282 ON GROWTH AND OVERGROWTH 



Limitations of the Parasitic Theory 



Accepting this as our definition — and I may say that other 

 attempted definitions agree in including or tending to include 

 the same provisions— then immediately we are compelled to 

 recognize that all true tumours are not of parasitic origin. There 

 is the group of teratomata, due to the inclusion within the 

 tissues of one individual of tissues derived from another. These 

 tissues become organically connected with those of the host, 

 and grow at the expense of the host, throughout subserving no 

 function of use to that host. They form a well-recognized group, 

 which I need not here describe more fully. 



Thus, then, on the one hand there are tumours which assuredly 

 are not of parasitic origin, but, on the other hand, we cannot go 

 to the opposite extreme and say that no tumours are due to the 

 action of parasites in the tissues. It may be laid down that 

 physical and toxic agencies, when acting below the point neces- 

 sary to induce cell exhaustion and cell destruction, may act as 

 stimuli rather than as irritants, and doing this may bring about 

 increased cell proliferation. It may be — it has been — argued 

 that these bacteria and their products cannot be the direct 

 cause of cell proliferation. Weigert and his pupils, for example, 

 strenuously deny such direct action, but the fact remains that 

 the bacterial toxins can be the initiating, indeed in certain cases 

 an essential, cause ; and the recent studies by Mallory, 1 more 

 especially upon the changes occurring in the endothelium of 

 vessels and lymphatics during typhoid and other infectious 

 disease, seem to prove this with absolute clearness. 



Bacterial toxins, which when concentrated lead to excessive 

 cell necrosis, in a less concentrated condition may induce active 

 cell growth. In the central portion of the abscess, where there 

 are abundant bacteria and their products, there is abundant 

 evidence of cell necrosis. At the periphery, where by sundry 

 mechanisms the bacteria are hindered from existing, and where 

 the toxins diffusing out are less concentrated, there is evidence 

 of active mitosis and cell growth. 



All the tissue components, it is true, are not alike in their 

 reaction. The simplest forms of cells may be stimulated to 



1 Mallory, Journ. of Exper. Med. iii., 1898, p. 611, and Trans. Assoc. 

 Amer. Phys. xv., 1900, p. 224. 



