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Information about the physiology of C fibers has been based largely upon studies of visceral nerves; and the properties revealed have, therefore, been those of the unmedullated axons of the postganglionic components of the sympathetic nervous system. An indication that the C fibers in mammafian skin nerves might have properties different from those of their sympathetic analogues first appeared in 1938 (Grundfest and Gasser), when it was observed that the curve of recovery of excitability of C fibers in the saphenous nerve of the cat was different from the one found in the examination of visceral nerves. Subsequent investigation has brought to light additional points of difference, to such an extent that it is now necessary to divide the C group of fibers into two subgroups. Much further work will be required to supply a full description of these fibers. Consequently, in connection with the principal subject of this paper, which is the form of the compound action potential and its relationship to the fibers which produce it, it will be necessary to bring forward in a preliminary way mention of characteristics still awaiting more complete analysis. Information about these characteristics is required both for the interpretation of the action potential and for support of the thesis that there is a group of unmedullated fibers, afferent in function, specialized in their properties, and systematized in the same sense in which the medullated fibers in mammalian skin nerves are known to he systematized.To meet the demands of the problem two separate lines of approach to it were required: a study of the functional behavior of the fibers, and a study of their morphology. The results of the two studies will be described in the order mentioned; and finally there will be pointed out the parts of the two studies which can be brought into relationship. SECTION I THE ACTION POTENTIALMet~ds.--The principal technical difficulty arose in the maintenance of a steady base line, the need for which will become apparent as soon as the records are examined. Three imposed experimental conditions converged toward the production of unsteadiness: the strong shocks required for stimulation, the high gain required for recording,

Unmedullated nerve fibers physiologically carrying signals in the afferent direction became of interest as a group apart, when it was learned that those in the skin nerves, having their origin in cells of the dorsal root ganglia (d.r.C), possess properties different from those with cells of origin in the sympathetic ganglia (s.C). It has long been known that the fibers of the olfactory nerve are unmedullated. Therefore, the question naturally suggested itself as to whether the olfactory fibers are relatable to the d.r.C fibers. In as much as the fibers in mammals are too short for investigation with the methods used for other fibers, it was decided to start with observation of their dimensions with the aid of the electron microscope. A picture was revealed unlike that of any nerve previously examined: extremely small fibers in great numbers. How such fibers might arise from the bipolar cells of the olfactory mucosa was a problem that could only be answered by an examination of the mucosa itself. In the course of the examination characteristics of the mucosa, as yet undescribed, came into view as a by-product. Descriptions are here given of the fibers in relation to the mucosa; and finally of some observations on the olfactory nerve of the pike, which will show that the fibers do not correlate with the d.r.C fibers.Although classification of the olfactory fibers with the d.r.C fibers is not permi.~sible each provides the best morphological analogy for the other. Embryologically both arise from thickenings of the ectoderm, placodes by the forebrain for the olfactory fibers, the neural crest for the d.r.C fibers. As the neurons form, an axonal projection is directed toward the central nervous system, and a dendritic projection toward the periphery, there to terminate in free endings of great fineness. Small as are the diameters of the cilia on the olfactory dendrites they are matched by the terminals of the d.r.C fibers in the skin (Gasser, 1954). In a previous paper it was shown that the axonal branches of the d.r.C fibers are much smaller than the dendritic branches, and that they are often connected in bunches to the Schwann membrane by their mesaxons. The pattern first revealed in connection with the d.r.C neurons is * In the preparation of the electron microscope pictures used in the references to the cytology of the olfactory mucous membrane. 473

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