Mini-atlas of the Marmoset Brain http://marmoset-brain.org Aya Senoo Tokyo University of Agriculture and Technology Hironobu Tokuno Tokyo Metropolitan Institute of Medical Science Charles Watson Curtin University Version 1.3 Dec 8, 2014 All rights reserved
CEREBRAL CORTEX occipital lobe visual CEREBELLUM deep cerebellar nuclei 4V solitary nucleus 12N sensory trigeminal RHOMBENCEPHALON pyramidal decussation 2
Level P 4.0 The hindbrain at the level of the pyramidal decussation This section shows the caudal end of the hindbrain, just before it joins the spinal cord. The hindbrain consists of the rhombencephalon and a small segment called the isthmus which joins the rhombencephalon to the mesencephalon. The cerebellum is also part of the hindbrain, because it grows out of the rhombencephalon and the isthum. The cerebellum lies between the rhombencephalon and the occipital pole of the cerebrum. In the center of the hindbrain is the prominent crossing of the pyramidal tract (pyramidal decussation). The fibers in the pyramidal tract arise in the cerebral and at this level they cross the midline to reach the opposite side of the spinal cord. The large trigeminal nuclei, which receive touch, pain, and temperature sensations from the face, are found in the lateral part of the hindbrain. This part of the trigeminal sensory complex is called the spinal trigeminal nucleus because it extends into the cervical spinal cord. Most of the remainder of the green area in this section is occupied by the reticular nuclei, which extend the whole length of the hindbrain. The large solitary nucleus lies in the floor of the fourth ventricle (4V). It receives taste and other visceral sensations from the head and internal organs of the thorax and abdomen. Just below is the hypoglossal nucleus (12N) that sends motor fibers to the tongue. The ventricular system is represented here by the caudal end of the fourth ventricle. Cerebrospinal fluid (CSF) can escape from the ventricular system via small holes in the roof of the fourth ventricle to reach the subarachnoid space. The cerebellum, which is very large in monkeys and other primates, lies dorsal to the fourth ventricle. In developmental terms, the cerebellum grows out from the roof of the hindbrain. It consists of an outer layer of cerebellar and a core of white matter (fibers). The deep cerebellar nuclei are embedded in the white matter of the cerebellum. The occipital pole of the cerebrum sits above the cerebellum. The primary visual lies on the medial side of the occipital pole. 3
visual CEREBRAL CORTEX occipital lobe CEREBELLUM 4V deep cerebellar nuclei vestibular nuclei sensory trigeminal RHOMBENCEPHALON icp sp5 cochlear nuclei inferior olive pyramidal tract 4
Level P2.5 The hindbrain at the level of the vestibular and cochlear nuclei This section shows the hindbrain (rhombencephalon) below and the cerebellum and cerebrum above. The large trigeminal nuclei are found in the lateral part of the hindbrain. The large vestibular nuclei lie in the floor of the fourth ventricle (4V) and the cochlear (auditory) nuclei are lateral to the vestibular nuclei. The inferior cerebellar peduncle (icp) lies between the vestibular and cochlear nuclei. It contains fibers that travel from the spinal cord and inferior olive to the cerebellum. Below the vestibular nuclei is the sensory trigeminal nucleus. This part of trigeminal sensory complex is called the spinal trigeminal nucleus and the trigeminal nerve fibers lateral to it are called the spinal trigeminal tract (sp5). A large fiber bundle, the pyramidal tract, lies on either side of the midline on the ventral margin of the hindbrain. The fibers in the pyramidal tracts arise in the cerebral and travel down to cross to the opposite side of the spinal cord. Above each pyramidal tract is the inferior olive, which is functionally connected with the cerebellum of the opposite side. Most of the remainder of the green area of the hindbrain in this section is occupied by the reticular nuclei, which extend for the whole length of the hindbrain. The fourth ventricle contains cerebrospinal fluid (CSF), which has flowed down from its origin in the lateral (cerebral) ventricles through the third ventricle and aqueduct to reach the hindbrain. The cerebellum is concerned with coordination of movement. It consists of an outer layer of cerebellar and a core of white matter (fibers). The deep cerebellar nuclei are embedded in the white matter of the cerebellum. The deep cerebellar nuclei receive input from the cerebellar and send fibers to the brainstem and thalamus. The occipital pole of the cerebrum sits above the cerebellum. The primary visual lies on the medial side of the occipital lobe. 5
