The fourth ventricle of the brain

The fourth (IV) ventriculus (ventriculus quartus) is a derivative of the cavity of the rhomboid brain. In the formation of the walls of the IV ventricle, the oblong brain, bridge, cerebellum and isthmus of the rhomboid brain participate. In form, the cavity of the IV ventricle resembles a tent whose bottom is in the shape of a rhombus (rhomboid fossa) and is formed by the posterior (dorsal) surfaces of the medulla oblongata and the bridge. The border between the medulla oblongata and the bridge on the surface of the rhomboid fossa is the brain stria (IV ventricle) [striae medullares (ventriculi quarti)]. They originate in the region of the lateral corners of the rhomboid fossa, they go in the transverse direction and are immersed in the median furrow.

The roof of the IV ventricle (tegmen ventriculi quarti) in the form of a tent hangs over a diamond-shaped fovea. In the formation of the anteroposterior wall of the tent, the upper cerebellar legs and the upper cerebral sail stretched between them (velum medullare craniale, s. Superius) take part.

The back wall is more complicated. It consists of the lower cerebral sail (velum medullare caudate [inferius, S. Posterius]), which is attached to the sides of the scrap laterally. From the inside to the lower cerebral sail, represented by a thin epithelial plate (the remainder of the dorsal wall of the third cerebral vesicle - rhomboid brain), the vascular base of the IV ventricle (tela choroidea ventriculi quarti) is located. The latter is formed due to the invagination of the soft shell of the brain in the gap between the lower surface of the cerebellum at the top and the lower cerebral sail below.

The vascular base, covered from the cavity of the IV ventricle by the epithelial plate, forms the vascular plexus of the IV ventricle (plexus choroidea ventriculi quarti). In the posterior wall of the IV ventricle, there is an unpaired median aperture (apertura medidna ventriculi quarti, Magendi's opening). In the lateral parts, in the region of the lateral pockets of the IV ventricle, there is a paired lateral aperture (apertura lateralis ventriculi quarti, Lushka's hole). All three apertures connect the cavity of the IV ventricle with the subarachnoid space of the brain.

The diamond-shaped fossa

The rhomboid fossa (fossa rhomboidea) is a rhomboid depression, the long axis of which is directed along the brain. It is bounded laterally in its upper part by the upper cerebellar pedicels, in the lower one by the lower cerebellar pedicels. In the lower corner of the rhomboid fossa under the lower edge of the roof of the IV ventricle, under the latch (obex), is the entrance to the central canal of the spinal cord. In the anteroposterior corner there is an opening leading to the midbrain waterway, through which the cavity of the third ventricle communicates with the IV ventricle. The lateral corners of the rhomboid fossa form lateral pockets (recessus laterales). In the median plane along the entire surface of the rhomboid fossa, from its upper angle to the lower extends a shallow median sulcus (sulcus medianus). On each side of this furrow there is a pair medial eminence (eminentia medianus), bounded laterally by the marginal groove (sulcus limitans). In the upper sections of the elevation belonging to the bridge, there is a colliculus facialis corresponding to the nucleus of the abducent nerve (VI pair) lying in this part of the brain and to the envelope of the facial nerve, whose core lies somewhat deeper and lateral. The anterior (cranial) sections of the border groove, somewhat deeper and widening upward (anteriorly) form the upper (cranial) fovea (fovea cranialis, s. Superior). The posterior (caudal, inferior) end of this furrow continues into the faint caudal fossa (fovea caudalis, s., Inferior), which is hardly discernible on the preparations.

In the anterior (upper) parts of the rhomboid fovea, slightly off the middle elevation, a small patch is sometimes visible on fresh brain preparations, differing from the others in bluish color, which is why it was called a bluish locus (locus caeruleus). In the lower parts of the rhomboid fossa belonging to the medulla oblongata, the medial prominence gradually narrows, passing into the triangle of the sublingual nerve (trigonum nervi hypoglossi). Lateral to it is the smaller triangle of the vagus nerve (trigonum nervi vagi), in the depth of which lies the vegetative nucleus of the vagus nerve. In the lateral corners of the rhomboid fossa lie the nuclei of the pre-collar nerve. This site was called the vestibular (vestibular) field (area vestibularis). From this area originate the brain strips of the IV ventricle.

Projection of cranial nerve cores on a rhomboid fossa. The gray matter in the region of the rhomboid fossa is located in the form of separate clusters, or nuclei, which are separated from each other by a white substance. To understand the topography of gray matter, it should be recalled that the neural tube in the medulla oblongata and the bridge opened on the posterior (dorsal) surface and unfolded in such a way that its posterior parts became lateral parts of the rhomboid fossa. Thus, the sensitive nuclei of the rhomboid brain corresponding to the posterior horns of the spinal cord occupy a lateral position in the rhomboid fossa. Motor nuclei, corresponding to the anterior horn of the spinal cord, are located medially in the rhomboid fossa. In the white matter between the motor and sensitive nuclei of the rhomboid fossa are the nuclei of the autonomic (vegetative) nervous system.

The gray matter of the medulla oblongata and the bridge (in the rhomboid fossa) are the nuclei of cranial nerves (V-XII pairs). In the upper triangle of the rhomboid fossa lie the nuclei of V, VI, VII, VIII pairs of cranial nerves.

V pair, trigeminal nerve (n. Trigeminus), has 4 nuclei.

