Cancer 97 



tides, which is inherited from one particle to another. 



On the whole, however, it can be said that the virus theory of 

 cancer meets with great difficulties, except in those cases where a 

 virus agent can be demonstrated. In other cases it is implied either 

 that a dormant virus is normally present, which only becomes active 

 when stimulated in some way. or that one of the normal cell particles 

 becomes a virus when it is acted on by a carcinogenic substance. 

 There is no doubt that the carcinogenic substance can act on the 

 cell particles present in the cytoplasm, but very little is known at 

 present about how their behaviour is changed and whether such 

 changes could be transmitted from one cell to its descendants. 



(3) Metabolic Theories of Cancer. It is an undoubted fact that 

 cancer cells behave differently from the normal cells in which they 

 originate. Much work has been done on these differences, but few 

 really characteristic effects have been found. It is found, however, 

 that the activity of some enzyme systems is increased and that of 

 others is diminished. These differences are often so great that the 

 cancer cell lives in an essentially different way to the normal cells. 

 Warburg and others have shown, for example, that the cancer cell 

 obtains its energy, not like most normal tissues from the oxidation of 

 sugars, but by a less profound type of splitting of the sugar molecules, 

 known as glycolysis^ Warburg has therefore proposed that the car- 

 cinogenic agents are in fact cell poisons which interfere with the 

 normal mode of life of the cell and compel it to adopt a new mode in 

 order to live. The change from the normal to the cancer type on 

 this theory is the cell's effort to continue to live when its normal 

 mode of life is interfered with. One result of this effort is that it is 

 stimulated to an abnormal rate of cell division. 



There is no doubt that many cells are adaptable to changes in their 

 environment. It was shown by Hinshelwood that bacteria in particular 

 are very good at adapting themselves to a new way of life. They can 

 grow in the presence of dyes which interfere with their ordinary mode 

 of Uving. There is usually a 'lag' period in which the cells are adapting 

 themselves to the new conditions. During this, new systems of 

 enzymes are formed and new methods of synthesis are found to re- 

 place those which have been blocked by the dye. According to the 

 metabolic theory, the cancer cell is an adaptation to new conditions — 

 a response to the loss of normal functioning. According to this view, 

 carcinogenic substances act by interfering with enzyme systems 

 which the cell normally uses. They act indeed as selective poisons 

 and they compel the cell, in order to continue to exist, to find an 

 alternative way of living. 

 1 See p. 47. 



