The first device for stereotactic surgery was described in detail in 1908 by British neuroscientist and surgeon Sir Victor Horsley and British physiologist Robert Henry Clarke. This device, named the Horsley-Clarke apparatus, facilitated the study of the cerebellum in animals by enabling accurate electrolytic lesioning to be made in the brain. To ensure that a lesion would be introduced in the correct site, Horsley and Clarke created atlases containing pictures of the brains of the animals on which they experimented. Shortly thereafter, in 1918, the first stereotaxic apparatus for humans was designed by Canadian neurologist Aubrey Mussen. However, the first attempts at stereotaxic surgery in human subjects were not made until the 1940s; these attempts were pioneered by American neurologists Ernest A. Spiegel and Henry T. Wycis. Since then, a number of modifications and refinements have been made to stereotaxic devices, procedures, and atlases, and these advances have significantly improved the utility of stereotaxy.