73 THE DISEASES OF DOMESTIC ANIMALS. 



In contagious diseases the quantity necessary to infection can 

 not be measured. The elements of infection in contagious disease 

 multiply within the organisms into which they are introduced. 

 Poisons do not thus multiply of themselves. 



The quantity of poison remains the same, unless a second intro- 

 duction takes place. 



Infectious or contagious diseases have their period of incuba- 

 tion — that is, a period elapses before the infected organism shows to 

 us that anything has taken place. > Even in inoculation the action is 

 not immediately visible. In some diseases, as rabies, this period may 

 extend over weeks or months, while in others only a few days elapse. 



By poisons the action is immediate, provided the quantity intro- 

 duced is sufficient. 



Infectious diseases have their cycles, or stages. They have the 

 above period of incubation, their period of full development, and 

 that of reconvalescence, their " stadium accrementi and decrementi," 

 while poisons have no such course. 



The diseases which are known as contagious, or infectious, do 

 not by any means belong to a single class or group. 



We have the group of acute exanthemata, such as the variolse, 

 measles, scarlatina, certain forms of mange, the foot-and-mouth dis- 

 ease, the maladie du coit, and the pustulous eruption upon the genitals 

 of our domestic animals. These diseases are frequently accompanied 

 by catarrhal conditions of the respiratory or digestive tracts, with 

 cerebral, hepatic, or splenic disturbances ; but these latter do not 

 constitute the essentials of the disease. They are also generally ac- 

 companied by fever, which of itself is nothing specific, fever being 

 a general phenomenon accompanying all serious constitutional dis- 

 turbances. 



There is no such thing as specific fevers. Of specific causes 

 there are many. 



"We have also a group of infectious diseases known as the acute 

 intestinal, that tract being the chief seat of the same, though, as 

 with the above, other parts or organs do not escape complication. 

 Such are abdominal typhus, cholera, dysentery, and rinderpest. 



Then we have those of the respiratory tract, the influenzas — a 

 collective name — pharyngitis et laryngitis diphtheritica, tussis con- 

 vulsiva, pleuro-pneumonia, and the malarial influenza, or pneumo- 

 pleuro-enteritis of the horse. Also the group of septic diseases, 

 which embraces those classed under the general names of septicae- 

 mia or pyaemia, erysipelas, gangrena septica, phlegmonia, puerperal 

 fever, etc. 



