Medical brain notes 24

  Ventricles of the Brain The interior of the brain contains a series of cavities (Fig. 20.1). The cerebrum contains a median cavity, the third ventricle; and two lateral ventricles, one in each hemisphere. Each lateral ventricle opens into the third ventricle through an interventricular foramen.   The third ventricle is continuous, caudally, with the cerebral aqueduct which traverses the midbrain and opens into the fourth ventricle.   The fourth ventricle is situated dorsal to the pons and medulla, and ventral to the cerebellum. It communicates, inferiorly, with the central canal which traverses the lower part of the medulla and the spinal cord. The entire ventricular system is lined by an epithelial layer called the ependyma.

  The Lateral Ventricles The lateral ventricles are two cavities, one situated within each cerebral hemisphere. Each ventricle consists of a central part which gives off three extensions called the anterior, posterior and inferior horns (Fig. 20.1). The Central Part        The central part of the lateral ventricle is elongated anteroposteriorly. Anteriorly, it becomes con tinuous with the anterior horn, at the level of the interventricular foramen. Posteriorly, the central part reaches the splenium of the corpus callosum.         The central part is triangular in cross section (Fig. 20.2). It has a roof, a floor, and a medial wall. The roof and floor meet on the lateral side. The  roof  is formed by the trunk of the corpus callosum. The  medial wall  is formed by the septum pellucidum and by the body of the fornix. It is common to thetwo lateral ventricles. The  floor  is formed mainly by...

  The Third Ventricle       The third ventricle is the cavity of the diencephalon. It is a median cavity situated between the right and left thalami (Fig. 20.2). It communicates, on either side, with the lateral ventricle through the interventricular foramen (Figs. 20.1, 20.6). Posteriorly, it continues into the cerebral aqueduct which connects it to the fourth ventricle. The ventricle has two lateral walls, an anterior wall, a posterior wall, a floor and a roof.          Each  lateral wall  is marked by the  hypothalamic sulcus  (Fig. 20.6) which follows a curved course from the interventricular foramen to the aqueduct. Above the sulcus, the wall is formed by the medial surface of the thalamus. The two thalami are usually connected by a band of grey matter called the  interthalamic connexus,  which passes through the ventricle.  The lateral wall, below the hypothalamic sulcus, is fo...

  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 w...

  The Cerebrospinal Fluid       The cerebrospinal fluid (CSF) fills the subarachnoid space. It also extends into the ventricles of the brain, and into the central canal of the spinal cord. It is formed by the choroid plexuses of the ventricles. The CSF formed in each lateral ventricle flows into the third ventricle through the interventricular foramen. From the third ventricle it passes through the aqueduct into the fourth ventricle. Here it passes through the median and lateral apertures in the roof of this ventricle to enter the part of the subarachnoid space which forms the cerebellomedullary cistern. From here the fluid enters other parts of the subarachnoid space. In passing from the posterior cranial fossa into the upper (supratentorial) part of the cranial cavity the CSF traverses the narrow interval between the free margin of the tentorium cerebelli and the brainstem. It leaves the subarachnoid space by entering the venous sinuses through arachnoid ...