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The ventricular system:
The ventricles of the brain include the paired lateral ventricles, 3rd and 4th ventricles.

Lateral ventricles (ventriculus lateralis):

The two lateral ventricles are irregular cavities situated in the lower and medial parts of the cerebral hemispheres, one on either side of the midline. They are separated from each other by a median vertical partition, the septum pellucidum, but communicate with the 3rd ventricle and indirectly with each other through the interventricular foramena of Monro. They are lined by a thin membrane; the ependyma covered by ciliated epithelium and contain cerebrospinal fluid. Each lateral ventricle is a C- shaped cavity which extends from its anterior horn in the frontal lobe in a continuous curve posteriorly (central part), then inferiorly, and finally anteriorly, to end in the temporal lobe as the inferior horn. From its convex posterior surface a posterior horn extends backwards to a variable extent into the occipital lobe.
The size and shape of this ventricle is very variable. In the young, the walls lie almost in opposition, while with increasing age and loss of neural tissue the ventricle expands and may reach a considerable size without an increase in its internal pressure (Gray et al., 1995).

The anterior horn or cornu of the lateral ventricle curves inferiorly into the frontal lobe from the interventricular foramen. It is triangular in coronal section .The narrow floor is formed by the rostrum of the corpus callosum; the roof and anterior wall by the trunk and genu of the corpus callosum; the vertical medial wall by the septum pellucidum and column of the fornix; the lateral wall by the bulging head of the caudate nucleus (Drake et al., 2010).
The central part of the ventricle: Also called body of the lateral ventricle, it is roofed by the trunk of the corpus callosum. Its medial wall, which decreases in height as it is followed posteriorly, is formed by the fornix and septum pellucidum anteriorly, and by the fornix posteriorly.

The floor consists from lateral to medial of the following structures:

The caudate nucleus.

The thalamostriate vein runs anteriorly in the groove between thalamus and caudate nucleus.
The stria terminalis runs with the thalamostriate.

A narrow strip of the dorsal surface of the thalamus.

The choroid plexus.

The fornix: anteriorly it is a rounded bundle but posteriorly it becomes progressively flattened and extends laterally into the floor of the lateral ventricle.

The posterior horn or cornu begins at the splenium of the corpus callosum, and extends posteriorly into the occipital lobe, tapering to a point. The roof, lateral wall, and floor are formed by a sheet of fibers (tapetum) from the splenium of the corpus callosum.

The medial wall is invaginated by two ridges; the upper of these (bulb of the posterior horn) is formed by the fibers of the forceps major ( Skinner et al., 2009).
The inferior horn is the direct continuation of the ventricular cavity into the temporal lobe. It runs inferiorly, posterior to the thalamus, and then passes anteriorly, curving medially to end at the uncus. The lateral wall is formed by the tapetum of the corpus callosum. The roof, which can be seen on the inferior surface of the 'frontal' part of the brain, consists of the white matter which passes laterally into the temporal lobe. The floor is broad posteriorly where the inferior and posterior horns meet, and is often raised (collateral triangle) by the collateral sulcus. Anteriorly its floor narrows and its medial part is formed by a convex ridge produced by the hippocampus covered by a layer of nerve fibers (the aveus) which passes medially from the hippocampus to form a ridge on its medial border (fimbria of the hippocampus). The amygdaloid body is an oval mass of grey matter which overlies the tip of the inferior horn of the ventricle in the uncus (Drake JM, 1998).

Fig. (4): Ventricles of the brain (BradburyM.W.B,1985)

Fig. (5): Cast of the ventricles of the brain, left lateral aspect. In this brain, the right posterior horn is considerably longer than the left. The key drawing indicates related solid structures in parentheses .
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