CEREBRAL CORTEX occipital lobe visual LV CEREBELLUM medial lemniscus 4V sensory trigeminal scp vestibular nuclei RHOMBENCEPHALON Amb mcp icp sp5 cochlear nuclei inferior olive pyramidal tract 6
Level P 1.5 The hindbrain at the level of the middle of the fourth ventricle This section shows the hindbrain at the level of the middle of the fourth ventricle. The trigeminal nuclei (the spinal trigeminal nuclei) are found in the lateral part of the hindbrain. Lateral to the trigeminal nuclei are the sensory fibers of the trigeminal nerve, called the spinal trigeminal tract (sp5). A large fiber bundle, the pyramidal tract, lies on either side of the midline on the ventral margin of the hindbrain. Above each pyramidal tract is the inferior olive, which is functionally connected with the cerebellum of the opposite side via the inferior cerebellar peduncle (icp). Medial to the inferior olive is the medial lemniscus, which is a fiber pathway carrying touch and position sense information from the hindbrain and spinal cord to the thalamus. Above the lateral edge of the inferior olive is a small motor nucleus which supplies the muscles of the pharynx, larynx, and palate. It is called the ambiguus nucleus (Amb). Most of the remainder of the green area of the hindbrain in this section is occupied by the reticular nuclei, which extend for the whole length of the hindbrain. The vestibular nuclei lie at the junction of cerebellum and hindbrain in the floor of the fourth ventricle. They receive information from the position sense organs of the inner ear. At the lateral edge of the hindbrain under the cerebellum are the cochlear nuclei, which receive auditory sensations from hearing organ of the inner ear. Lateral and dorsal to the vestibular nuclei are three large fiber bundles that connect with the cerebellum. The fibers of the middle cerebellar pedunclem (mcp) arise in the basilar pontine nuclei and travel to the cerebellar. The fibers of the inferior cerebellar peduncle (icp) arise in the inferior olive and spinal cord and project to the cerebellar. The fibers of the superior cerebellar peduncle (scp) arise in the deep cerebellar nuclei and travel to the red nucleus and the thalamus. As in the previous two levels, the cerebellum lies between the rhombencephalon and the occipital lobe of the cerebrum, which contains the primary visual and the caudal tip of the lateral ventricle (LV). 7
CEREBRAL CORTEX occipital lobe dcw LV superior colliculus (visual) MESENCEPHALON inferior colliculus (auditory) cochlear nuclei vestibular nuclei 7N CEREBELLUM 4V 6N sensory tigeminal scp RHOMBENCEPHALON parabrachial nuclei mcp s5 inferior olive medial lemniscus pyramidal tract 8
Level P 0.5 The hindbrain at the level of the abducens and facial nuclei This section shows the hindbrain at the level of two notable motor nuclei - the abducens (6N) and the facial (7N) nuclei. The abducens nucleus lies below the floor of the fourth ventricle; it supplies the lateral rectus muscle of the eye. The facial nucleus lies ventral to the sensory trigeminal nucleus; it supplies the muscles of facial expression. The sensory trigeminal nucleus is found in the lateral part of the hindbrain. At this level we see the principal trigeminal nucleus, closely applied to the sensory root of the trigeminal nerve (s5). Lateral to the trigeminal sensory nuclei are the cochlear nuclei. A large fiber bundle, the pyramidal tract, lies on either side of the midline on the ventral margin of the hindbrain. Above each pyramidal tract is the inferior olive, which is functionally connected with the cerebellum. Medial to the inferior olive is the medial lemniscus, which is a fiber pathway carrying touch and position sense information from the hindbrain and spinal cord to the thalamus. Most of the remainder of the green area of the hindbrain in this section is occupied by the reticular nuclei. Lateral to the trigeminal nuclei are the cochlear nuclei. The vestibular nuclei lie lateral to the abducens nucleus (6N). Above the vestibular nuclei is the superior cerebellar peduncle (scp) which is surrounded by the parabrachial nuclei. Lateral to the superior cerebellar peduncle is the middle cerebellar peduncle (mcp). Dorsal to the fourth ventricle (4V) is the central part of the cerebellum. The cerebellum protrudes laterally to lie dorsal to the vestibular and cochlear nuclei. Above the cerebellum, the dorsal parts of the mesencephalon (midbrain) can be seen. These are the superior and inferior colliculi. Above the mesencephalon and the cerebellum is the occipital lobe of the cerebrum, which in this section consists of the cerebral, deep cerebral white matter (dcw), and the lateral (cerebral) ventricle (LV). 9
