188 A COURSE ON ZOOLOGY. 



tern from the attacks of the virulent microbe of the 

 same species as itself, but which has not been attenuated 

 by culture. An excellent example of this possible at- 

 tenuation of a microbe is the vaccine matter employed 

 as a preventive of small-pox. We know that it is suffi- 

 cient to take a small quantity of the liquid contained 

 in the pustules on the arm of a recently-vaccinated 

 child for the purpose of fresh vaccinations. These pus- 

 tules contain the microbes of small-pox, attenuated by 

 culture, the culture substances being in the organism of 

 the child. We give this example in order to help to an 

 understanding of the mechanism of the attenuation of 

 microbes, but we must add that while the bacteria of 

 anthrax, of cholera, and of other diseases are known, 

 the microbe of small-pox has not yet been isolated with 

 certainty. In this respect it is curious to note that the 

 use of the vaccine of variola as a protection against small- 

 pox was known long before there were the least notions 

 about microbes and their attenuation. 



CHAPTER XXI. 

 Principal Orders of Mammals. 



MAMMALS are vertebrate animals, having a constant 

 temperature, a double and complete circulation, and an 

 aerial, pulmonary respiration. Their skin, though it 

 may appear perfectly naked, always has hairs, which 

 often cover it entirely. Mammals produce living young, 

 which they suckle. 



The head is always articulated with the first cervical 



