Bridge

The bridge (pons; Varoli's bridge) at the base of the brain stem has the appearance of a transversely located ridge, which at the top (in front) borders the midbrain (with the cerebral peduncles), and at the bottom (at the back) - with the medulla oblongata.

The dorsal surface of the pons faces the fourth ventricle and participates in the formation of its bottom - the rhomboid fossa. Laterally, it can narrow on each side and passes into the middle cerebellar peduncle (pedunculus cerebellaris medius), which goes into the cerebellar hemisphere. The border between the middle cerebellar peduncle and the pons is the exit point of the trigeminal nerve. In the deep transverse groove separating the pons from the pyramids of the medulla oblongata, the roots of the right and left abducens nerves emerge. In the lateral part of this groove, the roots of the facial (VII pair) and vestibulocochlear (VIII pair) nerves are visible.

On the ventral surface of the bridge, which in the cranial cavity is adjacent to the clivus, a wide but not deep basilar (main) groove (sulcus basilaris) is noticeable. The artery of the same name lies in this groove.

A cross-section of the pons shows that the substance that forms it is not uniform. In the central sections of the pons section, a thick bundle of fibers running transversely and related to the auditory analyzer pathway is visible - the trapezoid body (corpus trapezoideum). This formation divides the pons into the posterior part, or pontine tegmentum (pars dorsalis pontis, s. tegmentum pontis) and the anterior [basilar] part (pars ventralis [basilaris] pontis). Between the fibers of the trapezoid body are the anterior and posterior nuclei of the trapezoid body (nuclei ventralis et dorsalis corporis trapezoidci). In the anterior (basilar) part of the pons (at the base), longitudinal and transverse fibers are visible. The longitudinal fibers of the pons (librae pontis longitudinales) belong to the pyramidal tract (corticonuclear fibers, fibrae corticonucleres). Here are also the cortical spinal fibers (fibrae corticopontinae), which end on the nuclei (proper) of the pons (nuclei pontis); they are located between the groups of fibers in the thickness of the pons. The processes of the nerve cells of the nuclei of the pons form bundles of transverse fibers of the pons (nbrae pontis transversae). The latter are directed toward the cerebellum, forming the middle cerebellar peduncles.

In the posterior (dorsal) part (pontine tegmentum), in addition to the ascending fibers, which are a continuation of the sensory pathways of the medulla oblongata, directly above the trapezoid body lie the fibers of the medial loop (liniscus medialis) and lateral to them - the spinal loop (liniscus spinalis). Above the trapezoid body, closer to the median plane, is the reticular formation, and even higher is the posterior longitudinal fasciculus (fasciculus longitundinalis dorsalis, s. posterior). Lateral and above the medial loop lie the fibers of the lateral loop.

The grey matter of the pons is represented by the nuclei of the V, VI, VII, VIII pairs of cranial nerves, providing eye movement, facial expressions, and the activity of the auditory and vestibular apparatus; the nuclei of the reticular formation and the proper nuclei of the pons, participating in the connections of the cerebral cortex with the cerebellum and transmitting impulses from one part of the brain to another through the pons. In the dorsal parts of the pons there are ascending sensory pathways, and in the ventral parts - descending pyramidal and extrapyramidal pathways. Here there are also systems of fibers providing two-way communication of the cerebral cortex with the cerebellum. In the cerebellum there are nuclei (centers) providing coordination of movements, maintaining body balance.