ASSIMILATING AND SELF-PURIFYING POWER OF BLOOD. 215 
albumen in the formation of all new tissue, its nuclear par¬ 
ticles being always found to include fat-granules. 
242. The presence of a due proportion of the foregoing 
substances in the blood is an essential condition of health; 
and we find it provided-for in the marvellous power which 
the blood, like any solid tissue, seems to have of making itself 
from the materials supplied to it, and of getting rid of what 
is superfluous or unsuitable. Thus an excess of albuminous 
matter in the food does not seem to produce more than a very 
limited increase in the quantity of albumen in the blood, the 
surplus being made to undergo changes within the body, 
which issue in its being removed by the excretory organs. 
An excess of any of the saline compounds is very speedily 
strained off (as it were) into the urine. And an excess of fatty 
matters is drawn off either by the formation of fat as a tissue, 
or by the augmented activity of the liver in producing bile. 
This conservative power is still more remarkably shown in 
the completeness with which the poisons that are generated 
in the body by the decay of its tissues, and which are received 
into the current of the circulation for the purpose of being 
conveyed to the several excreting organs, are drawn off from 
it, so as to leave the blood pure. Thus, carbonic acid is being 
continually produced in such large quantities, that its accu¬ 
mulation in the blood, even for five minutes, would be fatal; 
yet by the aerating process to which the lungs are subservient, 
it is got rid of as fast as formed, so that the blood is restored 
to its previous purity. In like manner, the urea, which is 
one of the products of the wear and tear of the muscles 
consequent upon their use, is so perfectly and constantly 
eliminated by the kidneys, that its detection in the circulating 
current is a matter of difficulty, although we know that it 
must always be passing through this. 
243. Thus the circulating current may be likened to a 
tidal river running through the midst of a large town, and 
supplying it with the water needed for the drink of its 
human and other inhabitants, as well as with that which is 
required for the various manufacturing and cleansing opera¬ 
tions carried on within its precincts; the same stream also 
receives the drainage of the town, and consequently becomes 
charged with the products of animal and vegetable decompo¬ 
sition, and the foul refuse of manufactories; and as the flow 
