Fibers in spinal nerves of tree frogs: Eleutherodactylus and Hyla.

This is a study on numbers and sizes of nerve fibers as determined by electron microscope mapping of the spinal nerves of two species (in different families) of free frogs. The study provides data for comparison of fiber populations in the grass frog, Rana pipiens. In both free frogs the greatest number of fibers in the ten pairs of spinal nerves are in the second and ninth rami. The combined counts of myelinated and unmyelinated fibers in the rami on one side of the cord in Eleutherodactylus and Hyla are respectively: to the forelimb, 3,147 and 4,460 fibers; to the trunk, 334 and 1,234 fibers; and to the hindlimb, 4,335 and 6,213 fibers. With a few notable differences this distribution of fibers to regional body parts correlates closely with that found in Rana. The arboreal frogs have a relatively smaller number of medium and especially large size fibers in comparison with the aquatic-terrestrial living grass frog. The importance is stressed of examining the spinal nerves of an entirely aquatic frog with different feeding habits, such as Xenopus, for further comparison.