EFFECTS OF FORMALDEHYDE ON BRAIN OF RAT 311 



increases the amount of swelling of the brain and has a corre- 

 spondingly bad effect on the cell structures. 



Keeping the formaldehyde solution at a temperature of 36°C. 

 produces marked changes in the entire brain. By the third day 

 the brains have a soft, gelatinous appearance which is not found 

 in brains kept in a solution of formalin at room temperature or 

 below. Preparations of these brains are practically useless for 

 histological purposes, as the nuclei are greatly distorted and only 

 faint traces of chromatin can be detected. A brain kept in a 

 neutralized solution at a temperature of 8 to 11 °C. for four weeks 

 shows a fixation of the cell structures about like that obtained 

 when the solution remains at laboratory temperature, but the 

 cell outlines are sharper and the cell contents stain much better. 



It is evident, from the results of these investigations, that 

 conditions which affect the amount of swelling of brains in 4 

 per cent fomialdehyde also affect the preservation of the cell 

 structures in the brain tissue. If, therefore, it is considered nec- 

 essary or advisable to preserve brains in formalin, the solution 

 should not be neutralized and it should be used at relatively 

 low temperature. This will insure a minimum amount of swell- 

 ing and permit good staining. A prolonged immersion in the 

 solution is unnecessary and decidedly injurious to the tissue cells. 

 If the brain is ever to be used for histological purposes it should 

 be transferred into alcohol as soon as it is fixed and hardened. 

 Nerve tracts are apparently not adversely affected by a formal- 

 dehyde solution, and material so preserved can be stained by 

 the Weigert method and used for investigations on the extent 

 of meduUation or of degeneration. A solution that swells brains 

 from 30 to 60 per cent of their original weight in three days is 

 obviously not an Meal cell fixative, and brain tissue preserved 

 in formalin is therefore unfit for cytological work. It is more 

 trouble, perhaps, to fix brains in Bouin's ('97) fluid or in the 

 solution of Ohlmacher ('97), both of which give very excellent 

 preparations of cell structures (King '11), but the superiority of 

 these fixatives over a simple aqueous solution of formaldehyde 

 cannot be questioned. 



