246 INTRODUCTORY. 



successfully, as would be the case wer6 they not temporarily 

 so reduced. 



The well-known insusceptibility of the negro to yellow fever 

 and malaria are examples of the comparative immunity 

 which a race may enjoy ; and among the lower animals the 

 same thing is noted of different species for instance, the frog 

 is immune against infection with anthrax, while the mouse, 

 guinea-pig, and many other animals are extremely susceptible. 



The comparative insusceptibility of an individual to a 

 second attack of such diseases as smallpox, scarlet fever, and 

 measles is an example of acquired immunity. Such immunity 

 may be conferred artificially, as is witnessed daily in the 

 practice of vaccinating against smallpox. 



Equally striking is the varying susceptibility of an indi- 

 vidual at various times in his life to the same disease. 



Exciting causes are those immediately responsible for the 

 onset of the disease ; among these may be mentioned trauma, 

 exposure to heat and cold, poisons, and, by far the most 

 important of all, animal and, more especially, vegetable 

 parasites of which latter bacteria are the most important. 



But a discussion of these topics would lead far beyond the 

 limits assigned us and must be thus dismissed. The study of 

 bacteriology has grown within a few years to large propor- 

 tions and volumes have been written concerning animal 

 parasites. 



Pathology, or pathological anatomy, with which the following 

 pages are more especially concerned, may be divided for con- 

 venience into two sections one, the general aspects of disease- 

 processes without reference to any individual part ; and the 

 other, diseases of special organs and systems. 



