This century has witnessed the creation of new sciences extending the frontiers of knowledge to an unprecedented degree. We have seen the birth of cybernetics and bionics, bringing together such apparently distantly related branches of science as neurohistology and automation, synaptology and electronics. The electron microscope has resolved tissues almost down to the molecular level, and histochemistry has led to the fine analysis of brain structure. However, before these and other new sciences can develop properly and scientifically, a precise knowledge of the structure of the material with which they are concerned is absolutely essential. That is why the need exists at the present time for a detailed study of the larger units, i. e. , the neurons, their interrelationships and the pathways by which excitation is conducted. Biologists, neurologists, physicists, and specialists in other technical disci­ plines will find this study highly useful. During recent years many advances have been made in knowledge of the central visual system and its pathways. Above all, it has been found that the visual system is very extensive. The optic tract is connected, not only with the lateral geniculate body, but with the superior colliculus and the pulvinar. Besides the discovery of these principal pathways, connections have also been studied with the hypothalamus, the pretectal region, the medial geniculate body, subthalamus, and other parts of the brain stem. The visual system is thus connected with the reflex apparatus, the autonomic nervous system, and the auditory and reticular systems.