About 18 years ago, in 1888, I began to collect animal brains on a larger scale in order to study their morphology in detail, particularly the arrangement of the grooves and convolutions, and to compare them with the characteristics of the human brain. My primary intention was to conduct comprehensive research on the monkey brain. Having long since decided to describe the monkey brains, of which I now possess about 250, in connection with my work on the human brain, I started to photograph a selection of these brains and have light print plates made from these images. This scientific book, featuring many illustrations, is an unaltered reprint of the original edition from 1906.