358 DISEASES OF CATTLE. 



protozoa. Nearly all the diseases of cattle for which a definite cause 

 has been traced are due to bacteria. Among these are tuberculo- 

 sis, anthrax, blackleg, and tetanus (or lockjaw). Some diseases, such 

 as Texas fever and nagana, are traceable to protozoa, while others, 

 like actinomycosis and aspergillosis, are caused by fungi. Those dis- 

 eases of which the cause is unknown or imperfectly worked out are 

 pleuro-pneumonia, rinderpest, foot-and-mouth disease, rabies, cow- 

 pox, malignant catarrh, and dysentery. 



Bacteria may be defined as very minute, unicellular organisms o! a 

 plant-like character. Their form is very simple, as may be seen from 

 an inspection of the various species depicted on Plate XXIX. The 

 description of these figures will be found at the end of this article. 

 The magnification there given will furnish the reader some idea of 

 their very minute size. They multiply in two ways. The bacterium 

 elongates and then divides in the middle to form 2 daughter cells. 

 These go through the same process at once, and thus 4 cells are pro- 

 duced. The division of these leads to 8, the division of 8 to 16, and so 

 on indefinitely. The rapidity with which this multiplication takes 

 place depends upon the nature of the bacterium. The bacillus of 

 tuberculosis multiplies very slowly, while that of anthrax multiplies 

 with great rapidity, provided both are in the most favorable condition. 

 Another mode of reproduction, limited to certain classes of bacteria, 

 consists in the formation of a spore within the body of the bacterium. 

 Spore formation usually takes place when the conditions pertaining to 

 the growth of the bacteria become unfavorable. The spores are much 

 more resistant to destructive agents than the bacteria which produced 

 them. The anthrax spore may live several years in a dried state, but 

 the anthrax bacillus perishes in a few days under like conditions. 

 This matter will be referred to again when we come to discuss the sub- 

 ject of disinfection. 



Of the protozoa which cause disease very little is at present known. 

 The one which produces Texas fever is pictured on Plate XL VII, in 

 figs. 4 and 5. These parasites have a more complex life history than 

 bacteria; and as they can not be grown in artificial media, their 

 thorough investigation is at present hampered with great difficulties. 



The differences in the symptoms and lesions of the various infectious 

 diseases are due to differences in the respective organisms causing 

 them. Similarly the great differences observed in the sources from 

 which animals become infected and the mannerin which infection takes 

 place are due to differences in the life history of these minute organ- 

 isms. Much discussion has taken place of late years concerning the 

 precise meaning of the words ' ' infection " and " contagion. " But these 

 words are now wholly inadequate to express the complex processes of 

 •infection, and it may be said that each species of bacterium or proto- 

 zoon has its own peculiar way of invading the animal body, differing 

 more or less from all the rest. There are, however, a few broad dis- 



