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 Eanvier (" ttranglements 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 Eanvier 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. Ean- 

 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 each 

 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. 153. Nerve - ceii lower vertebrates (fishes) there are several nuclei 

 (5-18, according to Key and Eetzius) in each 

 segment. In their finer structure the nuclei of 

 Schwann correspond essentially with other cell- 

 nuclei. While the sheath of Schwann forms a complete tube, 

 investing the fibre closely on all sides, and surrounding the axis- 

 cylinder even at the nodes of Eanvier, where the medullary sheath 

 is interrupted (infra), the latter exhibits segmentation apart from 

 the constrictions of Eanvier. After death, the double contour of 

 the medullary sheath is interrupted by oblique lines on either side 

 of the nodes of Eanvier, which give the appearance of longer or 

 shorter segments of medulla, fitting together with funnel-shaped 



Z? 



ganglion of rabbit, 



