316 THOMAS J. HELDT 



logical function of the cell. Secondly, since a physiological func- 

 tion is progressive, one must, in order to study this condition, 

 be prepared to interrupt that function at any given point. He 

 would interrupt the functional state and immediately preserve 

 the tissue in that vital condition until such time as would be 

 convenient to continue the technique. The physiological proc- 

 esses of the central nervous system proceed with great rapidity 

 and post-mortem changes enter in like manner, so for a physio- 

 logical-histological investigation he regards our general neuro- 

 cytological technique as unsuitable. 



Mollgaard's paramount aim is, therefore, the elimination of 

 post-mortem changes. To attain this end he takes the tissue 

 from the living animal and quickly drops it into a freezing mix- 

 ture in which it is at once frozen. Since at the low temperature 

 (ca. — 40°C.) of the freezing mixture practically all chemical reac- 

 tions are almost completely interrupted, Mollgaard maintains 

 that the tissue is preserved in the same vital condition in which 

 it was when committed to the fluid. The next step rests upon 

 the fact that the central nervous system has, at a temperature 

 of about — 20°C., a consistency like that of paraffin and may 

 be cut into sections of 5 to lO^u in thickness without previous 

 fixation or embedding. The sections as fast as made are con- 

 signed to a fixative of similar temperature which at once fixes 

 them and so permanently preserves the sections in a 'vitally- 

 fixed' condition. 



More in detail, Mollgaard's technique is briefly as follows: 

 From the living animal (rats, dogs, etc.) upon which with the 

 aid of an anaesthetic, he has previously performed a craniotomy 

 or a laminectomy, he excises a small piece of brain or cord tissue, 

 and immediately drops the same into a freezing mixture, con- 

 sisting of carbon tetrachloride, xylol, and absolute alcohol. The 

 temperature of this mixture has been reduced to about — 40°C. 

 by the addition of carbon dioxide snow. The animal is then 

 killed, generally by cutting its throat, and pieces of the brain 

 or spinal cord taken and placed in the freezing mixture at vari- 

 ous intervals after death for the purpose of later comparison. 

 After a sojourn of varying length in the freezing mixture, the 



