THE DIGESTIVE ORGANS. 53 



by exercise are only burning carbon enough to keep up the 

 temperature of the animal, and consequently the blood is over- 

 charged with this gas as well as with hydrogen, and the liver is 

 called upon to do extra duty ; but if neither liver nor lungs 

 efficiently perform this important office, then the task is thrown 

 upon the only parts yet undescribed, which are concerned in 

 relieving the blood of its noxious elements the used-up and 

 waste materials which are no longer wanted in the animal 

 economy. 



These organs are THE KIDNEYS, whose office is to remove the 

 soluble worn-out particles from the blood, and among them the 

 saline materials. But their peculiar office is to carry off out of 

 the blood the superfluous nitrogen, which has either been used in 

 combination with carbon, hydrogen, and oxygen, in forming the 

 muscles of the body, and afterwards, when no longer fit for use, 

 poured into the blood, or which, not being required to supply 

 the muscles, is in excess, and poisoning that fluid if existing in 

 sufficient quantity. Hence, the greater the quantity of plastic 

 or nitrogenous food, the more the kidneys are called upon, 

 whether the animal is in a state of activity or rest, whilst the 

 excess of carbonaceous food, in the shape of starch, sugar, or fat, 

 is partly burned in the lungs, and partly carried into the bowels 

 in the shape of bile. In all these excrementitious processes, with 

 the exception of the production of the heat necessary to the 

 existence of the animal, the grand object of nature seems to be 

 to keep up the healthy constitution of the blood. This being the 

 general food of all parts of the body being, indeed, the very life 



