284 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



A careful study of the cortex 1 based on Hammarherg's records, gives 

 9200 million cell-bodies in this region alone. Considering the amount of 

 gray matter present in the rest of the central system, an estimate of 13,000 

 million for the total Dumber in the entire central nervous system is probably 

 a conservative calculation. 



From the foregoing tacts, 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 indi- 

 vidual. In such enlargements the chief increase is due to the formation of 

 the axones, 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 

 central system, perhaps accounting in some cases for the very heavy brains 

 occasionally reported. 



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

 the percentage of water in the nerve-tissues is high, but becomes less at 

 maturity. In white rats at birth the percentage of water for the encephalon 

 is 89 per cent, and for the spinal cord 85.3 per cent. At maturity it is about 

 78 per cent, for the encephalon and 70.1 per cent, for the cord. This change 

 is correlated in some measure with the development of the medullary sub- 

 stance. 



For the gross physical changes which have thus been indicated as occur- 

 ring during growth, an explanation is to be found in the changes affecting the 

 constituent elements, and these have been set forth when describing the 

 growth of the individual cells. 



C. Organization and Nutrition of the Central Nervous System. 



What is here meant by organization maybe 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 quantita- 

 tive, flie 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 

 growth the system becomes capable of new reactions in the sense that its 

 various responses are controlled and directed bv a larger number of incoming 

 impulses, and thus the number, complexity, and refinement of the reactions 

 are increased, and in this sense it really attains new powers. 



\\ ith the change in the age of the central system there occurs from birth 

 up to the prime of life, if we may judge from general reactions, an increase 

 1 Thompson: Journal of Comparative Neurology, 1899, vol. ix. 



