Intraretinal (extracellular) injections of horseradish peroxidase were used to stain rabbit retinal ganglion cells. Five basic morphological ganglion cell classes were identified by quantitative analysis of dendritic branching patterns and computer reconstruction of dendritic ramification levels. Type 1 cells are characterized by a unistratified, radial dendritic morphology. The dendritic fields are of medium to large size. Subgroups ramify in either the outer or the inner part of the inner plexiform layer (i. p. l.). Type 2 cells have complex intricately branched dendritic morphologies with wide branch angles. They are comparable with type 1 cells in dendritic field size. Subgroups of this class include unistratified cells ramifying in the outer or inner part of the i. p. l. as well as cells with more complicated i. p. l. ramification schemes. Type 3 cells are somewhate similar to type 1 cells. A particular distinction is that they are much larger than type 1 cells at the same retinal eccentricity. Type 4 cells have a thin elliptical soma and a lobulate dendritic tree structure. Type 5 cells are a somewhat heterogeneous group with very small intricately branched dendritic fields. Since the number of anatomical groups is comparable with the number of physiological classes, it is tenable that the major physiological cell classes are associated with distinct dendritic morphologies.