CEREBRAL CORTEX superior colliculus (visual) MESENCEPHALON periaqueductal gray ReIC inferior colliculus (auditory) cornu ammonis LV 4n 4V scp vestibular nuclei 5N RHOMBENCEPHALON parabrachial nuclei mcp CEREBELLUM 5n superior olive medial lemniscus pyramidal tract 10
Level A 0.5 The hindbrain at the level of the superior olive Because this section is obliquely cut through the brain stem, both the hindbrain (below) and the midbrain (above) can be seen in the same section. The rostral part of the fourth ventricle lies between the vestibular nuclei of the hindbrain below and the periaqueductal gray of the midbrain above. Just above the pyramidal tract is the medial lemniscus, a fiber bundle carrying touch information from the body and face to the thalamus. Lateral to the pyramidal tract and the medial lemniscus is the superior olive, which is a processing center for auditory information coming from the cochlear nuclei. Dorsal to the superior olive is the motor trigeminal nucleus (5N), which controls the chewing (masticatory) muscles. The trigeminal nerve itself (5n) attaches to the hindbrain at this level. Lateral to the motor trigeminal nucleus is the middle cerebellar peduncle (mcp). The most dorsal part of the hindbrain in this section contains the vestibular nuclei medially and the superior cerebellar peduncle (scp) and parabrachial nuclei laterally. The parabrachial nuclei receive input from the solitary nuclei, which are concerned with taste and visceral sensation. The constricted rostral end of the fourth ventricle (4V) is largely enclosed by the vestibular nuclei. In the roof of the fourth ventricle, the trochlear nerve (4n) of each side crosses the midline. A small rostral part of the cerebellum likes lateral to the hindbrain at this level. In this section the midbrain consists mainly of the superior colliculus, the inferior colliculus, and the periaqueductal gray. In the center of the midbrain is the aqueduct, a small canal along which cerebrospinal fluid (CSF) flows from the ventricles of the forebrain to the fourth ventricle of the hindbrain. The aqueduct is surrounded by a thick layer of cells called the periaqueductal gray. Each superior colliculus receives visual sensory information from the opposite eye. Each inferior colliculus receives auditory information from the superior olive of both sides. Within the cerebrum we can see the lateral ventricle (LV) and the most caudal part of the hippocampus, called the cornu ammonis. The remaining parts of the hippocampus can be seen in the sections rostral to this level. 11
CEREBRAL CORTEX cingulate corpus callosum LV superior colliculus (visual) ISTHMUS aqueduct 4N MESENCEPHALON periaqueductal gray scp pulvinar fi subiculum dentate gyrus cornu ammonis RHOMBENCEPHALON mcp lateral lemniscus pyramidal tract medial lemniscus 12
Level A 2.0 The forebrain at the level of the caudal end of the corpus callosum. The caudal end of the corpus callosum is present in this section. This very large fiber sheet connects symmetrical parts of the two cerebral hemispheres. Above the corpus callosum is the cingulate. Below the corpus callosum, this section shows both the hindbrain and midbrain. It also reveals the first appearance of the diencephalon, represented by the caudal tip of the pulvinar of the thalamus. Within the cerebrum, we see the caudalmost part of the hippocampus, represented by the dentate gyrus, the cornu ammonis, and the subiculum. The hippocampus is a part of the cerebral that is vital for memory registration. The fibres projecting out of the hippocampus are called the fimbria of the fornix (fi). Lateral to the fimbria is the lateral ventricle (LV). As in more caudal sections, the pyramidal tract is found next to the ventral midline of the hindbrain (rhombencephalon). The small rostral part of the superior olive is seen lateral to the pyaramidal tract, and the lateral lemniscus (which consists of fibers connecting the superior olive to the inferior colliculus) can be seen above the superior olive. The medial lemniscus between the pyramidal tract and the lateral lemniscus; it is a large fiber bundle carrying touch information from the body and face to the thalamus. In the center of the brain stem is the trochlear nucleus (4N), which supplies the superior oblique muscle of the eye. This nucleus is an important landmark for the isthmus, a thin strip of rostral hindbrain that separates the rhombencephalon from the mesencephalon. The territory of the isthmus is often mistakenly included in the mesencephalon. Below and to the side of the trochlear nucleus is the superior cerebellar peduncle (scp). In this section the midbrain consists mainly of the superior colliculus and the periaqueductal gray. In the center of the midbrain is the aqueduct, a small canal along which cerebrospinal fluid (CSF) flows from the ventricles of the forebrain to the fourth ventricle of the hindbrain. The aqueduct is surrounded by a thick layer of cells called the periaqueductal gray. 13
