17 THE GROWTH OF THE BRAIN. 



excluding their neurons, do not certainly form more 

 than one-tenth of the entire weight of a sample of grey 

 substance. The cellular portion weighs therefore 6 per 

 cent., and the white portion, consisting of the neurons, 

 weighs 94 per cent, of the entire encephalon. Employ- 

 ing the previous calculations whereby the mass of the 

 medullary substance is about one-half of the mass of the 

 neuron, then one-half of this weight, or 47 per cent, is 

 medullary substance proper. At maturity, therefore, 

 this substance alone has a weight nearly twice that of 

 the entire encephalon at birth. But no special argu- 

 ment is needed to show the importance of this factor in 

 the determination of brain-weight. Its quantity is pro- 

 bably variable within wide limits, yet the degree of this 

 variation has never been investigated. 



That form of forced growth which is called regenera- 

 tion, the capacity for which is so early lost by mammals, 

 is very remarkable, and will repay attention as a property 

 of nerve cells. A distinction should be made, however, 

 between the regeneration which involves the reproduc- 

 tion of new specialised elements by cell division, and 

 the regeneration of a lost portion of a cell by that part 

 which still persists. When in a mammal, for example, 

 a peripheral nerve is cut, it can regenerate, and this 

 regeneration can involve the production of a large 

 quantity of material and also be successively repeated 

 as many as three times. It is a remarkable function of 

 the peripheral nerves, and contrasts sharply with the 

 conditions within the central system, where such regene- 

 ration does not occur. So far, then, as the central 

 nervous system is concerned, it cannot renew itself in 

 the sense of regenerating elements, either lost or injured, 

 whereas injured peripheral nerves can be repaired. 



From the facts concerning the number and size of the 

 structural elements, it is evident that the enlargement of 



