THE HORIZONTAL CELLS OF ARTIODACTYL RETINAE - A COMPARISON WITH CAJALS DESCRIPTIONS

The morphology of horizontal cells in ox, sheep, and pig retinae as ob served after Lucifer Yellow injections are described and compared with the descriptions of Golgi-stained cells by Ramon y Cajal (1893). Hori zontal cells in the retinae of less domesticated species, wild pig, fa llow and sika deer, mouflon, and aurochs were also examined. All these retinae have two types of horizontal cell; their morphologies are in common, although with some familial differences. Their basic appearanc e is as Cajal described; except in one important respect, a single axo n-like process could not be identified on the external horizontal cell s. It is concluded that external horizontal cells of artiodactyls corr espond to the axonless (A-type) cells of other mammals. Cajal's intern al horizontal cells have a single axon which contacts rods. This type corresponds to the B-type cells of other mammalian retinae. Artiodacty l A- and B-type horizontal cells differ from those of many other mamma ls in that the B-type dendritic tree is robust and the A-type dendriti c tree is delicate. Historically, this morphological difference betwee n orders of mammals has led to some confusion. The comparisons present ed here suggest that the morphological types of primate horizontal cel ls can be integrated into a general mammalian classification.