52 ANATOMY 



5. If rats' brains are subjected to the action of a solution of for- 

 maldehyde that is kept at a constant temperature of 36C., they under- 

 go a greater amount of swelling than is produced when the solution is 

 kept at a temperature of 8 to 11C. The maximum weight increase 

 in the brains is reached by the end of the first day in the former case, 

 and not until the third day in the latter case. 



6. When the conditions under which the solution acts are uni- 

 form, the maximum weight increase in rats' brains subjected to the 

 action of a 4 per cent solution of formaldehyde is attained in all cases 

 by the third day, and there is then a gradual decrease in weight. Brains 

 of very young animals tend to reach the maximum earlier than do those 

 of older animals. 



7. The percentage weight increase in rats' brains as the result of 

 the action of a 4 per cent formaldehyde solution tends to be greater 

 in the brains of young animals than in those of adults. 



8. In animals of the same age the larger brain does not show a greater 

 percentage weight increase after treatment with a solution of formal- 

 dehyde than does the smaller one. 



9. A 4 per cent solution of formaldehyde extracts solids from the 

 brains of rats of all ages. This is shown by the fact that the percentage 

 of solids in brains that have been subjected to the action of such a 

 solution is always less than that found in the fresh brains of animals of 

 the same age. Brains of very young rats lose much more of their solids 

 than do brains of older animals. 



10. Brains of animals infected with pneumonia show a slightly 

 greater percentage weight increase when treated with a 4 per cent 

 solution of formaldehyde than do the brains of healthy animals. 



11. Even under the most favorable conditions an aqueous solution 

 of formaldehyde is not a satisfactory fixative for the cell structures in 

 brain tissues, as it causes a pronounced distention of the nuclei and 

 gives a poor preservation of the nuclear contents. 



The more important data are given in tables 36, 37, 38, 39, 40. 



