280 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



growth of the encephalon appears to be an important factor in the cause of 

 death ; hence the larger brain-weight found at autopsies during these years. 

 While, in general, the individual may be supposed to follow, in the develop- 

 ment of his encephalon, the course here indicated by the curve, this premax- 

 imal increase must be excepted for the reasons given. 



Relation between Growth of Body and that of Encephalon. — On com- 

 paring the growth of the entire body with that of the encephalon, it is evi- 

 dent 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 at that time 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 propor- 

 tion 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 signifi- 

 cant, since iu both sexes this high percentage value of the central system is 

 mosi 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 bv 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 (p. 281), 

 which -hows 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. 

 I>irge ' 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 112 grams. In the case of the ventral 

 1 Birge : Archivfur Anatomie und Phyxiologie, suppl., 1S82. 



