286 SCIENCE PROGRESS. 



or destroyed there is no reproduction of more. Their 

 number is fixed, but individually they possess enormous 

 power of growth, developing through a long period (pro- 

 bably far on into adult life of the organism) both in 

 complexity and size. The length of some of the slowly 

 grown cell branches is more suitably measured by feet 

 instead of the thousandths of a millimetre used with 

 other cells. They are also remarkable for their length 

 of individual life ; in a centenarian we may consider 

 his nerve-cells have led unbroken individual existences 

 for more than a hundred years, though many within that 

 time have probably reached their span and quietly "run 

 down ". 



But though unable to produce new cells the nerve-cell 

 is able to reproduce its own branches when they have been 

 torn through or otherwise destroyed. Like the bits into 

 which Verworn 1 tore up the Radiolarian, the cell portions 

 containing no nucleus enter slow degeneration and die 

 (Wallerian degeneration) ; but that containing the nucleus 

 repairs itself and reforms a perfect cell. It grows its pro- 

 cesses again and with a tolerable accuracy. If education is 

 related to the throwing out of new " dendrons " and " col- 

 laterals " by nerve-cells, 2 some cells for the greater part of 

 life retain a power not merely to repair a cell branch but to 

 put forth new twigs. If so, a part of the mysterious 

 restoration of function following destructive lesions of 

 the nervous system may perhaps be due to actually 

 new structural connections. The existing nerve-cells 

 may not merely restore some of the old lines where 

 broken, but push out communications altogether new. 

 Fresh combinations thus obtained may lower or heighten 

 pre-existing resistances and will at least partially rearrange 

 them. 



Even these brief annotations will have sufficed to 

 indicate how difficult at present to describe the re- 

 actions of the cortical regions, without in the description 



1 Pfliiger's Archiv, vol. li., p. i, 1892. 



2 Ramon-y-Cajal Nuevo conceptod. L Histol. d. 1. Centros Nerviosos, 

 Barcelona, 1893. 



