Complex sentence: She was studied by the American neurologist Morton Prince between 1898 and 1904, and his 1906 monograph, "Dissociation of a Personality", describing her case, was a landmark in the study of what he described as multiple personality disorders.
Split into simpler sentences: She was studied by the American neurologist Morton Prince between 1898 and 1904. His 1906 monograph, "Dissociation of a Personality", describing her case, was a landmark in the study of what he described as multiple personality disorders.