EXCRETION 165 



containing muscle fibres. These fibres contract and relax con- 

 tinuously, causing the whole organ to beat, like the heart, only 

 much more slowly. The spleen contracts slowly about once a 

 minute. 



The intestine. Much waste matter is excreted by the large 

 intestine and here the physico-chemical mechanism is more 

 clearly indicated (see Chap. XXVII.). 



Skin glands sweat excretion will be considered under regula- 

 tion of temperature (Chap. XXXI.), and elimination by the lungs 

 under Transport (Chap. XXIII.). 



