CHAPTER XVI. 



ELIMINATION; GENERAL DESCRIPTION OF THE URINARY OR- 

 GANS; STRUCTURE AND BLOOD-SUPPLY OF KIDNEY; SECRE- 

 . TION OF URINE; COMPOSITION AND GENERAL CHARACTERS 

 OF URINE. 



IN the last four chapters we have seen that the blood is con- 

 stantly supplied by means of the respiratory and digestive 

 mechanisms, with all the chemical substances it requires to 

 maintain the life, growth, and activity of the body. These sub- 

 stances, entering the current of the blood, are carried to all the 

 tissues, and are incessantly combining with the chemical sub- 

 stances of which these tissues are composed. These combina- 

 tions are not left to chance ; each tissue has a special affinity 

 for the chemical substance in the blood which it requires for its 

 own growth and special form of activity ; the secretory cell of 

 the liver picks out substances from which it can manufacture 

 bile and glycogen ; the muscle fibre assimilates those that will 

 promote the changes upon which depends the power of con- 

 tractility. We know that the proteid compounds contain the 

 most essential elements for the formation of all kinds of tissue, 

 and that phosphate of lime is a necessary ingredient in the 

 hardening of bone, but we are utterly ignorant of how it comes 

 about that each tissue element is enabled to select the particular 

 material it needs and to reject that which it does not require. 



Our bodies are masses of changing atoms, some of which, if 

 we may so express it, are on the " up grade," to construct the 

 various tissues, and some are on the " down grade," to form the 

 waste matters which are the final products of the tissues' activ- 

 ity. These changes, which are incessantly going on while life 

 lasts, are described under the general term of metabolism ; the 

 constructive changes being spoken of as anabolic, and the de- 

 structive as katabolic, changes. The final products then of 



178 