CEREBRAL CORTEX parietal lobe cingulate corpus callosum 3V fi caudate DIENCEPHALON somatosensory thalamus red nucleus 3V 3N VTA VPM & VPL medial geniculate ml cp substantia nigra dentate gyrus lateral geniculate caudate LV temporal lobe basilar pons interpeduncular nucleus subiculum cornu ammonis 14
Level A 4.5 The forebrain at the level of the lateral geniculate nucleus This section shows the junction of the midbrain (below) with the diencephalon above. The main features of the midbrain at this level are the oculomotor nucleus (3N), the red nucleus, the substantia nigra, and the VTA (ventral tegmental area). Each oculomotor nucleus (3N) supplies four of the six muscles that move the eye. In addition, the oculomotor nerve supplies the main muscle of the upper eyelid and the muscles of the pupil and lens. Between the oculomotor nucleus and the medial lemniscus (ml) is the red nucleus. This large group of cells gives rise to a fiber bundle that travels down the spinal cord, called the rubrospinal tract. At this level, the pyramidal (corticospinal) fibers and other descending fibers form a bundle called the cerebral peduncle on the ventral aspect of the midbrain. At this level the pyramidal tract is embedded in the cerebral peduncle (cp). Coating the inside (medial) border of the fibers of the cerebral peduncle (cp) is the substantia nigra, a large cell group that is important for motor control. A large group of substantia nigra neurons contain dopamine; they project to the caudate and putamen in the subpallial part of the cerebrum. On the medial side of the medial lemniscus is another group of dopaminergic neurons, the ventral tegmental area (VTA); these neurons project to the accumbens nucleus in the subpallium. Medial to the VTA is the midline interpeduncular nucleus. The diencephalon at this level mostly consists of thalamic nuclei. The somatosensory thalamic nuclei receive touch and proprioception information from the medial lemniscus. They are called the ventroposterior medial (VPM) and ventroposterior lateral (VPL) nuclei. The thalamic medial geniculate nucleus receives auditory information from the inferior colliculus. Lateral to the medial geniculate is the huge lateral geniculate nucleus which receives input from the eye. The lateral geniculate is so large in primates that it lies lateral to the main part of the thalamus. Each thalamic nucleus projects to a specific part of the cerebral : VPM and VPL project to the somaotosensory ; the medial geniculate projects to the auditory ; and the lateral geniculate projects to the visual. Below and lateral to the lateral geniculate nucleus is the temporal lobe of the cerebrum. At this level it contains the main parts of the hippocampus (dentate gyrus, cornu ammonis, and subiculum), the inferior horn of the lateral ventricle (LV), and the tail of the caudate nucleus. The body of the caudate nucleus is seen above the lateral part of the diencephalon, lateral to the body of the lateral ventricle. Because the caudate nucleus is C-shaped, it is possible to cut it twice in the same section. The ventricle of the diencephalon is the third ventricle (3V), which at this level is connected to the body of the lateral ventricle by the interventricular foramen. The cerebral at this level is represented by the temporal lobe (including the hippocampus), the parietal lobe, and the cingulate. 15
CEREBRAL CORTEX cingulate lateral fissure ec corpus callosum 3V 3V LV fi thalamus HYPOTHALAMUS VMH fornix caudate internal capsule insular putamen SUBPALLIUM globus pallidus opt caudate somatosensory auditory claustrum dentate gyrus cornu ammonis amygdala subiculum temporal lobe entorhinal 16
