According to their degree of morphological specialization, the cell populations of the brain stem m a y be classified into three groups: isodendritic, alludendritic a n d idjodendritic. The isodendritic neurons, or gcncralized neurons, are the most frequently encountered. If one discards those isodendritic centers that by common definition are s e n s u y or motor, a n isodendritic core is left which displays very little histological variation throughout t h e whole extent of the brain stem. This core corresponds, with certain restrictions, to t h e regions t h a t are usually regarded as reticular formation. It constitutes a continuum of overlapping dendritic fieEds t h a t extends from the spinal cord to the diencephalon. In view of the similarities t h a t exist ISODENDRITIC CORE OF BRAIN STEM E. RamBn-Moliner a n d W. J. H. Nauta PLATE 1 EXPLANATTON O F FIGURES3 A neuron from the region of the nucleus prepositus hypoglossi. Note the generalized dendritic pattern and the free intermingling of its dendrites with the transversally cut fibers of the medial longitudinal fasciculus ( x 300).4 Nucleus vestibularis interstitialis. (See Ramon y Cajal, '@9a, vol. I, fig. 323.) Note the highly differentiated dendritic patterns and the slight tendency 01 the dendrites to mingle with passing fibers. In spite of its name, this cell group has probably little relationship with the vestibular nuclei properly speaking. The latter are characterized by generalized neurons ( X 300).

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