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Abstract

This chapter reviews observations from comparative neuroanatomy to stress a few points relevant to real nerve theories : (a) neuronal wiring may in some cases be very precise (fiber projections in the visual system of the fly); (b) the synaptic junctions may be functionally very diversified (lamina ganglionaris of the fly); (c) some of the wiring takes place while the nerve net is already operating (the growth of dendrites in the cortex of the mouse); and (d) there are inborn constraints to this kind of plasticity. The chapter stresses on the finding from the anatomy of the visual ganglia of insects, which shows that in certain instances the wiring is specified down to individual nerve fibers. The compound eye of the fly is composed of about 3000 nearly identical subunits, called “ommatidia,” each equipped with its own separate optics and containing 8 separate photosensitive elements, the rhabdomeres. Each rhabdomere is a specialized portion of one cell, the so-called retinula cell. The upper ends of 7 of these rhabdomeres in each ommatidium are arranged in a very regular pattern, localized in the focal plane of the inverting optical system.