Summary

Variations the size of the subarachnoid space have been revealed by ultrasonographic (US) measurements mainly in neonates, infants, and children computed tomography (CT) and MR studies have also been conducted. The subarachnoid space dimensions measured between the arachnoid and the pia on the anterior and posterior sagittal diameters and the right and left transverse diameters are symmetrical between the right and left sides. Variable arachnoid trabeculae that differ in their strength and density are also seen in other subarachnoid cisterns and else-where in the subarachnoid space. The arachnoid membranes vary greatly in appearance and configuration. The superior margin of the posterior communicating membrane (PCM) can be free or joined to the anterior choroidal membrane or the inferior aspect of the diencephalon by arachnoid trabeculae. The subarachnoid space displaces fascicles of the facial nerve and portions of the geniculate ganglion by dissecting between the perineural membrane covering these neural structures.

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