The aim of the present series of experiments is to try to clarify these controversial issues and to determine whether the dorsal root ganglion modified in any way the passage of sensory impulses. Besides suffering a delay under normal conditions, the sensory impulses are found to be blocked in their passage through the dorsal root ganglion during the early relatively refractory period of the afferent fibres. METHODIn most of the experiments the 9th or 10th dorsal root ganglion of the frog (Hyla aurea, Rana cate8biana and R. pipiens) was used. These ganglia were dissected intact and in continuity with the roots and the trunk. The roots were severed adjacent to the spinal cord. The trunk was either cut prior to its entry into the sciatic plexus, or followed into one or several of the branches of the plexus, e.g. (1) posterior femoral cutaneous nerve, (2) lateral crural cutaneous nerve, (3) tibial nerve, and (4) peroneal nerve (see Ecker, 1881). The fibres of the posterior femoral cutaneous nerve have been found to go mainly to the 10th dorsal root, while those of the lateral crural cutaneous nerve go mainly to the 9th. The preparations were always scrupulously cleaned to remove connective tissue and blood clots. Every effort was made to avoid injury ofthe preparation.In some experiments requiring a small number of nerve fibres the 11th dorsal root ganglion was used. This ganglion is very small, lying on the lateral surface of the columella. As a considerable segment of its nerve trunk is buried in the coccygeal muscle and as its roots are thin and penetrate * Part of the work was carried out in the Department of Physiology, University of Otago Medical School, Dunedin, New Zealand, and has been reported as short communications (J. cell. comp. Physiol. 38, 131-135, 1951).

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glioii aiid dorsal root, another pliase of iiiteractioll between iier~7c iiiipulscs in neiglibouring fibres can be clearly cleinonstrated. When the interval between tlie two stimuli is less t I i a i i 0.4 iiisec tile irnpulscs show piwloniiiit-iiitly syncliroiiization. IYitli longer intervals there appears to hc also a n effect of desynclironizatioii. These effects should be detectable all aloiig tlie nerve, but are more evident a t tlie points of bifurcation. Tlie negative results reported by voii Bruckc and Early ('41) were due to the fact that they recorded tlic potentials from one point alone and compared oiily tlie actual composite potential with the calculated sum of tlie two single volleys. IVitli each volley consisting already of a great number of iiiipulses, which also interact between tlieniselres, sncli kind of comparison is no longer sensitive enougli.

STUDIES of the action potentials in sensory nerve fibres have led to a grouping of sensory endings as slowly adapting and rapidly adapting, the former giving a long-continued and the latter a brief discharge to a persistent stimulus. Many of the receptors in the frog's skin are of the rapidly adapting type, the discharge to a continuous stimulus lasting less than 0-1 sec.; others adapt somewhat more slowly (the discharge lasting from 1 to 100 sec.), while the stretch receptors in mammalian muscle, lung and other organs adapt still more slowly, the discharge usually continuing many minutes or even several hours. It is generally assumed that these differences express the constitutional properties of the nerve endings, but there is another possibility which has never been fully explored. This is that the more rapid failure of the discharge is due merely to rapid failure of the condition which acts as the immediate stimulus to the ending. A small weight resting on the skin produces a continued deformation of the skin, and in that sense the tactile stimulus may persist; but owing to yielding and readjustment of the tissues the deformation of the sensory nerve endings might occur only when the weight is first applied. In fact the rapid adaptation of the cutaneous endings might be due merely to a rapid regaining of their shape or position in spite of a continued pressure on the skin.To test this point we have made preparations of the frog's skin in which we could identify a tactile ending giving a brief discharge to light pressure and have then stimulated it by stretching the skin laterally. Direct observation of the skin structures makes it appear unlikely that there will be any tendency for the endings to return to their normal position while the stimulus is in being. Since we find that rapid adaptation is still a feature of the nervous discharge, we conclude that it is due to the properties of the sensory endings and not of their surroundings.

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