AN ANALYSIS OF FIBRE-SIZE IN THE HUMAN OPTIC NERVE

Abstract

The optic nerve contains about 815,000-1,000,000 fine, closely packed, myelinated fibers, which do not have Schwann sheaths. The size of each fiber (inclusive of its sheath) was assessed by projecting the image of a graduated scale on to the section of the nerve prepd. by the Weigert-Pal method. The fibers vary in size from 0.7 to 10 microns, but 80% of them are less than 2 microns and the majority are less than 1 micron. In the direction of the larger fibers the frequency drops steeply although in the region of 5-7 microns there is a low rise. The optic fibers proceeding from the central area of the retina, and therefore destined for the visual cortex, are finer than those from the periphery. There is a relative increase in the number of fine optic nerve fibers and in the number of small cells in the central visual area of the lateral geniculate body in the ascending phylogenetic series. These facts are in conflict with the current idea that the relatively thicker and fast-conducting fibers convey impulses destined for the visual cortex while the thinner slow-conducting fibers only reach the tectum.