Pathogenesis 89 



suffering from smallpox may be delivered of infants with 

 marks indicative of prenatal disease. Some common 

 infectious agents, such as the tubercle bacillus, seem to infect 

 unborn animals with difficulty. The frequency of antenatal 

 tuberculous infection is, however, somewhat controversial 

 at present, Baumgarten having reached the opinion, exactly 

 the opposite of what is commonly believed, that most children 

 are subject to antenatal infection, though the bacilli sub- 

 sequently develop and cause disease in only a few of them. 



PATHOGENESIS. 



This subject can be understood only through a broad 

 knowledge of the metabolic products of micro-organisms. 

 In general it may be said that the ability of micro-organisms 

 to do harm depends upon the injurious nature of their 

 products. This alone, however, will not explain the phe- 

 nomena of infection, for in many cases the intoxication is 

 subsidiary in importance to the invasive power of the micro- 

 organisms. Some bacteria having but limited toxic powers 

 possess extraordinary powers of invasion, as Bacillus an- 

 thracis, and the intoxication becomes important only after the 

 organisms have penetrated to all the tissues of the body. 

 Others, with more active toxic properties, have but limited 

 invasive powers, and a few organisms, growing with difficulty 

 in some insignificant focus, excite actively destructive 

 reactions in the tissues with which they come into contact. 

 Still others, with limited invasive powers, eliminate active 

 toxic substances, soluble in nature, that enter the circulation 

 and act upon cells remote from the bacteria themselves, as 

 in diphtheria and tetanus. 



The invasive power, of the organisms depends upon their 

 ability to overcome the body defenses. This may indicate 

 activity of the infecting organism, or weakness of the defen- 

 sive mechanism. The relation of these factors is exceedingly 

 complex, only partly understood, and will be fully discussed 

 in the chapter upon Immunity. 



For convenience toxins may be described as intracellular 

 or insoluble, and extracellular or soluble. 



The intracellular toxins. These products are but little 

 known and have only recently begun to attract attention. 

 Their insoluble nature makes it difficult to isolate them, and 

 determines the limitations of their activity. Until the in- 



