Figure 7.

Scaling of the subcortical white matter. (A) Variation in the volume of the subcortical white matter as a function of numbers of neurons in the cerebral cortex in the two hyrax species (afrotherians, black), rodents (green) and primates (red). The hyraxes overlap with the power function that applies to rodents (exponent, 2.009 ± 0.065). (B) Variation in the surface area of the subcortical white matter (that is, the surface of the gray-white matter interface) as a function of numbers of neurons in the cerebral cortex. The hyraxes overlap with the power function that applies to rodents (exponent, 1.308 ± 0.038). (C) Variation in the volume of the subcortical white matter as a function of numbers of other cells that it contains. The hyraxes have a larger white matter volume than expected for its number of other cells compared to rodents (exponent, 1.185 ± 0.060). (D) Variation in the volume of the cortical gray matter as a function of its numbers of neurons. The hyraxes overlap with the power function that applies to rodents (exponent, 1.174 ± 0.051), but notice that the rock hyrax (black circle on the right) has twice as many neurons in the cortex as the tree hyrax (black circle, left). Data from Ventura-Antunes et al. (2013). Data on the cortical surface areas and volumes were not available for eulipotyphlans and the smaller afrotherians.