Though detailed knowledge of the autonomic nervous system is comparatively recent, the initial conceptions of its nature have undergone very important modifications during the time it has been known. This is mainly due to the fact that originally the anatomical discovery was taken up by investigators in various branches of clinical medicine, physiology and pharmacology, and made a special subject for detailed research. Especially did the dualism which is so remarkable a feature of the autonomic nervous system give rise to a number of theories about the relationship of the two components.