The openings of the central canal in the filum terminale internum of some mammals.

Openings of the central canal in the filum terminale internum of the rabbit, guinea pig, and rat have been studied by light and electron microscopy. There were two openings in the rabbit, two or three in the guinea pig, and one in the rat. They opened dorsally and were of two types; one type was without a pial covering, the other with a pial covering. In both types, a focal junctional apparatus associated with increased density of the subjacent cytoplasm was observed between the pial cell and the ependymal cell at the margins of the opening, where the basal lamina on the surface of the filum terminale showed a characteristic ending for each species. Injections of india ink into the lateral ventricle of the brain indicated that the cerebrospinal fluid of the ventricular system drains out of the central canal by way of these openings, into the subarachnoid space. Reissner's fibre of the rabbit and rat consisted of a mass of moderately electron dense fibrous material containing many small vesicles in the central canal of the filum terminale. This mass passed through the openings into the subarachnoid space and continued into the subdual space.