There are two ways in which this important subject may be treated. Of these, the more interesting and logical would be to systematically expose the results of anatomical researches and of physiological experiments which tend to demonstrate the organic independence and the functions of various parts of the nervous system, and to give a classified series of results of autopsies bearing on localization. This would be all the more satisfactory because the questions involved, although of much importance in practice, are in reality physiological. The localization of functions being known, the physician could from the symptoms (_i.e._ perverted or abolished functions) present make a deductive diagnosis of great exactness. A treatise on medicine, however, cannot allow the space necessary for such a treatment of the topic which is best suited for monographic writing. The other method of exposition, the one we will follow, is that of summary statement of the association of the symptoms with definite lesions, with occasional anatomical and physiological explanations. This will, after all, be a series of diagnostic propositions stated as concisely and classified as practically as possible. With this end in view we divide the subject into five parts: The localization of lesions in the peripheral nervous system (including the cauda equina); The localization of lesions in the spinal cord; The localization of lesions in the medulla oblongata; The localization of lesions in the encephalon. Cranio-cerebral topography. I. Localization of Lesions in the Peripheral Nervous System. In general terms, it may be said that lesions of peripheral (cerebro-spinal) nerves give rise to various sensory symptoms in the area of cutaneous distribution of the affected nerves, and to a flaccid atrophic paralysis in muscles supplied by the same nerves. These muscles almost always exhibit the De R. in varying degrees, and other trophic and vaso-motor symptoms are common. Many of these symptoms also occur in cerebral and spinal diseases, so that, after all, the diagnosis of peripheral localization depends largely on a correct knowledge of the course and {66} distribution of nerves; of the relative distribution of the sensory and motor filaments of nerve-trunks; and of the frequent anomalies which occur. The subject of collateral innervation at the periphery must also receive attention, as involving a source of error.