Relative Numbers of Pyramidal Tract Neurons in Mammals of Different Sizes

Existing counts of the number of pyramidal tract fibers at the level of the inferior olivary nucleus on 21 different species of mammals are brought together and examined as a function of body weight. Orderly relationships are shown to exist, with improvement when the mammals are subdivided into two groups, the first being those with extensive corticospinal tracts and the second being those with short and diminutive corticospinal tracts. The number of pyramidal tract fibers varies as the 4/9power of body weight, the constant for the 1st group of mammals being 4 times that of the 2nd group. The number of pyramidal tract fibers per unit of brain weight also varies as the 4/9power of body weight, the two groups of mammals falling on nearly the same line. Man and 2 gibbons fall markedly below their line, having only about half the number of pyramidal tract fibers per unit of brain weight expected for mammals of their body size. Mean axon diameters correlate poorly with body size and with brain size, though primates tend to have the largest axons and rodents the smallest.The relationships shown, taken together with other measurements on mammalian brains, are used to derive a statement that the number of pyramidal tract neurons of any mammal is a constant fraction of the total number of neurons in its cerebral cortex, with man and the gibbons as exceptions. The latter have about twice as many cerebral neurons per pyramidal tract fiber as the other mammals. Some of the implications of these findings are discussed.