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 th.e 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 



