Complex sentence: For many subsequent years it was thought that VRS were in free communication with the cerebrospinal fluid in the subarachnoid space, but it was later shown with electron microscopy that pia matter serves as separation between the two.
Split into simpler sentences: For many subsequent years it was thought that VRS were in free communication with the cerebrospinal fluid in the subarachnoid space. It was later shown with electron microscopy that pia matter serves as separation between the two.