Level A 7.5 The forebrain at the level of the middle of the hypothalamus This level shows all of the major elements of the forebrain - the diencephalon, the hypothalamus, the subpallium (caudate, putamen, globus pallidus, and amygdala), and the cerebral. Separating the thalamus from the cerebrum is the internal capsule, a very large sheet of fibers that connects the cerebral with the thalamus, brainstem and spinal cord. The caudal end of the internal capsule forms the cerebral peduncle, which contains the pyramidal tract. Between the internal capsule and the insular are the globus pallidus, putamen, and claustrum. The caudate nucleus is C shaped and appears twice in this section, once above the thalamus, and again below the putamen. The caudate, putamen, and globus pallidus can be referred to collectively as the basal ganglia. They are involved in motor control, particularly semi-automatic movements and locomotion. The diencephalon in this section is mainly represented by the thalamus. The hypothalamus lies ventral to the thalamus. The ventromedial hypothalamic nucleus (VMH) and the fornix are prominent landmarks in the hypothalamus. Between the hypothalamus and the amygdala is the optic tract (opt), a bundle of fibers running from the optic chiasm to the dorsal lateral geniculate nucleus of the thalamus. Part of the third ventricle (3V) is in the midline in the hypothalamus; the dorsal recess of the third ventricle lies above the thalamus. It is separated from the lateral ventricle (LV) by the fimbria. The lateral surface of the cerebral seen in this section contains the somatosensory (touch and proprioception). Below the somatosensory is the lateral fissure, which encloses the insular (taste and visceral sensation). On the lower lip of the lateral fissure is the auditory. The part of the cerebrum below the lateral fissure is called the temporal lobe. The medial half of the temporal lobe at this level is occupied by the hippocampus, which is made up of the dentate gyrus, cornu ammonis, subiculum, and entorhinal. Lateral to the hippocampus is the caudal end of the amygdala. 17
motor CEREBRAL CORTEX cingulate lateral fissure bed nucleus of stria terminalis corpus callosum septum anterior comissure 3V LV preoptic area opt caudate SUBPALLIUM ic putamen globus pallidus ec somatosensory insular auditory amygdala claustrum temporal lobe entorhinal 18
Level A 9.5 The forebrain at the level of the anterior commissure The anterior commissure lies immediately in front of the rostral pole of the thalamus. The anterior commissure is a large bundle of crossing fibers, which connects the olfactory bulb and parts of the temporal lobe with symmetrical areas on the opposite side. The anterior commissure is in the center of this section, with the septum above, the preoptic area below, and the bed nucleus of the stria terminalis applied to its dorsal and ventral margins. In the midline of the preoptic region is the third ventricle (3V). Below the lateral part of the preoptic area is the very large optic tract, indicating that the optic chiasm must be just rostral to this section. The thin layer of white matter lateral to the putamen is called the external capsule. The lateral ventricle lies between the septum and the caudate nucleus in this section. Between the insular and the lateral ventricle (LV) lie a number of components of the subpallium - the claustrum, putamen, globus pallidus and caudate nucleus. Between the caudate nucleus and the putamen and globus pallidus is the rostral part of the internal capsule (ic). The internal capsule is a large sheet of fibers that connects the cerebrum with the thalamus, brainstem, and spinal cord. The superolateral surface of the cerebral seen in this section contains the somatosensory and the motor. Below the somatosensory is the lateral fissure, which encloses the insular (taste and visceral sensation). On the lower lip of the lateral fissure is the auditory. The part of the cerebrum below the lateral fissure is called the temporal lobe. The medial half of the temporal lobe at this level is occupied by the amygdala. Just ventral to the amygdala is an outlying part of the hippocampus, called the entorhinal. The medial surface of the cerebrum, above the corpus callosum, is represented by the cingulate. 19
CEREBRAL CORTEX frontal lobe ic cingulate corpus callosum rcc LV dcw caudate SUBPALLIUM putamen nucleus accumbens ac ec claustrum olfactory tubercle lateral olfactory tract 20
