DISEASES OP THE BLOOD. 117 



periodically extends in whatever direction climatic con- 

 ditions, war, and trade may determine, sweeping over 

 wide tracts of country, and leaving desolation and terror 

 behind it. Unless introduced into such a country as 

 Great Britain, it does not occur there, but this is no proof 

 that in places with a very different state of climate, pro- 

 duce, and soil it might not be generated spontaneously. 

 Tubercle and cancer, though communicable by inoculation, 

 undoubtedly appear without any communication, either 

 by contagion, infection, or inoculation. But they are 

 hereditary disorders, and the immature cancer or tubercle 

 elements may have been transmitted from the parent, and 

 have remained in a dormant state, resembling ordinary 

 non-specific tissue elements until surrounding conditions 

 became favorable to the display of their full vigour. 

 This view seems to derive support from the fact that just 

 as in due time each feature of development appears in the 

 progeny as in the parent, so tubercle appears at a fixed 

 age, which is the same in each. Tissue elements, 

 hitherto normal, perhaps undergo change when subjected 

 to certain conditions ; thus, the epithelium of the lungs 

 may sometimes become converted into tuberculous 

 matter, such as may propagate the disease by inoculation, 

 and disease which is transmissible to the offspring may 

 be artificially introduced by non-specific means, but this 

 has not by any means been proved, for until quite recently 

 the characters of tubercle were not sharply defined, and 

 accumulations of ova of parasites, of Gregarinidse, of 

 degenerated epithelial cells — in fact, any mass tending to 

 calcareous or caseous change, was termed tubercle. On 

 this account experiments on the spontaneous origin of 

 tubercle are to be accepted with reservation. Both 

 cancer and tubercle are associated with special constitu- 

 tional conditions, cachexias, termed respectively the can- 

 cerous and the scrofulous diathesis. The lesions which 

 are observed in these disorders are considered to be 

 local manifestations of a constitutional disorder. The 

 time which elapses between the entry of a specific poison 

 into the system and the manifestation of its presence 



