108 L. V. HEILBRUNN 



viscosity changes and, during division, the viscosity may jump to four 

 or five times its original value and back again. When muscle and 

 nerve are thrown into activity, the protoplasmic viscosity almost cer- 

 tainly changes, and when a cell is subjected to an anesthetic like ether, 

 the viscosity may be profoundly affected. Indeed, at the present 

 time, the only plausible theory of stimulation and response is a col- 

 loidal theory that involves the assumption of marked viscosity change 

 within the protoplasm. Cells are aroused to activity by various di- 

 verse agents such as electric shock, ultraviolet radiation, sudden pres- 

 sure, etc. All these agents could scarcely induce any one type of 

 chemical reaction, nor could they all directly affect the speed of pro- 

 toplasmic reactions in any definite way. The fact that the chemical 

 reactions of cells are affected in much the same manner by widely dif- 

 ferent physical and chemical agents can best be explained by as- 

 suming that these varied agents cause some universal type of colloidal 

 change and that this change influences the reactions, throws them 

 into gear, as it were. Moreover, if one is to explain physical phe- 

 nomena like the contraction of muscle protoplasm, one must seek an 

 interpretation in physical terms. No matter how many interesting 

 and complicated chemical reactions may be described in muscle cells 

 or extracts made from them, somehow^ or other these reactions must 

 be related to physical change in the protoplasmic colloid, either as a 

 result or a cause. And if we are to study physical change in the 

 protoplasmic colloid, viscosity is the best index of such changes that 

 has yet been discovered. 



A point that cannot be emphasized too strongly is that viscosity 

 change of the protoplasmic colloid must be studied directly. It is 

 true that the protoplasm is made up of proteins, but the colloidal 

 behavior of the protoplasm is not like that of any known protein. 

 The very fact that the protoplasm is alive indicates that it must 

 have peculiarities of colloidal behavior, and, indeed, it has been 

 abundantly proved that in many important ways the protoplasmic 

 colloid behaves unlike typical nonUving proteins {cf. 1). Nor is it 

 permissible to extract pure proteins from a tissue like muscle and 

 then to assume that the behavior of these purified proteins outside the 

 cell is like that Avhich they would have within the living muscle. 

 Thus, the fundamental basis of much of Szent-Gyorgyi's recent work 

 on muscle is wrong, as can be clearly demonstrated by studies on liv- 

 ing, intact muscle cells {2). 



In the ordinary study of colloid chemistry, major changes in vis- 



