EFFECTS OF FORMALDEHYDE ON BRAIN OF RAT 309 



If we omit from table 11 the lowest record, both for the con- 

 trol and for the infected group, the remaining brain weights come 

 fairly within the limits of probable normal variation in the brain 

 weight of adult rats, as indicated in table 12. With this modifi- 

 cation in table 11, the brains of the infected animals are, on the 

 whole, slightly heavier than those of the controls, and the aver- 

 age percentage weight increase in these brains after treatment 

 with 4 per cent formaldehyde is 2.80 per cent greater than that 

 in the control group. Comparing the percentage weight increase 

 in the brain of each of the infected animals with that of the 

 control of the nearest brain weight, it is found that in every 

 case the percentage weight increase in the brain of the infected 

 animal is somewhat greater than that in the brain of the normal 

 animal. It has been shown that the brain of an animal infected 

 with pneumonia contains from 0.4 per cent to 0.5 per cent less 

 water than the brain of healthy animals of about the same age 

 (King '11). Pneumonia, therefore, not only decreases the per- 

 centage of water in the brain of the rat, but it also produces 

 changes which cause the brain to undergo a slightly greater per- 

 centage weight increase when subjected to the action of a 4 per 

 cent formaldehyde. 



The greater percentage weight increase in the brains of the 

 infected group of animals is not due to the fact that the brains 

 of these animals were larger than those of the controls. In ani- 

 mals of the same age there is often a considerable variation in 

 the brain weight, due to the fact that large animals have heavier 

 brains than do smaller ones. An examination of the records of 

 the 69 sets of brains of various ages used in these different series 

 of experiments show that in 34 cases only does the larger brain 

 undergo a greater percentage weight increase in 4 per cent form- 

 aldehyde. It is the age of the animal, not the size of the brain, 

 that is a factor in determining the amount of swelling that the 

 brain will undergo in a solution of formaldehyde. 



