Middle Brain

The median brain (mesencephalon), unlike other parts of the brain, is less complicated. It distinguishes the roof and legs. The cavity of the middle brain is the aqueduct of the brain.

The upper (front) border of the midbrain on its ventral surface is the visual tracts and mastoid bodies, on the back - the front edge of the bridge. On the dorsal surface, the upper (anterior) border of the midbrain corresponds to the posterior margins (surfaces) of the thalamus, the posterior (lower ) border to the level of the exit of the roots of the nerve block (n. Trochlearis, IV pair).

The roof of the midbrain (tectum mesencephalic, which is a plate of quadruple, is located above the aqueduct of the brain.) On the brain specimen, the roof of the midbrain can be seen only after the removal of the cerebral hemispheres.The roof of the midbrain consists of four elevations - hills, looking like hemispheres. The other two intersecting at right angles with grooves.The longitudinal groove is located in the middle plane, the upper (front) sections form a bed for the pineal body, and the lower divisions serve as a place m, from where the bridle of the upper cerebral sail begins.The transverse groove separates the upper colliculi (colliculi superiores) from the lower hills (colliculi inferiores) from each of the knolls in the lateral direction the thickenings in the form of a roller - the knob of the knoll - the handle of the upper mound (brachium colliculi cranialis, s. Superioris), is located posteriorly from the thalamus and is directed to the lateral geniculate body.The handle of the lower mound (brachium colliculi caudalis, s. Inferioris) is directed to the medial geniculate body.

In humans, the upper hills of the roof of the midbrain (quadruple) and the lateral geniculate bodies act as subcortical visual centers. Lower hills and medial geniculate bodies are subcortical centers of hearing.

The legs of the brain (pendunculi cerebri) are clearly visible on the basis of the brain in the form of two thick white, longitudinally striated ridges that emerge from the bridge. These fibers are directed forward and laterally (diverge at an acute angle) to the right and left cerebral hemispheres. The deepening between the right and left legs of the brain was called the intercostal fossa (fossa interpeduncularis). The bottom of this fossa serves as a place where blood vessels enter the tissue of the brain. After removal of the vascular membrane on the brain preparations in the plate that forms the bottom of the intercostal fossa, a large number of small holes remain. Hence the name of this gray color of the plate with holes is the rear perforated substance (substantia perforata interpeduncularis, S. Posterior). On the medial surface of each of the legs of the brain there is a longitudinal oculomotor sulcus (sulcus oculomotorius), or a medial groove of the pedicle of the brain. The rootlets of the oculomotor nerve (III pair) leave this groove.

The legs of the brain are anterior (ventral) from the aqueduct of the brain. On the cross-section of the midbrain, the black substance (substantia nigra) is distinctly distinguished in the brain stem by its dark color (due to the pigment of melanin). It extends in the stem of the brain from the bridge to the intermediate brain. Black substance divides the leg of the brain into two parts: the posterior (dorsal) - the covering of the middle brain (tegmentum mesencephali) and the anterior (ventral) department - the base of the peduncle of the brain (the basis of pedunculi cerebri). The midbrain covers the nuclei of the midbrain and pass the ascending conductive pathways. The base of the brain stem entirely consists of white matter, downward conducting paths pass here.

The aqueduct of the midbrain (aqueductus mesencephali, s. Cerebri, sylvia aqueduct) is a narrow canal about 1.5 cm long. It connects the cavity of the third ventricle with IV and contains cerebrospinal fluid. According to its origin, the brain drain is a derivative of the cavity of the middle cerebral bladder.

On the frontal section of the midbrain, it can be seen that the roof of the midbrain (hillocks) consists of gray matter (gray and white layers of the upper hillock and the core of the lower hillock), which is covered externally with a thin layer of white matter.

A central gray matter (substantia grisea centralis) is located around the aqueduct of the midbrain, in which the nuclei of two pairs of cranial nerves are in the region of the bottom of the aqueduct. At the level of the upper mounds, beneath the ventral wall of the mid-brain aqueduct, near the midline, is the paired nucleus of the oculomotor nerve (nucleus nervi oculomotorii). It takes part in the innervation of the muscles of the eye. The parasympathetic nucleus of the autonomic nervous system is localized more ventral to it - the additional nucleus of the oculomotor nerve (nucleus oculomotorius accessorius, the nucleus of Yakubovich, the nucleus of Westphal-Edinger). Fibers emerging from the additional nucleus innervate the smooth muscles of the eyeball (muscle, narrowing pupil, and ciliary muscle). Ahead and somewhat higher than the nucleus of the third pair is one of the nuclei of the reticular formation - the intermediate nucleus (nucleus interstitialis). The processes of the cells of this nucleus are involved in the formation of the reticulospinal path and the posterior longitudinal fascicle.

At the level of the lower hills in the ventral regions of the central gray matter lies the pair nucleus of the fourth pair - the nucleus of the nerve block (nucleus n. Trochlearis). From the brain, the nerve block leaves behind the lower hills, on the sides of the bridle of the upper cerebral sail. In the lateral sections of the central gray matter throughout the midbrain is the core of the median cerebral path of the trigeminal nerve (V pair).

In the tire, the largest and noticeable on the transverse section of the midbrain is the red nucleus (nucleus ruber). It is located somewhat higher (dorsal) than black matter, has an elongated shape and extends from the level of the lower hills to the thalamus. The lateral and above the red nucleus in the brain stem cover on the frontal section shows a bundle of fibers that form part of the medial loop. Between the medial loop and the central gray matter is the reticular formation.

The base of the brain stem is formed by downward conducting paths. The internal and external parts of the base of the legs of the brain form fibers of the cortical-bridge way (see "Conductive Pathways ..."). The medial 1/5 part of the base occupies the frontal-bridge way, the lateral 1/5 part is the temporomandibular-occipital-bridge way. The middle part (3/5) of the base of the pedicle is occupied by pyramidal pathways.

Cortico-nuclear fibers pass medially, laterally - cortico-spinal cord.

In the middle brain there are subcortical centers of hearing and sight, nuclei providing innervation of arbitrary and involuntary muscles of the eyeball, as well as the medullary nucleus of the V pair.

Extrapyramidal system includes a black substance, red and intermediate nuclei, etc., providing muscle tone and controlling automatic unconscious movements of the body. Passing (sensitive) and descending (motor) conducting paths pass through the middle brain.

The nerve fibers that form part of the medial loop are the outgrowths of the second pathway neurons of proprioceptive sensitivity. The medial loop (lemniscus medialis) is formed by internal arc-shaped fibers (fibrae arcuatae internae). The latter are the processes of the cells of the nuclei of the wedge-shaped and thin bundles and are directed from the medulla oblongata to the thalamus nuclei together with the fibers of general sensitivity (pain and temperature) forming the spinal cord (lemniscus spinalis) adjacent to it. In addition, the fibers from the sensitive nuclei of the trigeminal nerve pass through the middle brain cover, known as the trigeminal loop (lemniscus trigeminalis); they are also directed to the nuclei of the thalamus.

The processes of the nerve cells of some nuclei form in the middle brain the crossings of the tire (decussationes tegmenti). One of them - the dorsal cross-over of the tire - "fountain" (Meinert cross), belongs to the fibers of the lining-spinal cord; the other is the ventral crosshair of the tire (Cross forel), it is formed by the fibers of the monak's bundle, the red nuclear-spinal cord.

[1], [2], [3], [4]