REGULATION OF ENVIRONMENT 65 



cells. We cannot by any means force up indefinitely 

 the concentration of a substance in the urine; and if 

 the concentration in the blood of a constituent of urine 

 falls below a certain point, the secretion of that con- 

 stituent ceases. If, for instance, the concentration of 

 sodium chloride in the blood falls below normal, sodium 

 chloride disappears at once from the urine, though it 

 is still abundant in the blood. Sugar is not secreted 

 at all by the kidneys unless its concentration in the 

 blood exceeds the normal. In both these cases the 

 acceleration is in the opposite direction to secretion, 

 so that the passage of these substances is actively 

 prevented. 



The secretory action of the kidneys is strikingly 

 dependent in other ways on the environment of the 

 secreting cells. Their activity is easily abolished by 

 want of oxygen, for instance, or by minute doses of 

 various poisons, and may be increased by the admin- 

 istration of various drugs. 



When we look at other cases of secretion we find 

 that often enough some one or other of the sub- 

 stances secreted is not present as such in the blood, 

 but is formed in the secreting cells. Instances of this 

 are the formation and secretion of hippuric acid in 

 the kidney, of urea, bile acids and pigments in the 

 liver, or of casein and milk-sugar in the milk glands. 

 The constituents or precursors of these substances 

 are taken up from the blood, and their combination or 

 decomposition takes place in the secreting cell. The 

 resulting substances are then accelerated outwards 

 from the secreting cell to the duct, while their precur- 