Level A 12.5 The forebrain at the rostral end of the corpus callosum This section passes through the frontal lobe of the cerebrum at the level rostral end of the corpus callosum. This part of the corpus callosum is folded back on itself (see sagittal sections) to form the rostrum of the corpus callosum (rcc). The cingulate lies dorsal to the corpus callosum. The rostral part of the lateral ventricle (LV) separates the rostrum of the corpus callosum from the head of the caudate nucleus. The caudate nucleus is separated from the putamen by the internal capsule (ic). The caudate and putamen are continuous with the nucleus accumbens ventrally. The accumbens is continuous with the olfactory tubercle.together, these four entities (caudate, putamen, accumbens, olfactory tubercle) form the largest of the subpallial groups, called the striatum. Lateral to the putamen is the claustrum, which is separated from the putamen by the external capsule. The anterior limb of the anterior commissure (ac) is prominent bundle of fibers between the olfactory tubercle and the accumbens. Lateral to the olfactory tubercle is the lateral olfactory tract. 21
CEREBRAL CORTEX frontal lobe cingulate deep cerebral white matter (dcw) lateral olfactory tract 22
Level A 14.5 The forebrain at the level of the frontal pole of the cerebrum The section cuts through the rostral pole of the frontal lobe of the cerebrum. The center of the cerebrum is filled with the deep cerebral white matter. The medial wall of the cerebral is formed by the cingulate. The lateral olfactory tract lies against the ventral (orbital) surface of the frontal lobe. 23
CEREBRAL CORTEX occipital lobe corpus callosum frontal lobe CEREBELLUM 4V inferior colliculus (auditory) 4V 6N superior colliculus (visual) RHOMBENCEPHALON pyramidal tract periaquaductal gray MESENCEPHALON ISTHMUS thalamus (p2) DIENCEPHALON pretectum (p1) basilar pontine nuclei fornix ac preoptic area septum ST SUBPALLIUM HYPOTHALAMUS VMH och 2n red nucleus 24
Sagittal Level L 1.75 Sagittal section close to the midline This section shows all major regions of the marmoset brain. In the forebrain, we can see the occipital and frontal lobes of the cerebral and a number of subpallial areas (septum, bed nucleus of stria terminalis, and preoptic area). Three forebrain commissures are present; the corpus callosum, the anterior commissure (ac), and the fornix (hippocampal) commissure. The hypothalamus and diencephalon are present. Within the hypothalamus, the VMH (ventromedial hypothalamic nucleus) is prominent and the optic chiasm is attached to the ventral border of the hypothalamus. The diencephalon is made up of three segmental components - the prosomeres. Prosomere 2 (mainly thalamus - p2) and prosomere 1 (pretectum - p1) are seen here, but prosomere 3 (prethalamus - p3) is not labeled. The brain stem consists of the mesencephalon and the three parts of the hindbrain - the isthmus, the cerebellum and the rhombencephalon. The mesencephalon includes the superior and inferior colliculi, the periaqueductal gray and the red nucleus. The thin strip of isthmus separates the mesencephalon from the rhombencephalon. In the rhombencephalon, the basilar pontine nuclei, the abducens nucleus (6N), and the pyramidal tract can be seen. The fourth ventricle (4V) lies between the cerebellum and the rhombencephalon. 25
CEREBRAL CORTEX corpus callosum LV caudate thalamus SUBPALLIUM superior colliculus (visual) DIENCEPHALON ic ac putamen pretectum CEREBELLUM inferior colliculus (auditory) prethalamus pallidum deep cerebellar nuclei 4V sensory trigeminal inferior olive ISTHMUS cornu ammonis dentate gyrus amygdala subiculum 26
Sagittal Level L 3.75 Sagittal section 3.75 mm lateral to the midline This section is some distance from the midline, but the major features are similar to those seen in the previous section. In the forebrain, we can see the subpallium consisting of the caudate nucleus, putamen and pallidum. Between the putamen and the pallidum is the anterior commissure (ac). At this level, the temporal lobe of the cerebrum contains the main parts of the hippocampus (cornu ammonis, dentate gyrus, and subiculum) and the amygdala. The diencephalon is made up of three segmental components (prethalamus, thalamus, and pretectum). The brain stem consists of the mesencephalon (not labeled here) and the three parts of the hindbrain the isthmus, the cerebellum, and the rhombencephalon. Their dorsal surface is covered by the cerebral. The mesencephalon consists of the superior and inferior colliculi. The thin strip of isthmus separates the mesencephalon from the rhombencephalon. In the cerebellum, we can see deep cerebellar nuclei. In the rhombencephalon, the sensory trigeminal nucleus lies from anterior to posterior. Below the sensory trigeminal nucleus is the inferior olive. The fourth ventricle (4V) lies between the cerebellum and the rhombencephalon. 27