THE CAUSATION OF DISEASE. 25 



If we are satisfied that the S of the tissue in which the 

 morbid process starts is normal, we may for all practical purposes 

 speak of its mal-E as the sole agent in causation. (I say for 

 all practical purposes, for disease being an abnormal interaction 

 of S and E, we can never logically exclude the S from causa- 

 tion.) This E may be spoken of as the primary mal-environment. 

 As the result of this primary morbid action the E of tissues 

 other than that primarily affected is modified, and there occurs 

 in them also a morbid interaction of cell and cell-environment. 

 This may be spoken of as the secondary mal-environment. In 

 this way one morbid action is set up after another in tissue 

 after tissue, and there is established a complex chaiu of morbid 

 actions, which it should be our business to trace back, link by 

 link, to its origin. We have seen that disease is a morbid in- 

 teraction of cell and cell-environment, and we had then in 

 mind a simple unicellular organism, but in a complex organism 

 built up of many cells, disease consists of many such morbid 

 interactions, and any given disease may be defined as the 

 morbid interaction of the primary mal-environment and the 

 tissue primarily affected plus all those morbid interactions of 

 the secondary mal-environments and the tissues secondarily 

 affected ; the name of the disease connoting the sum of these 

 morbid interactions. 



As regards the primary mal-environment, it may be defined 

 as that mal-environment of a tissue which is not due to disease 

 of another tissue. It may consist of an external agent acting 

 upon a tissue and either (1) producing disease in it, or (2) in 

 some other tissue through it, the tissue first acted upon escaping 

 disease. External agents setting up disease in the surface- 

 tissues (skin and mucous membranes) afford examples of the first 

 order. Thus are produced wounds, typhlitis, scabies, silicosis : dis- 

 eases in which the action of the external agent is disseminated 

 throughout the tissues also come more or less under this head 

 ■ — e.g., trichinosis, tuberculosis, specific fevers — but not entirely. 

 Pneumonia affords an example of the second kind of primary 

 mal-environment, this disorder being sometimes brought about 



we should then have the fullest explanation of causation which the case 

 admits of. We are for the present, however, only concerned with the influ- 

 ence of E. The influence of S will be considered hereafter. 



