The nerve fibre composition of the cat optic nerve.

The optic nerve has always been of particular interest to neurophysiologists because, despite its appearing to leave the brain in the manner of a peripheral nerve, it is a central tract of fibres. As such, its electrophysiological properties have been studied in considerable detail. But histological studies of the optic nerves of vertebrates are few, and not always in agreement. Studies of this central tract (G. H. Bishop, 1933; G. H. Bishop & O'Leary, 1940, 1942; P. 0. Bishop, Jeremy & Lance, 1953; G. H. Bishop & Clare, 1955; Chang, 1956; Lennox, 1958; Sefton & Swinburn, 1964) have shown that it contains groups of fibres differing from each other in threshold, conduction velocity, and destination in the brain; it might be expected that those fibre groups differing in conduction velocity would also form distinct groups when arranged according to fibre diameter (Hursh, 1939; Rushton, 1951). Fibre-diameter analysis of the optic nerve of the cat has been attempted by various workers (P. 0. Bishop et al. 1953; Chacko, 1954; G. H. Bishop & Clare, 1955; Chang, 1956; van Crevel & Verhaart, 1963); of these, Chacko, G. H. Bishop & Clare, and van Crevel & Verhaart describe a unimodal distribution of fibre diameter with a peak at 2-5 ,um, 1 0-1 *5 ptm, and 2 0-4 0 pum respectively. P. O. Bishop et al. describe peaks at 1 0-1 5 /am and 4 0-45 ptm (ill-defined), and Chang describes three peaks, at 10, 4-0 and 9-0 pm. There is thus some discrepancy between the results of various workers with regard to the number of peaks and the position of the first peak. The second discrepancy appears to reflect differences in measuring technique, some workers measuring axonal diameter (P. 0. Bishop et al. and, presumably, G. H. Bishop & Clare, and Chang), while the others measured total fibre diameter. The first discrepancy is most probably due to inadequate sampling. Van Crevel & Verhaart present evidence that the nerve fibres of the cat optic nerve are not randomly distributed, and thus the variation between estimated total number of fibres presented by different workers (Bruesch & Arey, 1942 (119 000); P. 0. Bishop et al. 1953 (120 000); van Crevel & Verhaart, 1963 (64000)) may be a reflexion of this non-homogeneity of the fibrediameter distribution. All these studies (except for that of Bruesch & Arey) have used histological methods which stain only myelinated fibres. The possibility that there may be unmyelinated fibres in the cat optic nerve cannot be overlooked. However, recent electron micro-

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