereas the lateral olfactory stria extends posterolaterally. Each stria is accompanied by a gyrus, and only the lateral olfactory stria is easily discernable. It continues posteriorly into the piriform lobe, which is separated from the rest of the cerebrum by the rhinal sulcus (Figure 7.74).
Diencephalon
The greatly expanded cerebrum covers the roof of the diencephalon, so that without dissection only the hypothalamus, or floor of the diencephalon, may be clearly observed (Figure 7.76). The optic chiasm, at the anterior end of the hypothalamus, represents a partial decussation of the optic nerves. Just posterior to the optic chiasm, a thin, delicate stalk, the infundibulum, suspends the hypophysis (Figure 7.77). In prepared specimens, the hypophysis and infundibulum may be missing. Remove them if present in your specimen. The opening for the infundibulum is a continuation of the third ventricle (see below). The area of the hypothalamus adjacent to the opening is the tuber cinereum (Figure 7.76). Immediately posterior to the tuber are the paired mammillary bodies (which may appear as a single rounded structure), which mark the posterior end of the hypothalamaus.
The dorsal part of the diencephalon is formed by the epithalamus. To see it, observe the brain in dorsal view, spread apart the cerebral hemispheres, and make a longitudinal cut through the corpus callosum. The epithalamus lies mainly ventral to the latter’s posterior
FIGURE 7.77 Sagittal section of sheep brain.
FIGURE 7.78 Left lateral view of the cat brain and cranial nerves, with dura mater and arachnoid mater removed.
FIGURE 7.79 Ventral view of the cat brain and cranial nerves, with dura mater and arachnoid mater removed.
discernable in sagittal section (Figure 7.77). The rounded pineal gland lies posterior to the commissure.
portion. Remove its roof, the tela choroidea, to reveal the third ventricle, the narrow cavity of the diencephalon. The slightly thickened posterolateral margin of the ventricle forms the habenula on each side.
These margins converge posteriorly toward the midline, and form the habenular commissure, which is more readily The walls of the thalamus, the lateral portions of the diencephalon, are mainly lateral to the dorsal margins of the third ventricle, but their extent is difficult to appreciate. Part of the thalamus, the intermediate mass (see below), extends across the third ventricle to connect the left thalamus and right thalamus (Figure 7.77). You may have cut through it during exposure of the third ventricle. The positional relationships of these structures will be examined again in sagittal view.Mesencephalon
The tectum, or roof, of the mesencephalon was partially exposed during dissection of the third ventricle. To see it more completely, spread the cerebral hemispheres from each other, as well as from the cerebellum. The tectum is characterized by two paired, prominent swellings that together form the corpora quadrigemina, which sit on the lamina quadrigemina (the latter will be seen in sagittal section). The two anterior swellings, the superior colliculi, are the larger, well-rounded structures that lie just posterior to the pineal gland. The inferior colliculi are the smaller, flattened structures that protrude from the posterior end of the superior colliculi. The trochlear nerves arise from the dorsal surface of the mesencephalon just posterior to the inferior colliculi.
Examine the brain in ventral view (Figure 7.76). The floor of the mesencephalon is formed by the cerebral peduncles, slightly elevated regions lateral and posterior to the mammillary bodies. The relatively wide and flattened oculomotor nerve arises from the surface of each cerebral peduncle.
Metencephalon
The metencephalon consists mainly of two regions. Dor- sally is the cerebellum (Figures 7.74 and 7.75). Its surface is highly folded into folia, which are separated by sulci. The median part of the cerebellum is the vermis. The cerebellar hemispheres lie on either side of the vermis.
A flocculonodular lobe may be distinguished on the ventrolateral part of each hemisphere. On the ventral surface, the metencephalon consists of the pons, a rounded region of transverse fibers posterior to the cerebral peduncles. On each side, the trigeminal nerve arises laterally from the posterolateral part of the pons. Distally the trigeminal has three main branches: the ophthalmic, maxillary, and mandibular nerves, which are often respectively abbreviated as V1, V2, and V3 (Table 7.4).Myelencephalon
The myelencephalon consists of the medulla oblongata, which forms the brain posterior to the metencephalon and connects to the spinal cord (Figures 7.76 and 7.77). The ventral fissure is the median ventral groove of the medulla. To either side are narrow longitudinal bands termed pyramids. Lateral to the anterior part of each pyramid, and just posterior to the pons, lies a trapezoid body.
The remaining cranial nerves are mainly associated with the medulla oblongata, but some of them may be difficult to discern if the meninges have been stripped. Thus, identify the stumps of these nerves before removing the meninges. The nerves to be identified are the abducens, facial, vestibulocochlear, glossopharyngeal, vagus, accessory, and hypoglossal nerves (Figures 7.74, 7.76, and 7.80).
An abducens nerve arises from the anterior part of the medulla, between the trapezoid body and pyramid. The facial nerve arises lateral to the trapezoid body just posterior to the trigeminal nerve, and the vesticulocochlear nerve arises slightly more dorsally, from beneath the flocculonodular lobe. The glossopharyngeal, vagus, and accessory nerves arise in sequence and more posteriorly from the lateral surface of the medulla. The hypoglossal nerve arises further posteriorly and ventrally. If the meninges have been stripped from the medulla, however, it will be extremely difficult to identify these nerves.
One method of exposing the nerves is to make a mid- ventral cut through the meninges using fine scissors.
