function in the rabbit. While presenting an overview of current understanding of a system, it is always wise to trace the historical roots of our knowledge and concepts. On the one hand, we may find that many established concepts derive their strength more from convention and convenience than from rigorous evi­ dence. Such traditional ideas should be challenged by critical experiments, using the best available techniques. On the other hand, some of the older publications remain a source of inspiration, because they contain valid ideas which generate fruitful experiments even today. A remarkable example of this category is Ter Braak's work on optokinetic nystagmus, published in 1936.