Further observations on the conditions determining the number and arrangement of the fibers forming the spinal nerves of the frog (Rana virescens)

1 The results obtained in a previous paper are corroborated in the following particulars: a The number of fibers in the ventral nerve root of the frog decreases as the fibers pass from their cells of origin in the spinal cord. b The number of fibers in the dorsal root decreases as the fibers pass from their cells of origin in the spinal ganglion. c The section of the nerve trunk taken immediately distal to the spinal ganglion (dorsal branches excluded) contains a greater number of nerve fibers than a section of the trunk taken further distal. These relations are explained as due to growth or to the processes by which a larger frog acquires the greater number of fibers in its spinal nerves than a smaller one. d In frogs of increasing weight, the fibers of the dorsal root increase more rapidly than do those of the ventral root. e The sum of the fibers in the trunk and dorsal branches combined, exceeds and by a considerable amount, the sum of the fibers contained in the two roots. We assume the addition of fibers on the distal side of the spinal ganglion to correspond to that found on the proximal side, but, owing to the mixing of the dorsal and ventral root-fibers in the distal end of the ganglion, nothing can be demonstrated. In addition to the corroborated results just enumerated, it has been found: 2 The excess of the sum of the trunk and dorsal branches over the sum of the two roots can not be due to a double counting of the same medullated sympathetic fibers running in the trunk and then passing out in the dorsal branches, because the greater the number of fibers composing the dorsal branches as compared with the number contained in the corresponding nerve trunk, the less does the sum of the trunk and dorsal branches exceed the sum of the two roots. Neither can it be largely due to the splitting of ventral root fibers in the region of the ganglion, for the number of fibers constituting the excess is in many cases greater than the entire number of fibers in the ventral root. 3 The average percentage increase in the number of fibers per millimeter of length is greater for the dorsal root than for the ventral root. 4 The average percentage increase of fibers per millimeter of length tends to be greater for the trunk than it is for either root. 5 The average rate of growth or increase of fibers per millimeter of length is greater for the smaller specimens than, for the larger ones. 6 In proportion to its weight, the smaller frog has a greater number of fibers in its ventral root, dorsal root, and its nerve trunk. In both the large and the small frog the proportions of fibers in the three localities are quite similar. 7 As the animal increases in weight, for each gram gained in weight, it gains in its 6th. spinal nerves alone, about 2.7 ventral root fibers, 4.7 dorsal root fibers and 10.4 fibers in the trunk and dorsal branches. Thus it may be seen that the frog gains dorsal root fibers more rapidly than ventral root fibers, and, on the other hand, gains fibers on the distal side of the spinal ganglion, more rapidly than in either root. 8 There is a general tendency toward a more rapid addition of fibers during the warmer months of the year. That the results are not more decided, may be due either to the fact that the frogs used in winter were not in the normal hibernating condition, or that some ingrowing fibers may not reach their destination during a single season.