Figure 3.

Left: Schematic diagram of the concept of tortuosity, shown for a porous medium (see text). Right: (a) Exponent of the displacement-versus-time curve (figure 2) for the ensemble of particle tracks for each of eight different mice. (b) Enlargement of the same plot, showing a clustering around a mean value of 0.946 (wide bar) with standard error of the mean 0.0597 (narrow bars). This shows that the particle motion is mostly unimpeded (exponent 1.0), as in an open space, rather than diffusive (exponent 0.5), as in a porous medium. (c) Average tortuosity of particle tracks from the same eight mice. The overall mean tortuosity is 1.0674 (wide bar) with standard error of the mean 0.0294 (narrow bars). A tortuosity of 1 corresponds to motion in a straight line. Here, the tortuosity is slightly greater than 1, probably because of curvature of the periarterial space. A much greater tortuosity would be expected for motion in a porous medium.