Bridge

The bridge (pons; variolium bridge) on the base of the trunk portion of the brain has the form of a transversely located cushion that borders the middle brain (with the legs of the brain) at the top (front) and the lower (behind) with the medulla oblongata.

The dorsal surface of the bridge faces the IV ventricle and participates in the formation of its bottom - a diamond-shaped fossa. In the lateral direction, on each side it can narrow and pass into the middle cerebellar pedunculus (pedunculus cerebellaris medius), which extends into the hemisphere of the cerebellum. The border between the middle cerebellar pedicle and the bridge is the place of the trigeminal nerve exit. In the deep transverse furrow that separates the bridge from the pyramids of the medulla oblongata, the roots of the right and left retracting nerves come out. In the lateral part of this furrow, the roots of the facial (VII pair) and the pre-cochlear (VIII pair) nerves are visible.

On the ventral surface of the bridge, which lies in the skull cavity to the clivus, a wide but not deep basilar (sulcus basilaris) is noticeable. In this furrow lies the eponymous artery.

On the cross section of the bridge it is seen that the substance forming it is not homogeneous. In the central sections of the bridge cut, a thick bundle of fibers running transversely and belonging to the conductive path of the auditory analyzer is seen, a trapezoid body (corpus trapezoideum). This formation divides the bridge into the back, or the bridge cover (pars dorsalis pontis, s. Tegmentum pontis) and anterior [basilar part] (pars ventralis [basilaris] pontis). Between the fibers of the trapezoid body are the anterior and posterior cores of the trapezoid body (nuclei ventralis et dorsalis corporis trapezoidci). In the anterior (basilar) part of the bridge (at the base), longitudinal and transverse fibers are visible. The longitudinal fibers of the bridge (librae pontis longitudinales) belong to the pyramidal pathway (cortex-nuclear fibers, fibrae corticonucleres). Here, there are cortical core fibers (fibrae corticopontinae), which terminate on the nuclei of the (own) bridge (nuclei pontis); they are located between groups of fibers in the thickness of the bridge. The processes of the nerve cells of the bridge nuclei form bundles of transverse fibers of the bridge (nbrae pontis transver-sae). The latter are directed toward the cerebellum, forming the middle cerebellar legs.

In the posterior (dorsal) part (bridge cover), in addition to the fibers of the ascending direction, which are the continuation of the sensitive conductive pathways of the medulla oblongata, the fibers of the medial loop (liniscus medialis) and, laterally, of the cerebrospinal loop (liniscus spinalis) lie directly above the trapezoidal body. Above the trapezoid body, closer to the middle plane, is the reticular formation, and even higher - the posterior longitudinal fasciculus (fasciculus longitundinalis dorsalis, S. Posterior). Sideways and above the medial loop lie the fibers of the lateral loop.

The gray substance of the bridge is represented by the nuclei of V, VI, VII, VIII pairs of cranial nerves, providing eye movement, facial expressions, the activity of auditory and vestibular apparatus; nuclei of the reticular formation and the own nuclei of the bridge, participating in the cortical connections of the cerebral hemispheres with the cerebellum and transmitting impulses from one part of the brain to the other through the bridge. In the dorsal sections of the bridge follow the ascending sensitive pathways, and in the ventral - the descending pyramidal and extrapyramidal pathways. There are also systems of fibers that provide two-way communication between the cerebral cortex and the cerebellum. In the cerebellum there are nuclei (centers) that provide coordination of movements, maintaining the balance of the body.