142 PRINCIPLES OF PHYSIOLOGY 



concretions formed in the bile ducts and in the 

 gall bladder The origin of this substance is 

 still imperfectly known, but it is formed from 

 metabolism of tissue. It is present in all cells, 

 and when these are broken down, it is not 

 thrown out as a waste product, but is used up 

 to form new cells. Thus, red blood corpuscles 

 are disintegrated in the liver and cholesterin 

 appears in the bile. It is then re-absorbed, 

 probably with other substances, and is carried 

 to the tissues to form new cells. This is a 

 striking example of physiological economy. 



75. By those various processes of excretion 

 waste matters and injurious matters are 

 removed from the blood, as has already been 

 explained. This fluid is therefore maintained 

 in a condition of physiological equilibrium, 

 and this is more remarkable when we consider 

 that it is constantly the seat of exchanges. 

 It is almost momentarily receiving matters 

 on the one hand and giving them up on the 

 other. In a sense, everything removed from 

 the blood must more or less alter its quality. 

 From this point of view the growth and 

 development of the epidermic cells on the 



