Abstract
A study has been made of the origin and course of the centrifugal fibres in the visual pathway of the pigeon using the Nauta method. Lesions in the mid-brain involving the isthmo-optic nucleus result in fibre degeneration which can be traced through the isthmo-optic tract to the chiasma and thence into the contralateral optic nerve and retina. In the retin a severe degeneration is found throughout the optic nerve layer, and occasional degenerating fibres can be traced through the ganglion cell layer to the inner aspect of the bipolar cell layer. Here they terminate in endings similar to those described by Cajal (1889) and Dogiel (1895) in relation to amacrine cells. The projection to the retina is completely crossed. Counts of the number of cells in the isthmo-optic nucleus indicate that the number of centrifugal fibres is approximately 10000; they form 1 % of the total number of fibres in the optic nerve. The isthmo-optic nucleus receives afferents from the tectum, and in this projection there would appear to be a well-defined organization.
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