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Example paths followed by microspheres tracing the motion of cerebrospinal fluid in a murine surface periarterial space. The artery (stained with FITC-dextran dye) is visible in the greyscale image. Flow was smooth, with little resemblance to the random walks characteristic of diffusion or to the directional random walks characteristic of flow through porous media, suggesting an open space. These measurements span 16 min immediately after cardiac arrest.
