74 THE GROWTH OF THE BRAIN. 



as the skeletal muscles are concerned, hyperplasic growth 

 as a normal occurrence is generally denied. Certain it 

 is that ordinary exercise fails to increase the bulk of 

 the muscles after a comparatively early period namely, 

 the twenty-fifth year, or the probable time of the cessa- 

 tion of growth, and that during the growing period the 

 increase in weight is largely if not entirely the result of 

 an increase in the size of the original fibres. If we are 

 to judge from the activities of those elements, which by 

 division give rise to the nerve cells, it is at a time some 

 months before birth that the hyperplasic growth in the 

 central nervous system of man terminates. But in 

 attempting thus to sharply separate these two processes 

 a false idea must not be conveyed. Strictly interpreted, 

 hyperplasia implies the formation of new cells. Yet at 

 birth, and for a long time after, many systems contain 

 cell elements which are more or less immature, not 

 forming a functional part of the tissue, and yet under 

 some conditions capable of further development. The 

 changes caused by the continued growth of these cells 

 are not, strictly speaking, to be classed under either of 

 the two heads just given, although for practical 

 purposes organs enlarging in this way may be con- 

 sidered as undergoing hyperplasic increase. 



It is evident from this brief survey how complex a 

 process increase in weight may be : it may be due to a 

 swelling of the cells already functional, the develop- 

 ment of immature cells, or the production of new cells, 

 and these processes may be combined in any proportion, 

 while accompanying them in every organ is a variable 

 increase in the connective tissue framework and the 

 system of nutritive channels. 



Associated with the study of these changes are two 

 statistical difficulties of some importance. The obser- 

 vations based on the weight of portions- of the body are 



