54 APPLIED BIOLOGY 



cell-substance), mineral substances, and peculiar substances 

 containing nitrogen and called nitrogeneous excretions. 

 Three of these water, carbon dioxide, and mineral matter 

 are easily shown to be present when animal matter is heated 

 and burned in a pipe or tube ( 13). Also it could be proved 

 by a careful test that one of the gases given off when the meat 

 is heated is ammonia ; and this gas contains nitrogen, which 

 is a constituent of the nitrogeneous excretions formed when 

 cell-substance oxidizes in a living cell. It is true, then, that 

 all the substances found when we analyze animal matter by 

 heating and burning are also present in the excretions formed 

 by oxidation which takes place at the temperature of the 

 frog's body. The chief difference is that oxidation in the 

 test-tube is rapid and at high temperature while in the frog's 

 body it is slow and at low temperature. 



50. Removal of Excretions. The excretions formed in 

 all the living cells of the frog's body are poisonous if allowed 

 to accumulate. Hence they must be eliminated from the 

 body. Kidneys, skin, and lungs are the organs of excretion 

 or excretory organs. Dissection of the frog showed that 

 many cells are at such a distance from these organs of ex- 

 cretion that the poisonous substances cannot be absorbed 

 directly by these organs, thus making it necessary that 

 the blood should absorb excretions from the cells, and, 

 flowing to the excretory organs, give up these excretions to 

 be eliminated from the body of the animal. In the frog, 

 the carbon dioxide is carried by the blood from the cells, 

 where it is formed, to the skin and lungs, where it is given 

 off to the air or to water. The nitrogeneous excretions are 

 first absorbed from the cells by the blood and then carried 

 to the kidneys, where, along with water, these excretions are 

 removed from the blood, passed into the ducts (ureters), 

 and thence to the exterior. In some simple aquatic animals, 

 the excretions are absorbed directly from the cells by the 

 water in which the animal lives ; and just as in the case of 



