To the Editor.—
The CNS is nourished by CSF, which Cushing referred to as the "third circulation."1 It is the only organ with such a circulation, aside from the eye and the internal ear, which actually are part of the CNS. The closure of the neural groove to form a tube is completed on day 26, which is two weeks prior to the development of the choroid plexuses. The first fluid that progressively distends the neural tube is secreted by the cells lining its lumen. This primitive CSF contains a quantity of protein that increases in concentration during these intervening two weeks during which time the somites develop. The resulting increase in colloidal osmotic tension of this primitive CSF causes the neural tube to expand progressively during the presomite and somite stages. If this distention becomes pathologic in degree, which is not surprising, overexpansion of the primitive cord
Gardner WJ. Spinal Deformities: A Neurosurgeon's Viewpoint. Arch Neurol. 1979;36(5):322. doi:10.1001/archneur.1979.00500410100023
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