ABSTRACT
The cells of such an animal as a sponge or a sea-anemone are exposed to fluids of rather variable composition, for the water and salts of their environment penetrate all through their bodies. In man the cells are bathed in the plasma or fluid part of the blood, whose composition is very constant. Most of our bodily activities can be regarded as more or less successful attempts to keep our milieu interieur uniform. The lungs, under the direction of the respiratory centres in the brain, regulate its dissolved gases, the kidneys its content of water and salt. Experiment on animals made it clear that too high a proportion of water to salt leads to convulsions and death as inevitably as too high a concentration of strychnine. In certain types of kidney disease salt excretion becomes very difficult; the patient drinks water to keep the composition of his blood nearly constant, and swells up, developing dropsy.
