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- 
