Dr. Probst begins by remarking that he can only find two cases before that described by Rokitansky, in 1858, in which the corpus callosum was deficient, one by Reil in 1812, and one by Ward in 1846. He apparently has overlooked the case mentioned in Solly's book on the brain in 1827, and another by Paget in 1846. If Dr. Probst had lighted upon the excellent paper by Dr. Alexander Bruce, recorded in the Proceedings of the Royal Society of Edinburgh, vol. xv, pp. 320—341, he would have been able greatly to add to the sixteen cases which he has mentioned. I have myself seen an instance of complete deficiency of the corpus callosum in the brain of a deaf woman who died in the Stirling District Asylum. There are at least six cases (Probst only knows of two) in which, without any mental deficiency or loss of sensory or motor power being observed during life, the corpus callosum was found to be entirely wanting.