The striatopallidonigral connection is a very particular one. It engages the totality of spiny striatal axons. Estimated numbers are 110 million in man, 40 in chimpanzees and 12 in macaques. The striato-pallido-nigral bundle is made up of thin, poorly myelinated axons from the striatal spiny neurons grouped into pencils "converging like the spokes of a wheel" (Papez, 1941). It gives its "pale" aspect to the receiving areas. The bundle strongly stains for iron using Perls' Prussian blue (in addition to iron it contains many heavy metals including cobalt, copper, magnesium and lead).