216 THE GROWTH OF THE BRAIN. 



the t\vo nerve cells, unlike in the mass of their neurons, 

 except that the nerves impulse has a shorter path in the 

 shorter neuron. It must, however, be left undecided 

 whether a large neuron is either an advantage to the 

 nerve cell by adding to the quantity of cytoplasm which 

 can store energy, or a disadvantage as representing an 

 outgrowth which requires to be maintained. Perhaps 

 future experiments will gain something in definiteness 

 if in measurements the comparison be made not alone 

 between the cell-bodies, but between the volume of 

 these and also of the entire cell and the nucleus. 



Mason has studied the size of the nucleus in the frog 

 and certain reptiles. 1 His results indicate among the 

 nuclei relations similar to those found for the cell-bodies 

 of mammals examined by Kaiser. The animals repre- 

 sented in his table had widely different body-weights, 

 nevertheless there is no correlation between their body- 

 weights and the diameters of the nuclei as there 

 described. The explanation which Mason proposes is 

 based on the connection of the larger nuclei with the 

 cells controlling the most active and most bulky 

 muscles. Scattered observations on the nerve cells of 

 mammals indicate similar relations. 



Physiological reactions and the mass of nerve elements 

 are interdependent. Gaskell has emphasised the rela- 

 tion between the diameter 2 of the fibres and their func- 

 tion, when describing the bundle of extremely fine 

 fibres which in mammals passes out by the ventral roots 

 of many spinal nerves, and which he was able to trace 

 into the sympathetic system. It has long been a 

 familiar fact, also, that the different tracts within the 

 spinal cord are characterised by fibres of different calibre, 

 but in order to interpret this it is necessary to compare 



1 Mason, Journ, of Neru. and Ment. Dis., 1880, 1881. 



2 Gaskell, Journ. of PhysioL, vol. vii., 1885. 



