1. Brain stem
The brain stem is a small part of the central nervous system between the spinal cord and the diencephalon. It consists of the medulla oblongata, pons and midbrain from bottom to top. The back of the medulla oblongata and pons are connected with the cerebellum, and the chamber between them is the fourth ventricle. The chamber is connected with the central canal of the medulla oblongata and spinal cord downward and with the aqueduct of the midbrain upward. If the junction between the cerebellum and the brain stem is cut off and the cerebellum is removed, the bottom of the fourth ventricle, that is, the upper part of the medulla oblongata and the posterior part of the pons, is rhombic, so it is called rhombic fossa. There are three main types of internal structures of the brain stem: nerve nuclei, long fiber bundles and reticular structures. The latter is a specific area where various neurons and fibers are staggered and relatively dispersed.
2. Cerebellum
The cerebellum occupies most of the posterior cranial fossa. Its upper surface is flat, close to the cerebellar tentorium formed by dura mater (see back), its lower middle part is concave, its sides are hemispherical, and its convex surface rests on the base of posterior cranial fossa. The narrow part of the middle cerebellum is called vermis; The swollen parts on both sides are hemispheres. The cerebellum is connected to the back of the brain stem through three pairs of cerebellar feet in front, and the inferior cerebellar foot from the spinal cord and the inferior olivary nucleus is located inside the middle foot (the boundary with the middle foot is not easy to distinguish). Superior cerebellar peduncle is mainly composed of cerebellar efferent fibers, which are thin and located in the front, and there is an upper medullary sail between the left and right upper feet. The inferior sail of the medulla oblongata connects the choroidal tissue of the fourth ventricle downward from the cerebellum.
The total volume of cerebellum accounts for about 10% of the whole brain, but the number of neurons contained in it exceeds half of the total number of neurons in the whole brain. A large number of neurons are concentrated on the surface of the cerebellum, forming a cerebellar cortex, and many roughly parallel transverse grooves can be seen on the surface of the cortex, which divides the cerebellum into many transverse slices called leaves. The white matter cortex of the cerebellum is wrapped by the center of the medulla oblongata, and there is also a gray matter nucleus buried in the medulla oblongata, which is called cerebellar nucleus or central nucleus. The cerebellar nucleus is the part where the cerebellum sends out efferent fibers, which consists of three pairs of nuclei: the cerebellar fastigial nucleus is located above the top of the fourth ventricle; Human intermediate nucleus can be divided into spherical nucleus and embolic nucleus; -We; The outer side of the intermediate nucleus is the dentate nucleus, which is bag-shaped and has the largest volume.
Despite receiving a lot of sensory information, the function of cerebellum is mainly related to motor control, that is, maintaining human balance and coordinating the movement of skeletal muscle. Minor brain injury will not cause voluntary movement's loss (paralysis), but it can show imbalance and muscle tension, especially motor coordination disorder. With the evolution of vertebrates, the size of cerebellum is increasing, reaching its peak in humans. This is related to the ability of higher animals, especially people, to engage in precise and complex movements.
3. diencephalon
The diencephalon develops from the forebrain and is located between the brain stem and telencephalon. Its volume is less than 2% of the central nervous system, but its structure and function are very complicated, second only to the cerebral cortex. The two sides and back of diencephalon are covered by highly developed cerebral hemisphere, and only the optic chiasma, optic tract, gray nodule, funnel, pituitary gland and nipple on the ventral part are exposed at the bottom of brain.
The diencephalon can be divided into five parts: dorsal thalamus, superior thalamus, hypothalamus, posterior thalamus and subthalamus.
The diencephalon cavity is a narrow space located in the median sagittal plane, called the third ventricle, and its top is formed by choroidal tissue; The fundus consists of optic chiasma, gray nodule, funnel and optic papilla. The front boundary is the end plate; After passing through the midbrain aqueduct; The lateral walls are dorsal thalamus and hypothalamus.
4. telencephalon
Both telencephalon and diencephalon are developed from forebrain. Telencephalon is the highest part of the brain, connected by the corpus callosum. In phylogeny, telencephalon function is related to sense of smell, starting from fish. With the advanced development of animals, telencephalon has more functions than sense of smell since reptiles. The order of reappearing species in human telencephalon can be divided into protocortex, architectural cortex, paleocortex and neocortex. Protocortex and old cortex are related to olfactory and visceral activities; Neocortex is highly developed, accounting for more than 96% of the cerebral hemisphere cortex. It has become the highest regulator of various life activities of the body, pushing the protocortex and the old cortex to lower and lower parts of the inner surface of the hemisphere, forming marginal lobes.
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