84 DEVELOPMENT OF BLOOD. 



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 ef- 

 fected 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 supplied 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 particulars 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 inser- 

 tion 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 main- 

 tained ; 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 new 

 blood, formed after the vaccination, is made like the blood as 

 altered by the vaccine virus ; in other words, the blood exactly 

 assimilates to its altered self the materials derived from the 

 lymph and chyle. In health we cannot see the precision of 

 the adjustment of the blood to the tissues ; but we may imagine 

 it from the small influences by which, as in vaccination, it is 

 disturbed ; and we may be sure that the new blood is as per- 

 fectly assimilated to the healthy standard as in disease it is as- 

 similated to the most minutely altered standard. 1 



How far the assimilation of the blood is affected by any for- 

 mative power which it may possess in common with the solid 

 tissues, we know not. That this possible formative power is, 

 however, if present, greatly ministered to and assisted by the 

 actions of other parts there can be no doubt; as 1st, by the di- 

 gestive and absorbent systems, and probably by the liver, and 

 all of the so-called vascular glands ; and, 2dly, by the excre- 

 tory organs, which separate from the blood refuse materials, 



1 Corresponding facts in relation to the maintenance of the tissues 

 by assimilation will be mentioned in the chapter on NUTRITION. 



