312 RETROGRESSIVE CHANGES 



great practical importance in causing death from fever, for 

 although 47 C. (117 F.) is probably never reached in man, 

 yet application of much lower temperatures, even 42 (108 F.), 

 for a few hours will cause coagulation of these proteids (all pro- 

 teids coagulate at less than their ordinary coagulation point if 

 the heating is continued for a long time). It would seem from 

 the above observation that heat causes cell death through coag- 

 ulation of the proteids. Whether the cell death is in any way 

 dependent upon destruction of the enzymes by heat has not 

 been ascertained ; but as most enzymes are not destroyed much 

 below 60 70, it seems improbable that they are greatly 

 injured at the temperatures at which cells are killed. It is 

 possible, however, that under the conditions in which enzymes 

 exist in the cell they may be more susceptible to heat than 

 under normal conditions. Just how coagulation of cell globu- 

 lins can determine the death of a cell is difficult to understand, 

 unless the physical conditions of the cell are greatly altered 

 thereby. Ordinarily we have in the cell an equilibrium between 

 colloids in solution and colloids in the solid or gel state ; if the 

 colloids are rendered insoluble by heat, so that this equilibrium 

 is destroyed, serious alterations in the mechanism of all metab- 

 olism must result (Mathews). 



Different tissues show unequal susceptibility to heat. "Wer- 

 hovsky l found the blood most affected by raising the tempera- 

 ture of living animals, next the liver, kidneys, and myocardium 

 in order, the other tissues being little or not at all structurally 

 injured. 



Cold is well withstood by unicellular forms, and relatively 

 poorly by more complex organisms, particularly by those with a 

 highly developed circulatory system ; this is because individual 

 cells are not greatly affected by freezing, whereas the circulatory 

 channels are readily blocked by this cause. Bacterial cells are 

 not killed by exposure for long periods to the temperature of liquid 

 air 2 ( 190). Reduction of the temperature of plant cells to 

 13 may result in a granular transformation of the cytoplasm, 

 often with rather serious structural alterations. Cytoplasm seems 

 to be more affected than the nucleus, for mitosis may occur slowly 

 in plant cells at 8, and Uschinsky 3 noted that in animal tis- 

 sues the nuclei were less affected by cold than the cytoplasm. 

 Blood seems little affected by freezing temperature, for du Cornu 

 found that dog's blood kept on ice for five to ten days could be 



1 Ziegler's Beitr., 1895 (18), 72. 

 2 MacFadyen, Lancet, 1900 (i), 849. 

 8 Ziegler's Beitr., 1893 (12), 115. 



