336 THE BEGINNINGS OF LIFE. 



the mouth nearly always contains myriads of them l ? 

 It cannot be replied that Bacteria are unable to live 

 in the blood during life, since the observations of 

 Davaine, Vulpian and others clearly show that they can 

 flourish in the blood of man and also in that of several 

 of the lower animals during their life. There seems 

 then to be no reasonable alternative, and v/e are com- 

 pelled to fall back upon the assumption, that the Bacteria 

 met with in our observations had been evolved out of 

 the blood plasma and other fluids of the body, just as 

 we have seen that they arise in previously heated 

 organic infusions 2 . 



During life, and under the influence of all the varied 

 activities of the living body, the plasma of the blood, 

 rich in nutritive materials, is probably giving birth con- 

 stantly to living particles which speedily develop into 

 leucocytes. These amoeboid corpuscles are the organ- 

 isms into which such new-born living matter invariably 

 tends to develop in the healthy living body 3 . But when 

 death has supervened, then all is changed; the mole- 

 cular composition of the fluid may have altered, whilst 

 the activities of the tissues which formerly influenced 

 it have ceased to act. It is, however, a fluid still rich 

 in albuminoid materials; and when released from 



1 See pp. 345, 346- 



2 More especially since the more recent investigations of Dr. Sander- 

 son have led him to the conclusion that the blood does not naturally 

 contain either visible or invisible Bacteria. (' Thirteenth Report of 

 Medical Officer of Privy Council,' 1870, p. 65.) 



3 See vol. i. p. 226. 



