The intermediate brain develops from the caudal part of the anterior cerebral bladder. In the process of ontogeny, it undergoes significant changes. In it the ventral and dorsal walls are thinned and the lateral walls thicken considerably. The cavity of this segment of the neural tube widens considerably, acquiring the shape of a gap located in the median plane. It is called the third ventricle.
It should be noted that the dorsal (upper) wall of the third ventricle is represented only by the ependymal epithelium. Above the ependymal epithelium is the outgrowth of the vascular membrane of the brain, which delimits the intermediate brain and the structures of the terminal brain (vault and corpus callosum). The lateral parts of the intermediate brain from the lateral side are directly fused to the structures of the terminal brain.
On the lateral wall of the cavity of the embryonic neural tube there is a border groove, which corresponds to a subthalamic groove in an adult. It is located on the side wall of the third ventricle and is the boundary between the ventral and dorsal parts of the midbrain.
The dorsal part of the lateral wall of the midbrain develops from the pterygoid plate and is called the thalamic brain.
The ventral part of the lateral wall of the midbrain, which is below the subthalamic groove, develops from the main plate and is called the hypothalamus, or subtalamic region.
Thus, the midbrain contains the thalamic brain and hypothalamus. Its cavity is the third ventricle.
In the thalamic brain, three parts are distinguished: the thalamus, or visual cusp, epithalamus (nadalamic region), and metatalamus (ctalamic region). The listed structures of the thalamic brain are accessible only from the dorsal surface of the brainstem after removal of the hemispheres (Figure 3.14).
Fig. 3.14. Dorsal (posterior) surface of the midbrain and midbrain:
1 - the medial geniculate body; 2 - lateral geniculate body; 3 - soldering leashes; 4 - the leash; 5 - triangle of leashes; 6 - caudate nucleus; 7 - the third ventricle; 8 - the thalamus; 9 - Pineal body; 10 - upper mound; 11 - the lower mound; 12 - upper cerebral sail; 13 - middle cerebellar pedicle; 14 - median sulcus
The thalamus (the visual hillock) is ovoid in shape. The medial and dorsal surfaces of the thalamus are free, the ventral and lateral surfaces are fused to the structures of the terminal brain. The anterior end is pointed and called the anterior tubercle of the thalamus; The posterior end is thickened and called the thalamus pillow. The dorsal surface of the thalamus is covered with a thin layer of white matter. Laterally on this surface is a narrow terminal strip that separates the optic tuber and caudate nucleus.
On the medial edge of the dorsal surface of the thalamus, there is a white scallop called the thalamus medulla, which behind the posterior bounds a small triangular area - the triangle of the leash that pertains to the supralamic region. Most of the dorsal surface of the thalamus is covered with a vascular plate, over which is located the vault, referring to the final brain.
The medial surface of the thalamus faces the cavity of the third ventricle. Its lower border is the subthalamic groove. Between the medial surfaces of the visual hillocks there is a heavy interthalamic fusion. It is formed again as a result of the approach of thalamuses.
Epithalamus (the subthalamic region) is located posteriorly from the thalamus and is, as it were, its continuation. To it carry a pineal gland, leashes, a spike of leashes and triangles of leashes.
Pineal gland resembles a squeezed pine cone in shape. It is located in the furrow between the upper hills of the midbrain. The pineal gland is a gland of internal secretion.
At the base of the gland there is a pineal deepening, which is a small cavity, which is a continuation of the third ventricle. Bottom of the pineal gland is limited to posterior spike of the brain, from above it is a spike of leashes.
The triangle of the leash is a small triangular field that lies between the leash, the thalamus and the upper mound. Under a thin layer of white matter here is the core of the leash.
The metatalamus (the cataract region) is represented by medial and lateral geniculate bodies. The medial geniculate body has the form of a small elevation (7 × 5 mm) located ventrally of the cusp of the visual hillock (Figure 3.15). Together with the lower hills of the midbrain, the medial geniculate bodies are subcortical centers of hearing. The nuclei of the medial geniculate body play the role of communication centers for nerve impulses that go to the cortex of the cerebral hemispheres. On the neurons of the cores of the medial geniculate body, the fibers of the lateral loop terminate.
The lateral geniculate body is the elevation of an elongated shape (12 × 5 mm), which terminates the visual tract. It is located on the lower lateral surface of the cushion of the visual hillock, anterior to the medial geniculate body. The cranial bodies are separated by a wide furrow. The lateral geniculate bodies together with the upper hills and the cushion of the visual hillock are subcortical centers of vision. The nuclei of the lateral geniculate body are communication centers in which the paths that conduct nerve impulses to the visual centers of the cortex of the cerebral hemispheres are interrupted.
Fig. 3.15. The hypothalamus and the cataract area. The midbrain incision. View from the ventral surface:
1 - the aqueduct of the brain; 2 - red core; 3 - middle brain; 4 - black substance; 5 - mastoid body; 6 - anterior perforated substance; 7 - funnel; 8 - visual crossover; 9 - optic nerve; 10 - gray hillock; 11 - visual tract; 12 - rear perforated substance; 13 - the legs of the brain; 14 - lateral geniculate body; 15 - the medial geniculate body; 16 - pillow of the thalamus; 17 - roof plate
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