THE PHENOMENA OF INFECTION 455 



tization is that life processes manifested through ferment 

 action are modified and may be modified at will by changes 

 in environment. The blood-serum and organ extracts of 

 normal guinea-pigs do not digest egg-white, but these 

 fluids from an animal sensitized to this protein do have 

 this action. The virus of smallpox is pathogenic to the 

 man who has never had smallpox, and has not been vacci- 

 nated, but to the man who has had the disease or been 

 properly vaccinated the virus of smallpox is non-pathogenic. 

 We explain this, and in our opinion, the experiments of 

 Pirquet have so demonstrated, that this is due to the fact 

 that the ferments of the man's body cells have been so 

 influenced by the disease or by vaccination that they have 

 acquired a new function that of digesting and thus 

 destroying the virus of the disease. If this explanation be 

 true, it opens up a wide field for the possible extension of 

 the beneficial effects of preventive treatment. 



There is another point of difference between intracellular 

 and extracellular ferments, which is of the greatest impor- 

 tance in a study of the phenomena of infection. The extra- 

 cellular ferments are comparable to those of the digestive 

 juices of the alimentary tract in the higher animals. They 

 roughly prepare foods for the cells. Their function is 

 solely a lytic one. They break up complex proteins into 

 simpler bodies, but the products thus formed are not, 

 without further treatment, ready to be built into the struc- 

 ture of the cell. Proteins in the medium are rendered 

 soluble by the extracellular ferments. They are so altered 

 that they may be taken into the cell, but they are not so 

 patterned that they are ready to be built into the structure. 

 They are fitted for absorption, but are not ready for assimi- 

 lation. The extracellular ferments are in a sense destructive 

 agents. They break down complex molecules into simpler 

 structures. The intracellular ferments are constructive. 

 They are cell builders. They shape the material brought 

 them and fit it into place. They build up specific proteins. 

 They convert the raw material brought them into specific 

 proteins, bacterial, vegetable, or animal. This does not 