Begin posteriorly and work your way forward. When you reach the level of the abducens nerves, carefully reflect the meninges. As you peel the meninges back, look for a series of fine rootlets arising from the ventrolateral surface of the posterior part of the medulla. These rootlets merge to form the hypoglossal nerve. Separate it from the meninges and continue peeling the latter. Soon you will note the glossopharyngeal, vagus, and accessory nerves. The accessory will probably be the most prominent at first, because it forms a longitudinal nerve adjacent to the surface of the medulla. Follow it forward, and you will find the stumps of the three nerves arising very close to each other. The glossopharyngeal and vagus arise by a series of small rootlets, so they may not be readily discernable. The accessory nerve arises as a series of very fine rootlets along the posterior part of the medulla. Posterior to it, your specimen may preserve the 1st spinal nerve. Its origin appears similar to that of the hypoglossal nerve.Examine the dorsal surface of the medulla. A good part of its roof is covered by a tela choroidea. Remove it to expose the fourth ventricle, the cavity within the medulla that continues forward under the cerebellum. This part of the roof of the fourth ventricle is covered by a separate membranous structure, the medullary velum.
Sagittal Section of the Brain
With a new scalpel blade, make a sagittal section through the brain by extending the section made earlier to expose the third ventricle. Carefully continue the
FIGURE 7.80 Cranial nerves and the structures they innervate.
section ventrally and posteriorly using the pineal body, superior colliculi, and middle of the vermis as guides to the sagittal plane of the brain.
Examine one of the halves in sagittal view (Figure 7.77) and note the great extension of the cerebrum. The corpus callosum (see also above), the structure allowing communication between the cerebral hemispheres, is also clearly revealed. Its anterior, curved part is the genu, its posterior part the splenium, and between them dorsally is the trunk of the corpus callosum. The fornix curves anteroventrally from near the splenium. The septum pellucidum is a thin, vertical, shiny partition between the corpus callosum and fornix. Most sagittal sections do not coincide precisely with the sagittal plane, and so the septum remains on one or the other half of the brain. If it is absent on the half you are examining, you will see a cavity, the lateral ventricle of the cerebral hemisphere; check the other half for the septum. You may break through the septum into the other lateral ventricle.
Below the ventral part of the fornix is the anterior commissure, a small, rounded structure representing a group of fibers. The lamina terminalis, forming the anterior wall of the third ventricle, extends ventrally from the anterior commissure to the optic chiasma, which is oval in section. The third ventricle lies mainly ventral to the fornix and splenium of the corpus callosum. Note that it is narrow but dorsoventrally high. The circular massa intermedia extends across the third ventricle and connects the thalami that form the right and left sides of the diencephalon. Each lateral ventricle communicates with the third ventricle through the interventricular foramen, located just anterior to the massa intermedia. Identify the mammillary body ventrally on the hypothalamus and, more posteriorly, the cerebral peduncle and pons.
Identify the pineal body and the habenular commissure in this section. Note the posterior commissure extending between the pineal body and the lamina quadrigemina (mentioned above) of the mesencephalon. Posterior to the massa intermedia, the third ventricle communicates with the cerebral aqueduct, a narrow passageway that extends posteriorly beneath the lamina quadrigemina and then into the fourth ventricle. The latter, in turn, extends back into the central canal of the spinal cord.
Examine the section through the cerebellum and observe how the white matter, the arbor vitae, branches into the surrounding gray matter. The roof of the fourth ventricle is formed by the medullary velum anteriorly and the tela choroidea posteriorly, but these structures were probably removed during exposure of the fourth ventricle. Note how the cerebellum attaches to the rest of the brain on each side of the fourth ventricle via a series
TABLE 7.5 Cranial nerves and associated foramina of the sheep
*The main part of the facial nerve passes through the inner ear and middle ear before emerging from the stylomastoid foramen.
**As the vestibulocochlear supplies structures of the inner ear, it does not leave the skull.
of fiber tracts or peduncles. Make a frontal section through the peduncles to remove the cerebellum. The posterior curved part of the section consists of the posterior cerebellar peduncle. Anteriorly there are two peduncles. The middle cerebellar peduncle is slightly larger than and lateral to the anterior cerebellar peduncle.
Cranial Nerves
The stumps of the 12 cranial nerves have already been identified during the dissection of the brain, but their peripheral distribution cannot easily be followed. Figure 7.80 provides a summary of the cranial nerves, their origin from the brain, and their peripheral distribution. Table 7.5 indicates the foramina through which the cranial nerves leave the skull. Review the names and numbering sequence for the nerves, as well as the foramina through which they pass.
Key Terms: Brain and Cranial Nerves
abducens nerve accessory nerve anterior commissure arachnoid mater arbor vitae
central canal
cerebellar hemispheres
cerebellar peduncle, anterior, middle, and posterior
cerebellum cerebral aqueduct
(aqueduct of Sylvius) cerebral hemispheres cerebral peduncle cerebrum corpora quadrigemina corpus callosum diencephalon dura mater epithalamus facial nerve flocculonodular lobe folia fornix genu glossopharyngeal nerve gyrus (plur., gyri) habenula habenular commissure habenular commisure hypoglossal nerve hypophysis hypothalamus inferior colliculi infundibulum intermediate mass interventricular foramen
(foramen of Monro) lamina quadrigemina lamina terminalis lateral olfactory stria lateral ventricle longitudinal cerebral fissure mammillary body intermediate mass medial olfactory stria medulla oblongata medullary velum
meninx (plur.,
meninges) metencephalon myelencephalon oculomotor nerve olfactory bulbs olfactory tract optic chiasma optic nerves pia mater pineal body pineal gland piriform lobe pons posterior commissure pyramids rhinal sulcus septum pellucidum splenium sulcus (plur., sulci) superior colliculi tectum tela choroidea of diencephalon tela choroidea of myelencephalon telencephalon thalamus third ventricle trapezoid body trigeminal nerve trochlear nerves trunk, of corpus
callosum tuber cinereum vagus nerve ventral fissure vermis vestibulocochlear nerve
(auditory nerve, octaval nerve)