8 FIRST PRINCIPLES. 



to obtain materials for its other requirements, it receives 

 nutrition in the form of food, water and oxygen. Food to be 

 utilised has to be digested, water and soluble salts are absorbed 

 unchanged, and oxygen, as we have just seen, is taken into the 

 system by the lungs from the air. 



Regarding the body as practically made up of water, nitro- 

 genous matter, fat, and mineral matter ; we find that the 

 water and soluble salts which have served their purpose, or 

 which are in excess of the requirements of the system, can be 

 removed (excreted) without change. Water is discharged 

 chiefly from the kidneys, lungs and skin ; and mineral matters 

 from the kidneys and to a slight extent from the skin, 

 especially in the case of common salt. The kidneys act as 

 filters which have selective power in removing waste material 

 along with water from the blood. This impure water (urine) 

 is carried by two tubes (the ureters] from the kidneys into the 

 bladder, from which it is finally discharged through the 

 urethra. The insoluble mineral matters of the body, though 

 fairly stable, gradually undergo change by becoming converted 

 into soluble salts. Broken up nitrogenous matter yields 

 compounds which are removed along with the urine, and fat 

 which by uniting with the oxygen brought to the tissues 

 by the blood, is changed into grape sugar and finally into 

 carbonic acid and water ; the carbonic acid being given off by 

 the lungs into the atmosphere. The fat which has accu- 

 mulated in the tissues undergoes the same changes, when it 

 becomes broken up, as the fat which results from the disinte- 

 gration of nitrogenous matter. A portion of the waste fat, 

 as we all know, is excreted unchanged in perspiration and 

 in other secretions. As many of the waste materials have a 

 poisonous influence on the body ; it is necessary for the 

 maintenance of health that the organs (lungs, kidneys, intes- 

 tines, skin, liver, etc.) which excrete them, should be in 

 perfect working order, and that the system should have a 



