Topological analysis of the brain stem of the lamprey Lampetra fluviatilis

The investigations of His, Herrick, Johnston and many others have led to the doctrine that the central nervous system fundamentally consists of a number of longitudinally arranged zones or columns, some of which are delimited from each other by distinct ventricular sulci. In order to facilitate the study of this basic morphological pattern in the brain stem, a procedure has been developed, making it possible to survey in one figure the neuronal structures and their relationship to the ventricular grooves in this region. Essentially this procedure involves two steps: first the cell masses and large individual cells are projected upon the ventricular surface, and next the ventricular surface is flattened out, i.e., subjected to a one‐to‐one continuous topological transformation. Such a topological analysis was carried out on the brain stem of the lamprey, Lampetra fluviatilis, and yielded the following results: The sulcus limitans of His extends throughout the rhombencephalon, but does not enter the midbrain. The rhombencephalic basal plate can be clearly divided into a medial somato‐motor, and a lateral viscero‐motor zone. The somato‐motor zone consists of a rostral extension of the spinal motor column and three reticular nuclei. Rostrally this zone is continuous with the tegmentum mesencephali, which contains another reticular nucleus and the superficial and deep nuclei of III. The reticular nuclei contain groups of large cells. On either side seven of these large cells, three in the midbrain, one in the isthmus and three in the hindbrain, are significantly larger than the others, and are designated as Müller cells. The viscero‐motor zone is confined to the hindbrain. It consists of a continuous chain of discrete cell masses: the motor nuclei of V, VII, IX and X. This zone contains in addition the large Mauthner neuron, which is situated between the nuclei of V and VII. In the rhombencephalic alar plate, separate viscero‐sensory and somatosensory zones could not be recognized. The delimitable nuclei in this region all belong to the somato‐sensory octavo‐lateralis complex. The latter complex extends rostrally into the very small cerebellum which, apart from a zone of periventricular gray and a nucleus cerebelli, harbours the motor nucleus of IV. The sensory centers of the midbrain, i.e., the torus emicircularis and the tectum are caudally continuous with the gray substance of the alar plate of the hindbrain. Large, presumably primary sensory, elements occur in the caudal and ventral part of the rhombencephalic alar plate. Such “dorsal cells” are also present in the spinal cord, but a midbrain equivalent, i.e., nucleus mesencephalicus of V, is entirely lacking.