6o8 THE NERVE CELL. 



a similar way. 1 This conjecture opens up a wide field of speculation, since it 

 is possible to extend it so as to embrace the explanation of many physiological 

 and psychical conditions. 2 Such speculations, however, unless they have been 

 tested experimentally, do not lie within the scope of this work. No one has 

 been able to see any amoeboid movement in the nerve cells of vertebrates, and 

 the only direct evidence of such movement in nerve cells, that we have any 

 knowledge of, has been furnished by Wiedersheim, 3 who observed slow changes 

 of form to occur in ganglion cells of the central nervous system of a minute 

 crustacean. It is true that various observers have described in Golgi prepara- 

 tions, appearances in the processes of nerve cells after excitation, or after the 

 action of anaesthetics and other drugs, which they interpret as indicating a 

 withdrawal or retraction of the finer processes. But it cannot yet be accepted 

 as proved that the changes which are described are invariable concomitants of 

 alterations in the functional condition of the cells, still less that they are the 

 cause of the functional conditions. 4 



Every nervous path is formed of a chain of nerve cells. The chain 

 of the simplest reflex process may be conceived as composed of only 

 two cells, the sensory and the motor, or afferent and efferent, root- 

 cells, and in such a chain there would be only one place of adjunction, 

 where the central process of the sensory cell comes in contact with 

 the dendrons or the cell body of the motor cell. On the other hand, 

 a path which includes any of the higher nerve centres or any complex 

 nerve processes, must have a chain of several cells, with a synapse 

 at the place of contact between each two links in the chain. There is 

 reason to believe that the additional delay (" lost time"), which is charac- 

 teristic of the passage of nervous impulses through the nerve centres, 

 is due to a block at each synapse ; that, in fact, the nervous impulses are 

 momentarily arrested at these places of contact of the nerve-cells with 

 one another. And it is not improbable that the relative number of 

 these blocks will furnish a key to the differences which are found to 

 obtain in the reaction time for different reflexes and psychical processes. 5 

 The differences of reaction time are too great to be accounted for simply 

 by the fact that the nervous impulses are sent along paths of different 



1 It appears certain that nice-tin, in blocking the passage of nervous impulses along a 

 nerve path, acts upon the synapses (Langley). 



2 The production of both natural and hypnotic sleep has been supposed to be due to 

 retraction of processes of the nerve cell, leading to a partial separation of the synapses 

 between some cells (Lepine, Compt. rend. Soc. de biol., Paris, 1895 ; Duval, ibid., 

 1895 ; and " L'amoeboidisme du systeme nerveux," Rev. scient., Paris, 1898) ; Rabl-Ruck- 

 hard, Neurol. Centralbl., Leipzig, 1890, Bd. ix. S. 199). Lugaro (Riv. di patol. nerv., 

 Firenze, 1898), on the other hand, considers that the condition of sleep is concomitant 

 with a general diffusion or protraction of nerve cell processes, and the state of attention 

 with retraction of some of the processes and gemmules, so that the paths of conduction for 

 nerve impulses are thereby narrowed, and the impulse waves become intensified. 



3 Anat. Anz., Jena, 1890, Bd. v. S. 673. 



4 See on this subject v. Kolliker, Sitzimgsb. d. phys.-med. Gesellsch. zu Wiirzburg, 

 1895 ; Ramon y Cajal, Arch. f. Anat. u. Entwcklngsgesch. , Leipzig, 1895 ; R^nant, Presse 

 mid., Paris, 1895, p. 297 ; Monti, Arch. ital. de biol., Turin, tome xxiv. ; Demoor, " De la 

 plasticity morpholog. des neurones," Arch, de biol., Gand, 1896, tome xiv. ; Stefanowska, 

 Compt. rend. Soc. de biol., Paris, 13th Nov. 1897 ; Manouelian, ibid., and Annie 

 Psychologique, 1898 ; Heger, Bull. Acad. roy. de mid. de Brig., Bruxelles, 1898, tome ix. 

 p. 831 ; Pupin, "Le neurone," These, Paris, 'l898 ; Deyber, These, Paris, 1898 ; M. Duval, 

 Compt. rend. Soc. de biol., Paris, 1895 ; Rev. d. sc. mid., Paris, 1898, 12 Mars ; Soukhanoff, 

 Cellule, Lierre et Louvain, 1898, tome xiv. ; Havet, ibid., 1899, tome xvi. 



5 The relative strength of the blocks in different individuals may also be the physical 

 cause of the individual differences in reaction time for the same stimulus. As is well 

 known, these differences gave rise amongst astronomers to the term ' ' personal equation " 

 being applied to the difference in noting the time of an observation which is found to 

 obtain, with a considerable degree of constancy, between any two observers. 



