The cerebrospinal fluid occupies the subdural and subarachnoid spaces of the brain and cord and also the ventricular cavities of the brain.

The average quantity is about two ounces (Landois and Stirling). Its specific gravity is about 1010, It is of a very pale yellow colour, and presents many of the characters of ordinary lymph, but differs from lymph in not being coagulable, as it does not contain either fibrin factors or fibrin ferment. It contains a substance which acts on Fehling's solution like dextrose, but which is not a sugar (Foster).

The cerebro-spinal fluid is derived in part from the lymphatic vessels which open into the subarachnoid and subdural spaces, but is also believed to be secreted by the epithelial cells which cover the choroid plexuses. These cells are cubical in form, and resemble secreting cells ; a process of the choroid plexus covered by these cells has been aptly compared by Foster to ' an everted alveolus of a secreting gland, with the epithelium outside and the blood-vessels within.

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