. Electro-physiology. Electrophysiology. 42 ELECTRO-PHYSIOLOGY CHAP. thread, with a simple contour, which is constricted here and there at irregular intervals, known after their dis- coverer as nodes of Ranvier (" dtranglements annulaires") (Figs. 152, 153). The length of segment between the nodes is con- siderably greater in the inferior vertebrates (fish, amphibia) than in the higher, so that in the former there are fewer nodes in the same tract; corresponding per- haps with varying consumption of materials, if, as Ranvier believes, the nodes are to be regarded as points of entranc
. Electro-physiology. Electrophysiology. 42 ELECTRO-PHYSIOLOGY CHAP. thread, with a simple contour, which is constricted here and there at irregular intervals, known after their dis- coverer as nodes of Ranvier (" dtranglements annulaires") (Figs. 152, 153). The length of segment between the nodes is con- siderably greater in the inferior vertebrates (fish, amphibia) than in the higher, so that in the former there are fewer nodes in the same tract; corresponding per- haps with varying consumption of materials, if, as Ranvier believes, the nodes are to be regarded as points of entrance for the nutritive fluids. It may also be remarked that the electrical nerves of Torpedo, as well as embryonic nerves, have invariably shorter and more numer- ous segments than fully-developed fibres. Ran- vier's nodes, again, are found at all dividing points of peripheral medullated nerve - fibres, while in the central elements their existence is doubtful. Near the constrictions, in fresh, peripheral, medullated fibres, are the long nuclei of the sheath of Schwann, which lie at the side of the fibres, and seem to be embedded in the medullary sheath. The fact that (in higher vertebrates) one nucleus is placed centrally be- tween each pair of nodes has, along with other facts (infra), given rise to the view that eacli nerve - fibre is due to the fusion of several cells, a theory which can hardly be maintained in the face of embryological researches. In the FIG. las. — Nerve - ceii lower vertebrates (fishes) there are several nuclei "r£enSSIE (5-18, according to Key and Retzius) in each ganglion of rabbit, segment. In their finer structure the nuclei of Schwann correspond essentially with other cell- nuclei. While the sheath of Schwaun forms a complete tube, investing the fibre closely on all sides, and surrounding the axis- cylinder even at the nodes of Ranvier, where the medullary sheath is interrupted (infra), the latter exhibits segmentation apart from the constricti
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