300 HELEN DEAN KING 



weights given in tables 4 and 5. Where the solutions were neu- 

 tralized the form of the graphs is practically the same whether 

 40 or 20 cc. of the solution was used, but graph C falls consid- 

 erably below graph A at every point. Where the solutions were 

 not neutralized the form of the graphs show more variation, but 

 graph D, for most of its length, runs higher than graph B. Since 

 the diluent action of the tissue fluids is undoubtedly greater 

 when 20 instead of 40 cc. of solution is used, it is evident that 

 a stronger neutralized solution of 4 per cent formaldehyde causes 

 more swelling in brain tissue than does a weaker neutralized solu- 

 tion, whereas the reverse is the case where the solutions are not 

 neutralized. 



Series 5. Since the temperature at which a solution acts is 

 known to have a marked effect on the rate at which the solution 

 will be absorbed, a final series of experiments was made to ascer- 

 tain how different temperatures would affect the swelling of rats' 

 brains in 4 per cent formaldehyde. In all of these experiments 

 each brain was put into 40 cc. of a solution that was freshly made 

 and neutralized when wanted for use. The bottles containing 

 one set of brains were kept in a water bath at a constant temper- 

 ature of 36°C. for four weeks: the corresponding set of brains 

 remained at a temperature of 8 to 11 °C. for the same length of 

 time. 



The data for the brains kept at the higher temperature are 

 given in table 8. 



In these experiments, as shown in the table, the maximum 

 weight increase was reached in every case at the end of the first 

 day. The decrease in weight at three days was practically the 

 same for the brains of all ages, amounting to about 8 per cent. 

 Subsequent weight changes were comparatively slight and, except 

 in the very young brains, the final weighings differed but little 

 from those noted for the third day. In this instance, also, there 

 is a direct relation between the age of the animals and the per- 

 centage increase in brain weight, but the average percentage gain 

 for the entire set of brains is considerably less than that found 

 in the experiments in which the brains were kept in a neutralized 

 solution at room temperature (table 4). These results accord 



