Figure 1.

Long-distance axons in the mammalian neocortex. a, Sagittal view of macaque brain with corpus callosum indicated in white. The gray rectangles indicate rostral (r) and caudal (c) locations sampled for transmission electron microscopy. b, Representative micrographs of callosal tissue. Scale bar, 1 μm. c, The fraction of axons that are myelinated increases with brain diameter (slope, 1.04 ± 0.14% myelination per centimeter of brain diameter). Each symbol represents pooled values from one animal. The gray bars indicate ranges of previous adult measurements. d, Volume-filling efficiency of callosal axons. The fraction of volume filled by axons was determined by measuring the fraction of each electron micrograph occupied by myelinated and unmyelinated axons. e, Distribution of axon densities. Box plots indicate median, 25th and 75th percentiles, and dots indicate measurements outside this range. The gray bar indicates the range of previous measurements (Swadlow et al., 1980; LaMantia and Rakic, 1990).