NERVOUS SYSTEM 



115 



but undergoes a regular change in weight and volume. The 

 average percentage gain of each pair of brains in an age group 

 at five intervals up to 75 days is given in table 78. 



These data show a rapid increase in weight at seven days, 

 followed by a slow steady loss of such a nature that after 75 

 days in the fluid the weight of the brain is from 23 to 35 per 

 cent more than when fresh. Age is the main condition which 



TABLE 78 



Percentages of gain of pairs of albino rat brains in Muller's fluid arranged 



according to age and weighed at intervals from twenty-four hours to 



seventy-five days 



modifies this reaction. The brains of the older rats gain more 

 and retain a greater gain at 75 days. The initial weight of the 

 brain also affects the result, since at like age the lighter brain 

 gains more during the early part of its stay in Muller's fluid, 

 but this difference tends to disappear later. 



The increase in the weight of the brain is due mainly to the 

 taking up of water, but the percentage of salts deposited in the 

 fixed tissues is much greater than that in the fixing solution. 

 The increase of the myelin with age explains some of the dif- 

 ferences here observed. 



