The Invasion of the Body by Micro-Organisms 85 



but subsequent to these early manifestations comes depressant 

 action on the nervous cells with palsy, peculiar to the products of 

 the diphtheria bacillus, and therefore specific. 



It is upon the peculiar specific reactions of the bacterio-toxins 

 and the peculiar susceptibility of certain cells to this action that the 

 production of distinct clinical manifestations depend. 



THE INVASION OF THE BODY BY MICRO-ORGANISMS 



Some bacteria whose invasiveness is insuflELcient to enable them 

 successfully to maintain life in healthy tissues, occasionally get a 

 foothold, in diseased tissues and assist in morbid changes. This 

 is seen in what is described as sapremia, in which various sapro- 

 phytic bacteria, possessing no invasive powers, by growing in the 

 putrefying tissues of a gangrenous part, give rise to poisonous sub- 

 stances which when absorbed by the adjacent healthy tissues pro- 

 duce such constitutional disturbances as depression, fever, and the 

 like. 



Bacteria with hmited invasive powers and intracellular toxins 

 can at best occasion local effects. Such organisms not infrequently 

 vary, however, and when of unusual vitahty may survive entrance 

 into the blood and lymph circulations and occasion bacteremia, or, • 

 as it is more frequently called, septicemia, a morbid condition 

 characterized by the presence of bacteria in the circulating blood. 

 When bacteria entering the circulation are unable to pervade the 

 entire organisms, they may collect in the capillaries of the less re- 

 sisting tissues, producing local metastatic lesions, usually purulent 

 in character. This results in what is surgically known as pyemia. 



The mode by which the entrance of bacteria into the circulation 

 is effected differs in different cases. Kruse* believes that they some- 

 times are passively forced through the stoma ta of the vessels when 

 the pressure of the inflammatory exudate is greater than that of the 

 blood within them; that they may sometimes enter into the bodies 

 of leukocytes that have incorporated them; that they may actually 

 grow through the capillary walls, or that they reach the blood cir- 

 culation indirectly by first following the course of the lymphatics. 



Toxemia results from the absorption of the poisonous bacterial 

 products from non-invasive bacteria, as in tetanus. 



THE CARDINAL CONDITIONS OF INFECTION 



Infection can take place only when the micro-organisms are 

 sufficiently virulent, when they enter in sufficient number, when 

 they enter by appropriate avenues, and when the host is susceptible 

 to their action. 



Virulence. — ^Virulence may be defined as the disease-producing 

 power of micro-organisms. It is a variable quality, and depends 



* Flugge, "Die Mikroorganismen," vol. i, p. 271. 



