Infectious Agents. 6y 



tonitis as a consequence being- generally a dilluse one. Tuber- 

 cle bacilli having effected an entrance into the pleural or peri- 

 cardial cavities multiply in the lymph and are actually rubbed 

 into the whole serous surface by the movements of the heart and 

 lungs. For this reason there usually occurs in these regions a 

 disseminated, dense eruption of tubercles involving the whole 

 pleura, pericardium and epicardium. So, too, currents in the 

 secretory fluids on mucous surfaces are favorable to the dissemina- 

 tion of microorganisms ; and the covering of these surfaces permits 

 migration and generalization, especially of the motile forms. 

 Tubercle bacilli may thus reach the larynx in the bronchial secre- 

 tions from cavities in the lungs, and if retained may occasion 

 laryngeal ulceration ; pyogenic bacteria from the pharynx may pass 

 to the Eustachian tubes and middle ear in horses ; and in case of 

 infection of the milk ducts of the nipple the dift'erent kinds of bac- 

 teria in the milk and milk passages advance into the parenchyma 

 of the udder and give rise to mastitis. 



The pathogenic action of bacteria is, as already indicated, for 

 the most part a toxic process, but also in some degree a mechan- 

 ical one; it is governed both by the properties and the quantity 

 of the e.vfrcniely varied metabolic products elaborated by the 

 individual microbes, but is also dependent upon factors deter- 

 mined by the place of infection and by the predisposition of the 

 tissues. Local and general eft'ects are distinguishable ; first by the 

 fact that at the original point of infection and at metastatic locali- 

 ties anatomical lesions are originated (inflammations, degenera- 

 tions, necroses, proliferations, depending upon the nature of the 

 microbe, both in the sense of a mere foreign body and of the pecu- 

 liarity of its constituent materials) ; and, second, by the develop- 

 ment of general metabolic disturbances, especially fever, from the 

 generalization of the infectious agents and their products. In 

 each case the phenomena are expressions of tissue reaction to the 

 noxious foreign microbe undergoing multiplication from time to 

 time in the system (v. chapters on fever, limiting inflammation). 

 Each kind of pathogenic microorganism manifests a fixed mode 

 of action, determined by its manner of multiplication in the tis- 

 sues and the particular nature of its toxic product (specific 

 action), and therefore produces a specific disease. In case of 

 other causes of disease there is no uniformity of action, but, on 

 the contrary, a variability ; or, better, it may be said that other 

 or non-infectious diseases mav occur in the same form from the 



