152 THE GROWTH OF THE BRAIN. 



meter. It is known, for example, that continuous fibres 

 pass from the cortex of the cerebrum to the lumbar 

 enlargement of the spinal cord, and it therefore follows 

 that certain neurons must extend over this distance, 

 which, within the central nervous system, is the longest 

 distance that is thus traversed. In the peripheral system 

 the fibres stretch from their point of origin in the cord 

 to the most distant portions of the limbs, and in man 

 this is a greater distance than any within the central 

 system. In each instance, be it remembered, the fibre 

 reaching for this distance is the continuous outgrowth 

 from a single cell-body. 



Confining attention to the cell-bodies only, it is found 

 that the volume of the germinal cell-body in the cord of 

 the fcetus as compared with that of the nerve cell-body 

 in the cord of the adult, may be as I to 100. From this 

 cell-body proceeds the neuron, entirely wanting in all 

 germinal cells, but which later on may, in some cases, 

 acquire a length of three feet. Quantitatively this out- 

 growth is the most important portion of the cell ; for 

 even when the axis-cylinder alone is considered it may 

 have a volume of more than 500 times that of the adult 

 cell-body. There is thus developed a relation between the 

 cell-body and its outgrowth which is quite unique, since 

 the outgrowth is very many times more bulky than the 

 body which gives origin to it. It will be necessary to 

 use numerical statements in the subsequent argument, 

 and it will therefore be desirable to give a record of the 

 manner in which they have been obtained. The unit of 

 measure employed by histologists is the micron. Its 

 value is O'OOi mm., and its symbol the Greek letter p. So 

 far as possible, the following measurements will be given 

 in this unit, since the figures will thus be most easily 

 compared. In the accompanying table the value of the 

 linear, square, and cubic millimeter is given in microns, 



