The cell masses in the brain stem of the turtle Testudo hermanni; α a topographical and topological analysis

The ventricular sulcal pattern and the cytoarchitectonic organization of the brain stem of the turtle Testudo hermanni have been studied in transversely cut, Nissl stained serial sections. Four longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans and the sulcus medianus superior could be distinguished. With the aid of the usual cytoarchitectonic criteria 54 cell groups have been delimited; 12 of these are primary efferent or motor nuclei, 13 are primary afferent or sensory centers; 10 of the nuclei distinguished are considered as belonging to the reticular formation. The remaining 19 cell masses can be indicated as “relay” nuclei. In order to provide a basis for experimental hodological work the topographical position of the nuclei is illustrated in photomicrographs of representative levels and in graphical reconstructions. In order to elucidate the morphological pattern of the cell masses and their relationship to the ventricular sulci the brain stem was subjected to a topological analysis. Essentially this method requires that (1) the cell masses are projected upon the ventricular surface and that (2) this surface, with its sulci marked on it and with the outlines of the subjacent cell masses projected upon it, is flattened out. This analysis yielded the following results: The sulcus limitans extends throughout the rhombencephalon, dividing this brain part into a basal plate and an alar plate. In the basal plate the sulcus intermedius ventralis marks the boundary between an area ventralis and an area intermedioventralis containing, respectively, the somatic motor nuclei of primary and higher order, and the visceral motor centers. Yet the terms somatic motor zone and visceral motor zone are not applicable to these two areas as a whole since both contain a number of non‐motor centers. Thus the intermedioventral zone harbours two general somatic sensory centers, i.e., the spinal‐ and princeps nuclei of V. The medial parts of the tegmentum isthmi and of the tegmentum mesencephali contain somatic motor centers of primary and higher order, and thus may be considered rostral continuations of the rhombencephalic ventral zone. The rhombencephalic alar plate contains visceral sensory, general somatic sensory and special somatic sensory centers, but these do not show a simple ventrodorsal arrangement. A prominent feature is that the special somatic sensory nuclei of termination of VIII, the cerebellar nuclei, and a