732 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Estimates of the Volume of the Central Nervous System. Encephalon and Spinal 



Cord at Different Ages. 



Three-quarters of this volume is assumed to represent the nerve-elements proper. For the 

 first two records I am indebted to Professor F. P. Mall. The third is estimated. 



From the foregoing facts, together with those bearing on the cell-elements, 

 it is possible to get some conception of the growth-processes in the central 

 system, and to see how they are due to an enlargement of the nerve-elements 

 which have been formed at a very early stage in the life-history of the 

 individual. In such enlargements the chief increase is due to the formation 

 of the neurons, and in them, in turn, about half the substance is represented 

 by the medullary sheaths. 



In all probability these sheaths are no exception to the rule according to 

 which all parts of the body are variable, not only in their absolute but also in 

 their relative size, and therefore it is possible that the quantitative variation in 

 this constituent is a very important factor in modifying the weight of the cen- 

 tral system. 



Change in Specific Gravity with Age. During fetal life and at birth the 

 specific gravity of the nerve-tissues is low, but becomes higher at maturity. 

 This change is correlated in some measure with the development of the medul- 

 lary substance. 



For the gross physical changes which have thus been indicated as occurring 

 during growth an explanation is to be found in the changes affecting the con- 

 stituent elements, and these have been set forth when describing the growth 

 of the individual cells. 



0. ORGANIZATION AND NUTRITION OF THE CENTRAL NERVOUS 



SYSTEM. 



What is here meant by organization may be easily illustrated. When, for 

 example, by later growth new tissue is added to the liver or the skin is in- 

 creased in area or a muscle enlarged, there is caused by the addition of new 

 substance a change in the powers of these tissues, which is mainly quantitative. 

 The larger organ exhibits the same capabilities that the smaller organ exhibited, 

 but does so in a greater degree. 



In the central nervous system, on the other hand, it appears that with 



