CENTRAL NERVOUS SYSTEM. 



185 



stimulation of the cord does give rise to an impulse in the afferent spinal 

 nerve, nevertheless the impulse is continually within the limits of one cell- 

 element. This shows that the reversed impulse can pass the spinal ganglion, 

 and in doing this it probably traverses the cell-bodies there located. There 

 is, however, no evidence that the stimulation of 

 the dorsal columns of the cord produces out- 

 going impulses in the dorsal nerve-roots except 

 when the stimulus is applied to the axones, 

 which are outgrowths of the cells of the spinal 

 ganglia. 



In the case of the interpolation of the cell- 

 body in the course of the axones there is every 

 reason to think that the nerve-impulse traverses 

 the body of the cell itself. This is suggested 

 by the changes caused in the cell-body of the 

 spinal ganglion-cells as the result of stimulating 

 the peripheral axone. Moreover, some observers 

 report an appreciable delay (0.036 second) in 

 the passage of the nerve-impulse through the 

 cell-body in the case of those cells which form 

 the spinal ganglion. 1 This delay has recently 

 been denied. 2 



The observations of Steinach, 3 on the capacity 

 of the afferent nerves of the frog to conduct the 

 centripetal impulses through the region of the 

 spinal ganglion, indicate that impulses may pass 

 this region when the cell-bodies are very prob- 

 ably excluded from forming a part of the possi- 

 ble pathways, thus showing that tho two branches 

 of the T-process are physiologically continuous. 

 These results do not show, however, that the 

 centripetal impulses fail, under normal condi- 

 tions, to pass to the cell-bodies also. It may be 

 pointed out that this is another piece of evi- 

 dence in favor of the view that within the 

 limits of a single neurone or fraction of a neurone there is no limitation to the 

 passage of a nerve-impulse in all directions, wherever it is started. 



Double Pathways. — [f the view is correct, that in passing through the 

 spinal ganglion the normal impulse traverses the cell-body, then the nerve- 



Pro. 75.— A longitudinal section 

 of the cord to show the branshing 

 of incoming root-fihres in dorsal 

 columns. At the left are three (DR) 

 root-fihres, each of which forms two 

 principal branches. These give off 

 at right angles other branches, col- 

 laterals, 0>l, which terminate in 

 brushes. O C, central cells, whose 

 axones give off similar collaterals 

 (Ramon y Cajal). 



'Gad and Joseph : Archivf. Anatomie ». Physiologic, 1889. 



2 Moore and Reynolds: Proceedings of the Fourth International Physiological Congress, 

 held at Cambridge, 1898. Supplement, vol. xxiii, Journal of Physiology. These authors deny 

 the delay. 



3 Steinach: " Ueber die centripetale Erregungsleitnng im Iterciche des Spinalganglions," 

 Pfiiiger's Archiv, 1899, Bd. 78. 



