104 ASSIMILATION OF BLOOD. 



"Without doubt these little bodies have, like all other 

 parts of the organism, a tolerably definite term of existence, 

 and in a like manner die and waste away when the portion 

 of work alloted to them has been performed. Neither the 

 length of their life, however, nor the fashion of their 

 decay, has been yet clearly made out, and we can only 

 surmise that in these things they resemble more or less 

 closely those parts of the body which lie more plainly within 

 our observation. 



From what has been said, it will have appeared that when 

 the blood is once formed, its growth and maintenance are 

 effected by the constant repetition of the development of 

 new portions. In the same proportion that the blood yields 

 its materials for the maintenance and repair of the several 

 solid tissues, and for secretions, so are new materials sup- 

 plied to it in the lymph and chyle, and by development 

 made like it. The part of the process which relates to the 

 formation of new corpuscles has been described, but it is 

 probably only a small portion of the whole process ; for the 

 assimilation of the new materials to the blood must be 

 perfect, in regard to all those immeasurable minute par- 

 ticulars by which the blood is adapted for the nutrition of 

 every tissue, and the maintenance of every peculiarity of 

 each. How precise the assimilation must be for such an 

 adaptation, may be conceived from some of the cases in 

 which the blood is altered by disease, and by assimilation 

 is maintained in its altered state. For example, by the 

 insertion of vaccine matter, the blood is for a short time 

 manifestly diseased ; however minute the portion of virus, 

 it affects and alters, in some way, the whole of the blood. 

 And the alteration thus produced, inconceivably slight as 

 it must be, is long maintained ; for even very long after a 

 successful vaccination, a second insertion of the virus may 

 have no effect, the blood being no longer amenable to its 

 influence, because the hew blood, formed after the vaccina- 

 tion, is made like the blood as altered by the vaccine 



