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Summary of 16 experiments on the effect of stretching the nerve on the rate of the nervous impulse in the pedal nerve of Ariolimax columbianus.
five experiments (Table IV, Nos. 3, 10, 11, 14 and 16) the rate of the inipulse in the two conditions in the nerve is identical, and in two experiments (Table IV, Nos. 13 and 15) the rate in the stretched condition is slightly higher than that in the relaxed or contracted condition. The average rate of all the sixteen slugs is 35 cm. per sec. for the stretched nerve, and 37 cm. per sec. for the contracted nerve. But these differences are slight when one considers the difficulties in applying the peripheral electrodes to exactly the same point of the nerve throughout the successive alternations, and the difficulties in obtaining exact measurements of the length of the nerve, because of the necessary shifting of both pairs of electrodes, not to mention the difficulties in obtaining comparable tracings, due to the unequal relaxations of the muscular part of the preparation. The records leave no doubt in regard to the uniformity
of the rate in the nerve in the different states of extension and contraction within the limits indicated. In the pedal nerves of Ariolimax, stretching the nerve within physiological limits increases the transmission time for the whole nerve while contraction or shortening of the nerve decreases it, but in each change of length of the nerve the velocity in a unit of length is the same, that is the rate is the same in the two conditions.
It is obvious that if the change of the length of the nerve was due to the straightening out or the formation of folds and kinks either in the nerve as a whole or in elements in individual nerve fibers the transmission time between two constant points of the nerve would be the same, and the rate would appear greater in the stretched condition as compared to that of the contracted condition. But the fact that the transmission time between any two points increases with the stretching of the nerve seems to show that the stretching is accompanied by actual extension of the conducting substance, whatever that may be. And the fact that the actual rate remains the same in the two conditions of the nerve seems to prove that this rearrangement of the conducting substance does not change the rate of the conducting process.
Furthermore, this rearrangement of the molecules of the conducting substance within the wide limits represented by extending the nerve to twice in length does not appear to effect the functional properties of the nerve either in the above experimental conditions or in the normal conditions of the animal.
These facts are certainly evidence on the side of the view that the conducting substance in this nerve is in a liquid condition or at least in a semi-liquid condition.
These experiments also confirm measurements of the rate of the nervous impulse in the pedal nerves of Ariolimax reported by us in which the average rate was found to be 40 cm. per second. These records show an average rate of 36 cm. per second, the slightly lower figure in the latter case being in all probability due to the greater number of records used from each preparation, as it will be seen from Tables I, II and III that the rate decreases rapidly during the course of an experiment.
They also show that the particular amount of stretching of the nerve within the physiological limits does not need to be regarded as a source of error in determining the rate of the nervous impulse in this slug.
THE NERVOUS STRUCTURES IN THE PALATE OF
THE FROG: THE PERIPHERAL NETWORKS
By C. W. PRENTISS,
Instructor of Biology, Western Reserve University.
With 12 figures. On account of the doubts which have recently been thrown upon the neurone theory by the researches of APÁTHY, BETHE and others, especial attention has been drawn to the networks of cells and fibers which apparently form an important part of the peripheral nervous system in most Metazoa.
In his recent book on the nervous system BETHE (:03) discusses at some length the comparative histology and physiology of these structures. According to his own investigations and the observations of HESSE ('95), the brothers HERTWIG ('78), and EIMER ('79), the nervous system of the Medusae is composed largely of nerve cells and fibers which are united into a diffuse network. The neurofibrillae of this network form a basketwork about the nuclei of the cells, and are connected both with muscle-fibers and with sensory organs in the epithelium of the sub-umbrella. Smidt (:02) describes a subepithelial plexus in mollusks; both he and BETHE demonstrated its connection with sensory organs, and according to the physiological experiments of the latter it also sends motor fibers to the muscles.
Among the arthropods similar structures were first observed by HOLMGREN ('95). Later BETHE ('96) described peripheral networks in Crustacea, and his observations were verified by HOLMGREN ('98) and NUSBAUM and SCHREIBER ('97).
In the nervous system of vertebrates networks of cells and fibers have been studied chiefly in connection with the blood vascular system. They have been described by DogIEL ('93, '98), LEONTOWITSCH (:01), CAVALIÉ (:02), BETHE ('95, :03) and others. BETHE states that such networks are present throughout the whole integument of the frog. They form a close network about the blood vessels and a wide-meshed subepithelial plexus. DOGIEL (98) and LEONTOWITSCH (:01) have carefully studied the networks in the human skin. They assert that connections exist between these structures and the medullated fibers, but their figures do not convince one of this. Their statements have, however, been verified by Bethe (:03) who figures a medullated fiber continuous with a wide-meshed sub-epithelial plexus. In the vertebrate heart also, DogIEL ('98), HofMANN (:02) and BETHE (:03) have observed independently a network of cells and fibers surrounding the muscle bundles. This network resembles closely the diffuse nervous system of the Medusae and BETHE maintains that the structures he has seen are undoubtedly of a nervous nature.
It has been stated even recently by certain investigators, that both the cells and fibers composing these networks are nonnervous structures. BARDEEN (:03), among others, has exexpressed his doubts as to their nervous character; he criticises LEONTOWITSCH and suggests that the whole network described by the latter may be composed entirely of connective tissue. Even if the fibers are nervous structures the cells may be merely sheath cells.
It is important both to the teacher and student of neurology that these doubts be either confirmed or entirely removed. If networks of true nerve cells and fibers really exist in the integument of vertebrates, then the idea that the peripheral nerves originate only from ganglion cells in or near the central nervous system must be abandoned. If, however, the networks are proved to be nothing more nor less than connective tissue structures, the opponents of the neurone theory have lost one of their strongest arguments.
My research was begun in the Physiological Institute of