278 BLOOD. 



corpuscle, converted into a blood-cell ; for it is an incontrovertible 

 proposition in physiology, that like conditions acting on like 

 substrata, must give rise to identical results. If, however, the 

 formation of a cell depend upon the medium surrounding it, its 

 subsequent activity can only be developed in relation to this 

 medium ; hence the blood-cells and the plasma must stand in 

 a constant reciprocal relation to one another, precisely as the yeast- 

 cells do to the fermenting mixture. It remains, however, for future 

 inquiries to determine the metamorphoses w r hich result from this 

 reciprocal action. As far as we are at present able to form an 

 opinion on this subject, we think we shall not be deviating very 

 widely from the truth, if we regard the blood-cells as organs, that 

 is to say, as laboratories, in which the individual constituents of the 

 plasma are prepared for the higher function of aiding in the forma- 

 tion and reproduction of the tissues. As soon, however, as we 

 attempt to specify the individual constituents of the plasma, we 

 lose ourselves in a labyrinth of hypotheses. Thus, for instance, 

 some observers have conjectured that fibrin was elaborated from 

 albumen, which is possible, in so far as fibrin appears to be a sub- 

 stance ready elaborated for deposition in the tissues, but improbable, 

 since we also find in some cases that the fibrin is increased in an 

 extraordinary degree in blood which is very poor in corpuscles 

 (chlorosis). 



The blood-corpuscles, like all other cells endowed with vital 

 activity, have a definite period of existence. This limited duration 

 has been regarded by some philosophical inquirers as a specific 

 property of living beings, as if every physical or chemical process 

 must not in like manner have a definite period of duration limited 

 by a commencement and a termination. No one can doubt that 

 the activity of the blood-corpuscles has its boundary, and that 

 they perish, although the mode and course of their gradual de- 

 struction yet remains a mystery. All we know is, that in our 

 microscopico-chemical investigations, the cells of thesame blood vary 

 in the length of time during which they can resist chemical agents, 

 and hence it is conjectured that the more easily decomposed cells, 

 which, moreover, generally exhibit greater intensity of colour, are 

 the older, and that those which are not easily acted upon, are paler, 

 and appear in their granular contents to present the rudiments 

 of a nucleus, are of more recent origin. We have no certain 

 knowledge of the length of time that an individual cell continues 

 to exist. The observation made by Harless, that a frog's blood- 

 corpuscles entirely disappear after nine or ten alternations of 

 oxygen and carbonic acid, would enable us to form an average 



