An anatomical investigation has been carried out on the third optic ganglion of the fly, Musca domestica. Two systems of giant units, the dendritic arborizations of which are arranged orthogonally relative to each other, dominate the neuropile of this ganglion. The elements of the two systems have been reconstructed using a graphical procedure based on histological sections. One system branches predominantly in the dorso-ventral direction, the other one in the anterior-posterior direction. Both systems of the giant units have a twin system composed of elements smaller in diameter and strictly parallel to the main units. The two systems have been termed the Vertical and Horizontal Systems. The elements of the two systems of fibers project into the periesophageal region where they come into contact with other descending elements. Electron microscopic investigations show that the two systems are post-synaptic at the level of the ganglion from which they originate. The horizontal system has been shown to be post and pre-synaptic in nature during its course in the mid-brain and ultimately presynaptic at its endings in the periesophageal ring. The peculiar geometric arrangement of the two anatomical systems of fibers suggests a precise function in relation to the visual world and in particular to the detection of the direction of motion. The accuracy of the structural pattern displayed by the giant units in the lobular plate seems to suggest that this optic ganglion represents the ultimate orderly projection of the external world in the brain of the fly. A short review of the electrophysiological data concerning this ganglion has been tentatively correlated with some behavioral data related to the visual orientation and fixation in insects.