1. The motor nucleus of the trigeminal nerve (nucleus motorius nervi trigeminalis) is located in the upper parts of the rhomboid fovea, in the region of the upper (cranial) fossa. The processes of the cells of this nucleus form the motor spine of the trigeminal nerve.
2. The sensitive nucleus, to which the fibers of the sensitive root of this nerve fit, are 3 parts:
   • the bridge core of the trigeminal nerve (nucleus pontinus nervi trigeminalis) lies laterally and somewhat posteriorly from the motor nucleus. The projection of the bridge core corresponds to the bluish place;
   • the nucleus (lower) of the spinal cord of the trigeminal nerve (nucleus spinalis [inferior] nervi trigiinalis) is, as it were, an extension of the preceding nucleus. It has an elongated shape and lies throughout the medulla oblongata, it enters the upper (IV) segments of the spinal cord;
   • the nucleus of the medullary tract of the trigeminal nerve (nucleus [tractus mesencephalici] nervi trigeminalis) is located cranially (upward) from the motor nucleus of this nerve, next to the aqueduct of the midbrain.

The sixth pair, the abducens nerve (n. Abducens), has one motor nucleus of the nerve (nucleus nervi abducentis), located in the loop of the knee of the facial nerve, in the depth of the facial mound.

VII pair, facial nerve (n. Facialis), has 3 nuclei.

1. The nucleus of the facial nerve (nucleus nervi facialis) motor, large, lies quite deep in the reticular formation of the bridge, lateral to the same hillock (hillock). The processes of the cells of this nucleus form the motor spine. The latter moves in the brain at first in the dorsomedial direction, traverses the nucleus of the VI pair from the dorsal side, forming the knee of the facial nerve, and then goes in the ventrolateral direction.
2. The nucleus of the pathway (nucleus solitarius) is sensitive, common for VII, IX, X pairs of cranial nerves, lies in the depth of the rhomboid fossa, projected laterally beyond the border groove. The cells that make up this nucleus are already found in the bridge cover, slightly proximal to the level of the cerebral stria of the IV ventricle, and extend throughout the dorsal sections of the medulla oblongata up to the 1st cervical segment of the spinal cord. On the cells of this nucleus, fibers that conduct impulses of taste sensitivity terminate.
3. The upper salivary nucleus (nucleus salivatorius rostralis, s.superior) is vegetative (parasympathetic), located in the reticular formation of the bridge, somewhat superficial (dorsal) and lateral to the motor nucleus of the facial nerve.

VIII pair, pre-cochlear nerve (n. Vestibulocochlearis), has 2 groups of nuclei: two cochlear (auditory) and four vestibular (vestibular), which lie in the lateral sections of the bridge and are projected in the region of the vestibular field of the rhomboid fossa.

1. Anterior cochlear nucleus (nucleus cochlearis ventralis, s. Anterior).
2. Posterior cochlear nucleus (nucleus cochlearis dorsalis, S. Posterior). On the cells of these nuclei, the synapse ends the processes of the neurons of the cochlear node (spiral node of the cochlea), which form the cochlear nerve part. These nuclei lie one ventral to the other and to the side of the vestibular nuclei.

Vestibular nuclei receive nerve impulses from sensitive areas (ampullar scallops and spots) of the membranous labyrinth of the inner ear.

1. The medial vestibular nucleus (nucleus vestibularis, medialis; Schwalby nucleus).
2. The lateral vestibular nucleus (nucleus vestibularis lateralis, the nucleus of Deiters).
3. The upper vestibular nucleus (nucleus vestibularis rostralis, s. Superior, the nucleus of Bekhterev).
4. The lower vestibular nucleus (nucleus vestibularis caudalis [inferior], the core of Roller).

The nuclei of the last four pairs of cranial nerves (IX, X, XI and XII) lie in the lower triangle of the rhomboid fossa formed by the dorsal region of the medulla oblongata.

IX pair, glossopharyngeal nerve (n. Glossopharyngeus), has 3 nuclei, one of which (double, motor) is common for IX and X pairs of cranial nerves.

1. The double nucleus (nucleus ambiguus), the motor nucleus, is located in the reticular formation, in the lower half of the rhomboid fossa, and is projected in the region of the lower (caudal) fossa.
2. The nucleus of a single path (nucleus solitarius) is sensitive, common to VII, IX and X pairs of cranial nerves.
3. The lower salivary nucleus (nucleus salivatorius caudalis, s. Inferior) is vegetative (parasympathetic), located in the reticular formation of the medulla oblongata between the lower olive nucleus and the double nucleus.

X pair, the vagus nerve (n. Vagus) has 3 nuclei: motor, sensory and vegetative (parasympathetic).

1. The double nucleus (nucleus ambiguus) is motor, common for IX and X pairs of cranial nerves.
2. The nucleus of a single path (nucleus solitarius) is sensitive, common for VII, IX and X pairs of nerves.
3. The posterior nucleus of the vagus nerve (nucleus dorsalis nervi vagi) is parasympathetic, it lies superficially in the region of the triangle of the vagus nerve.

The XI pair, the additional nerve (n. Accessorius), has the motor nucleus of the accessory nerve (nucleus nervi accessorii). It lies in the thickness of the rhomboid fossa, below the double core, and continues in the gray matter of the spinal cord throughout the upper 5-6 segments (between the hind and anterior horn, closer to the front).

The XII pair, the hyoid nerve (n. Hypoglossus), has one nucleus in the lower corner of the rhomboid fossa, in the interior of the triangle of the hyoid nerve (nucleus nervi hypoglossi). The sprouts of its cells participate in the innervation of the muscles of the tongue and, together with the nerves that depart from the cervical plexus, in the innervation of the muscles of the anterior region of the neck (sublingual muscles).