Part I: Histological investigation
This is the first post in a three part series. Parts II and III are here and here.
The modern scientific study of white matter has its roots in the 19th century when links were being established between mental dysfunction and neuroanatomy. Correlations made between postmortem abnormalities in the brains of mental patients and clinical evaluations while they were living led to important discoveries. The identification of the arcuate fasciculus as a language pathway that connected the two language centers, the Broca and Wernicke regions, is one famous example. In that case, Carl Wernicke, who was developing language network models, made the association between lesions in the arcuate fasciculus and the various aphasias he had observed.
The impetus from these investigations crossed over to other developments. Theodor Meynert, the reputed neuroanatomist, had classified prominent white matter tracts or fasciculi, as they were known, based on the kinds of connections they made. Burdach and Déjérine published postmortem atlases, and both prominently included white matter dissections. New techniques for histopathological analysis were introduced. Notable among these was Camilio Golgi's staining method and Santiago Ramón y Cajal's use of it in his histolgical studies of nerve fibers.
Quantitative white matter fiber analysis benefited from these cumulative efforts which made studies in fiber thinning, demyelination and microstuctural damage possible. Today, postmortem dissections still give the most precise quantitative assessments.
Note of appreciation: This write-up was compiled based partly on Marco Catani's--I have pointed him out before--voluminous publications. He writes exceedingly well on the subject of language networks and related themes.