DEVELOPMENT OF NERVE ELEMENTS. iyi 



the nervous system is due in the first instance mainly to 

 an increase in the number of neuroblasts formed, and in 

 the second, to the size which they later attain, and that 

 to the increase in size are due those growth changes 

 which are usually recorded. The non-nervous tissues 

 in the central system are assumed to represent 25 per 

 cent, of its total volume, and to have the same proportion 

 at different ages, this being but a gross approximation, 

 allowable only in the absence of better data. The in- 

 terpretation now to be attempted is based on the notion 

 that in organic evolution structure precedes function in 

 the sense that the- slightly specialised elements are not 

 at once utilised, and that a partial development of nerve 

 cells may result in their presence in considerable masses, 

 without implying a corresponding physiological activity. 

 This being granted, the further inquiry is as follows : 



i . By wliat means does tJie brain of the new-born attain 

 the weight found in the adult and decrease again during 

 old age ? The weight of the male child's brain at birth 

 being taken as 372 grammes, and that of the adult male 

 as 1,360 grammes, there is an absolute increase of 988 

 grammes to be explained. The brain increases in weight 

 during this time by the increase of its non-nervous con- 

 stituents and by the enlargement of its nerve elements. 

 These latter changes are dependent on the increase in 

 the mass of the cell-body and cell outgrowths, especially 

 the neuron, and in the acquisition by the neuron of a 

 medullary sheath. Calculation shows that the absolute 

 mass of the medullary substance is the chief source of 

 increase in weight during this period. The weight 

 increase in the .nerve elements proper is due to the 

 enlargement of those cells which at birth are small, 

 the addition of new cells being excluded since their 

 formation ceased before birth. In answer to the first 

 question, it can therefore be stated that between birth 



