The Fourth Ventricle
For a proper understanding of the anatomy of the fourth ventricle, it is necessary that some features of the gross anatomy of the cerebellum and of related structures be clearly understood. Reference to Fig. 20.8 will show that the cerebellum is intimately related to the ventricle. The upper part of the ventricle is related to the superior (or anterior) medullary velum. When traced inferiorly (and posteriorly) the velum merges into the white matter of the cerebellum. The lower part of the ventricle is related to the nodule (Figs. 20.8, 20.9A). It will be recalled that the nodule forms the anterior-most part of the inferior vermis. Immediately lateral to the nodule there is the tonsil of the cerebellum. If the tonsil is lifted away, we see that the nodule is continuous laterally with a membrane called the inferior (or posterior) medullary velum (Fig. 20.9B). Posteriorly, the inferior velum merges intothe white matter of the cerebellum. This is seen in Fig. 20.10 which is a sagittal section along the axis XY shown in Fig. 20.9B. The inferior medullary velum has a thickened free edge which connects the nodule to the flocculus. This edge is the peduncle of the flocculus. In the intact brain, this peduncle is very near the posterior surface of the medulla, and is separated from the inferior cerebellar peduncle only by a narrow interval. With these facts clearlyrecorded we may now consider the anatomy of the fourth ventricle.
The fourth ventricle is a space situated dorsal to the pons and to the upper part of the medulla; and ventral to the cerebellum.
For descriptive purposes the ventricle may be considered as having a cavity, a floor, a roof and lateral walls.
The cavity of the ventricle is continuous, inferiorly, with the central canal; and, superiorly, with the cerebral aqueduct. It communicates with the subarachnoid space through three apertures, one median and two lateral (Figs 20.8, 20.13A). A number of extensions from the main cavity are described (Fig. 20.11). The largest of these are two lateral recesses, one on either side. Each lateralrecess passes laterally in the interval between the inferior cerebellar peduncle (ventrally), and the peduncle of the flocculus (dorsally) (Fig. 20.13A). The lateral extremity of the recess reaches the flocculus. At this extremity, the recess opens into the subarachnoid space as the lateral aperture.
Another recess present in the middle line, is called the median dorsal recess. It extends into the white core of the cerebellum and lies just cranial to the nodule (Figs 20.8, 20.11). Immediately lateral to the nodule, another recess projects dorsally, on either side, above the inferior medullary velum. This is the lateral dorsal recess (Figs. 20.10, 20.11).
Because of its shape, the floor of the fourth ventricle is often called the rhomboid fossa (Fig.20.12). It is divisible into an upper triangular part formed by the posterior surface of the pons; a lower triangular part formed by the upper part of the posterior surface of the medulla; and an intermediate part at the junction of the medulla and pons. The intermediate part is prolonged laterally over the inferior cerebellar peduncle as the floor of the lateral recess. Its surface is marked by the presence of delicate bundles of transversely running fibres. These bundles are the striae medullares.
The entire floor is divided into right and left halves by a median sulcus. Next to the middle line there is a longitudinal elevation called the median eminence. The eminence is bounded laterally by
thesulcus limitans. The region lateral to the sulcus limitans is the vestibular area which overlies the vestibular nuclei. The vestibular area lies partly in the pons and partly in the medulla.
The pontine part of the floor shows some features of interest in close relation to the sulcus limitans and the median eminence. The upper-most part of the sulcus limitans overlies an area that is bluish in colour and is called the locus coeruleus. (Deep to the locus coeruleus there is the nucleus coeruleus which extends upwards into the tegmentum of the midbrain. It is regarded as part of the reticular formation).
Somewhat lower down, the sulcus limitans is marked by a depression, the superior fovea. At this level the median eminence shows a swelling, the facial colliculus.
The medullary part of the floor also shows some features of interest in relation to the median eminence and the sulcus limitans. The sulcus limitans is marked by a depression, the inferiorfovea. Descending from the fovea, there is a sulcus that runs obliquely towards the middle line. Thissulcus divides the median eminence into two triangles. These are the hypoglossal triangle, medially; and the vagal triangle, laterally. Between the vagal triangle (above) and the gracile tubercle (below), there is a small area called the area postrema. Finally, mention must be made of two terms often used in relation to the medulla. The lowest part of the floor of the fourth ventricle is called the calamus scriptorius, because of its resemblance to a nib. Each inferolateral margin of the ventricleis marked by a narrow white ridge or taenia. The right and left taeniae meet at the inferior angle of the floor to form a small fold called the obex.The term obex is often used to denote the inferior angle itself.
The upper part of each lateral wall is formed by the superior cerebellar peduncle (Fig. 20.13B). The lower part is formed by the inferior cerebellar peduncle, and by the gracile and cuneate tubercles (Fig. 20.13C,D).
The roof of the fourth ventricle is tent-shaped and can be divided into upper and lower parts which meet at an apex (Figs. 20.8, 20.13A). The apex extends into the white core of the cerebellum. The upper part of the roof is formed by the superior cerebellar peduncles and the superior medullary velum (Figs. 20.13A,B). The inferior part of the roof is devoid of nervous tissue in most of its extent. It is formed by a membrane consisting of ependyma and a double fold of pia mater which constitutes the tela choroidea of the fourth ventricle (Fig. 20.13 A,C). Laterally, on each side, this membrane reaches and fuses with the inferior cerebellar peduncles. The lower part of the membrane has a large aperture in it. This is the median aperture of the fourth ventricle through which the ventricle communicates with the subarachnoid space in the region of the cerebellomedullary cistern. In the region of the lateral recess, the membrane is prolonged laterally and helps to form the wall of the recess. The inferior medullary velum forms a small part of the roof in the region of the lateral dorsal recess (Fig. 20.10). It may be noted that some authors describe the entire membranous structure, forming the lower part of the roof of the fourth ventricle, as the inferior medullary velum. The nodule is intimately related to the roof of the ventricle in the region of the median dorsal recess.
As stated above the tela choroidea of the fourth ventricle is made up of two layers of pia mater. The superior (or dorsal) layer lines the inferior vermis. On reaching the nodule (and more laterally, the inferior medullary velum), it is reflected on itself to form the inferior (or ventral) layer (Fig. 20.8). When traced laterally the dorsal layer is continuous with the pia mater covering the cerebellar hemispheres, while the ventral layer is continuous with the pia mater lining the medulla (Fig. 20.13C).
The choroid plexuses of the fourth ventricle are similar in structure to those of the lateral and third ventricles. They lie within the folds of pia mater that form the tela choroidea, and project into the cavity of the ventricle from the lower part of the roof (Fig. 20.13C). Each plexus (right or left) consists of a vertical limb lying next to the midline, and a horizontal limb extending into the lateral recess. The vertical limbs of the two plexuses lie side by side so that the whole structure is T-shaped. The lower ends of the vertical limbs reach the median aperture and project into the subarachnoid space through it. The lateral ends of the horizontal limbs reach the lateral apertures, and can be seen on the surface of the brain, near the flocculus.
1. The area postrema is believed to be the site of the vomiting centre.
2. Tumours (medulloblastomas) are common near the roof of the fourth ventricle.
3. In Arnold Chiari deformity the medulla and the tonsils of the cerebellum come to lie in the vertebral canal. Apertures in the roof of the fourth ventricle are blocked leading to obstruction to flow of CSF and internal hydrocephalus. Cranial nerves arising from the medulla are stretched. This is a congenital anomaly. It is often associated with syringomyelia.