The fourth ventricle

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The fourth ventricle (ventriculus quartus) is common cavity of аll parts of the hindbrain composing the rhombencephalon (the medulla oblongata, cerebellum, pons, and isthmus). It is shaped like а tent in which а floor and roof are distinguished.

The floor, or base, of the ventricle is shaped like а rhombus, so it is termed the rhomboid fossa (fossa rhomboidea). It is formed by the posterior surface of the medulla and pons. The apex of the roof juts out into the inferior surface of the cerebellum. In the posteroinferior angle of the rhomboid fossa the central canal of the spinal cord is opened, while in the anterosuperior angle the fossa communicates with the aqueduct. The lateral angles terminate as two blind lateral recesses of the fourth ventricle (recessus laterales ventriculi quarti) curving ventrallу around the inferior cerebellar peduncles.

The roof of the fourth ventricle (tegmen ventriculi quarti) is composed of two medullary veli: the velum medullare superius stretched between the superior cerebellar peduncles, and the velum medullare inferius, а paired structure adjacent to the floccular peduncles. Раrt of the roof between the veli consists of cerebellar mаttеr. The inferior medullary velum is supplemented by а layer of the pia mater, tela chorioidea ventriculi IV, covered on the inside by а layer of epithelium, lamina chorioidea epithelialis, the choroid plexus of the fourth ventricle is connected with it.

There are three openings in the cavity of the ventricle: а median aperture (apertura mediana ventriculi quarti), or Magendie's foramen. It is situated in the region of the inferior angle of the rhomboid fossa, and two smaller, lateral apertures in the region of the lateral recesses of the ventricle (aperturae laterales ventriculi quarti); the lateral apertures are also known as Luschka's foramina. Ву means of these apertures the fourth ventricle communicates with the subarachnoid space of the brain. So, the cerebrospinal fluid from the cerebral ventricles enters the intermeningeal spaces. Cerebrospinal fluid accumulating in the ventricles of the brain in constriction or obliteration of these apertures in inflammation of the meninges (meningitis) cannot drain into the subarachnoid sрасе and hydrocephalus develops.

The rhomboid fossa has four sides, two superior and two inferior. The superior sides are bounded by two superior cerebellar peduncles, the inferior sides are bounded by the two inferior cerebellar peduncles. On the midline of the rhombus, from the superior to the inferior angle, stretches the sulcus medianus. It divides the rhomboid fossa into right and left halves. On either side of the sulcusis an elevations- eminentia medialis.

Trigonum nervi hypoglossi is situated in the low angle of the rhomboid fossa. Lateral two the low part of this triangle trigonum nervi vagi is located. Above the eminentia medialis has a swelling colliculus facialis, produced by the root of the facial nerve.

In the region of the lateral angles, on either side is the vestibular area, the nuclei of the 8-th nerve lie here. Striae medullares ventriculi quarti is situated in the middle of rhomboid fossa. These striae divide the rhomboid fossa into а superior and

inferior halves and correspond to the junction of the medulla oblongata and the pons.

Topography of the Rhomboid Fossa Grey Matter

The grey matter of the spinal cord is continuous with the grey matter of the brain stem and partly spreads in the rhomboid fossa and on the walls of the aqueduct and partly breaks up into the nuclei of the cranial nerves or into nuclei of the bundles of the conduction tracts.

To understand the arrangement of these nuclei one must bear in mind that, as it is said above, the closed neural tube of the spinal cord opened on its dorsal side at the junction with the medulla oblongata and spread out to form the rhomboid fossa. As а result the posterior horns of grey matter of the spinal cord diverged, as it were. The somatic sensory nuclei embedded in the posterior horns acquired а lateral position in the rhomboid fossa while the somatic motor nuclei corresponding to the anterior horns remained in а medial position. As to the vegetative nuclei located in the lateral horns of the spinal cord, they, in accordance with the position of the lateral horns are situated between the somatic sensory and somatic motor nuclei in the rhomboid fossa.

Projection of the cranial nerve nuclei onto the rhomboid fossa.

The fifth pair, the trigeminal nerve (n. trigeminus), has four nuclei, one motor and three sensitive nuclei.

1. The superior sensory nucleus of the trigeminal nervе (nucleus sensorius

principalis n. trigemini) is projected onto the dorsolateral part of the superior part of the pons.

2. The nucleus of the spinal tract of the trigeminal nerve (nucleus tractus spinalis).

3.The motor nuclei of the trigeminal nerve (nucleus motorius n. trigemini),it is concerned with mastication.

4.The mesencephalic nucleus of the trigeminal nerve (nucleus tractus mesencephalicus).

The sixth pair, the abducent nerve (n. abducens), has one motor nucleus embedded in the loop of the facial nerve and the facial colliculus on the surface of the rhomboid fossa corresponds, therefore, tо this nucleus.

The seventh pair, the facial nerve (n. facialis), and the nervus intermedius, whose course is intimately connected with the facial nerve has three nuclei, one-motor, one sensitive, and one vegetative.

1.Nucleus motorius nervi facialis which lye in the reticular formation of the dorsal part of the pons.

2. The sensory nucleus of the tractus solitarius (nucleus tractus solitarii).

3. The vegetative (secretory) superior salivary nucleus (nucleus salivatorius superior) embedded in the reticular formation of the pons dorsal to the nucleus of the facial nerve.

The eighth pair, the auditory nerve (n. vestibulocochlearis), has numerous nuclei which are projected onto the lateral angles of the rhomboid fossa in the vestibular area. The nuclei are separated into two groups according to the two divisions of the nerve. One division of the nerve, pars cochlearis, the cochlear nerve or the nerve of hearing proper, has two nuclei:

1.Nucleus cochlearis dorsalis, and 2.Nucleus cochlearis ventralis, lying lateral to and in front of the dorsal nucleus.

The other division of the nerve, pars vestibularis, the vestibular nerve, has four nuclei (nuclei vestibulares).

1. The medial vestibular nucleus (main) (Schwalbe's nucleus).

2. The lateral vestibular nucleus (Deiters' nucleus).

3. The superior vestibular nucleus (Bechterew's nucleus).

4. The inferior vestibular nucleus.

The ninth pair, the glossopbaryngeal nerve (n. glossopharpngeus), also contains three nuclei.

1. The sensory nucleus tractus solitarius, common with the facial nerve.

2. The vegetative (secretory) inferior salivary nucleus (nucleus salivatorius inferior); its cells are scattered in the reticular formation of the medulla oblongata between the nucleus ambiguus and the olivary nucleus.

3. The motor nucleus ambiguus in common with the vagus and accessory nerves.

The tenth pair, the vagus nerve (n. vagus), has the following three nuclei.

1. The sensory nucleus of the tractus solitarius (nucleus tractus solitarii) common with the facial and glossofaringeal nerves.

2. The vegetative dorsal nucleus of the vagus nerve (nucleus dorsalis n. vagi), lies in the same region.

3. The motor nucleus ambiguus common with the nucleus of the ninth pair;

The eleventh pair, the accessory nerve (n. accessorius), has two nuclei (both motor): one is located in the spinal cord and is called the spinal nucleus of the accessory nerve (nucleus spinalis n. accessorii), the other is а caudal continuation of the nuclei of the tenth and ninth pairs of nerves and is called the nucleus ambiguus; it is located in the medulla oblongata dorsolateral to the olivary nucleus.
The twelfth pair, the hypoglossal nerve (n. hypoglossus), has а single, motor nucleus, located in the inferior part of the rhomboid fossa, deep in the hypoglossal triangle (trigonum n. hypoglossi).

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