CENTRAL NERVOUS SYSTEM. 727 



The table indicates a proportional relation at birth, and probably for a 

 short time after, different from that found at maturity, but this very early 

 approximates that found in the adult. 



Relation between Growth of Body and Encephalon. When the curve 

 of growth for the entire body is compared with that for the growth of the 

 encephalon, it is quite evident that the growth is more rapid in the central 

 nervous system than in the body at large, and that it is almost completed in 

 the former at the end of the eighth year, whereas the body has reached but 

 one-third of the weight which it will attain at maturity. 



A causal relation between a well-developed central system and the subse- 

 quent growth of the entire body is thus suggested, and also it is evident that 

 conditions which influence growth will at any time find the body on the one 

 hand, and the central system on the other, at quite different phases in their 

 development. 



The long-continued growth of the body brings it about that the central 

 system, which at birth may form 12 per cent, of the total weight of the indi- 

 vidual, is at maturity about 2 per cent, or less. For this change in proportion 

 the increase of the muscular system is mainly responsible. 



Further, the much smaller mass of the muscular system in the female is 

 the chief cause of the higher percentage value of the central system in the 

 female a relation which has been much emphasized, but which is really not 

 significant, since in both sexes this high percentage value of the central system 

 is most developed at birth, and becomes steadily less marked as maturity is 

 approached. 



Increase in the Number of Functional Nerve-elements. Having 

 thus briefly indicated the facts of growth so far as they can be detected by 

 the balances, it still remains to mention the series of changes which may be 

 studied by other means, such as micrometric measurements or enumeration. 

 The results obtained by these methods are somewhat complex and must be 

 treated with great care. Human embryology indicates that after the third 

 month of fetal life the number of cells in the central system is not increased. 

 With the cessation in the production of new cells the only remaining means 

 of increase in size is by enlargement of those cells already present. 



How this occurs is well indicated by the accompanying table (page 728), 

 which shows the change in the size of cell-bodies in a given locality in man. 



All vertebrates are not similar in respect to the manner of this change. 

 Birge * has shown that in frogs there is a gradual increase in the number of 

 the fibres forming the ventral and dorsal spinal roots, and that this goes on at 

 the rate of about fifty additional fibres in the ventral roots and seventy in the 

 dorsal, for each gram added to the total weight of the frog. The increase was 

 still apparent in a frog weighing one hundred and twelve grams. In the case 

 of the ventral root-fibres it was also determined that the cells of origin in the 

 ventral horns of the spinal cord increased in number in a similar manner. 

 Here is exemplified an instance of long-continued enlargement of the nervous 

 1 Birge : Archivfur Anatomic und Physiologic, Supplem., 1882. 



