126 PRINCIPLES OF PHYSIOLOGY 



been explained, is maintained in a state of 

 nutritive equilibrium. The separation and 

 elimination of waste matters are effected by 

 the process termed excretion. A secretion, 

 such as saliva, or gastric juice, is a fluid 

 holding matters in solution which have been 

 formed by the activities of the cells in secreting 

 glands, and it is intended to be used for some 

 other purpose in the economy of the living 

 body. Reference has already been made to 

 the uses of the saliva and of the gastric juice. 

 An excretion, on the other hand, is the separa- 

 tion from the body of such a fluid as urine 

 by the kidneys, or bile by the liver-fluids of 

 no further use. The chief excretive mechan- 

 isms will now be considered. 



66. Lungs. By the process of respiration, 

 carbonic acid is excreted by the lungs. This 

 substance, carbonic acid, is produced in 

 connection with the activities of all living 

 matter. It is formed in the tissues, especi- 

 ally in the glandular, nervous and muscular 

 tissues ; it is abundant in lymph ; it exists 

 in the blood in a state of loose chemico-physi- 

 cal combination with the potassium salts and 



