88 ORGANIC FUNCTIONING 



all bodily energy is ultimately derived from the oxidation of carbon 

 and hydrogen we expect that COo and OH, will be prominent 

 constituents of the excretions. 



22a. Origins of the Excreted Substances. The CO 2 and 

 OH 2 come mainly from the oxidation of the lactic acid produced 

 when the muscle-fibres pass into the state of tension. But CO 2 

 is produced in the activities of other tissue-elements — even from 

 nerve-cells and nerve-fibres. There is also waste in all tissues 

 that function : thus the numbers of " worn-out," red blood- 

 corpuscles that are destroyed in the liver every second are 

 very large, and so on. Nitrogenous residues come from 

 such waste and also from the utilization of proteins as energy- 

 sources. 



22b. Excretory Paths. CO 2 is eliminated from the respir- 

 atory organ (see Section 30). The latter is the lung, gill, general 

 integument, etc., but the physical process of excretion of CO 2 is 

 probably essentially the same in all higher animals. Water is 

 excreted via the kidneys, or renal organs in non-vertebrate animals, 

 or from the respiratory organs, or from the skin in many terrestrial 

 animals. Faecal matters are to some extent excretions, as they 

 consist of worn-out, essential intestinal bacteria, the pigments 

 and other materials eliminated from the body in the bile that passes 

 into the intestine from the liver, and of unabsorbed materials of 

 foodstuffs. 



22c. The Nitrogenous Residues are eliminated via the 

 kidneys or renal organs. They have a general chemical simi- 

 larity, consisting of urea, uric acid, hippuric acid, etc. (see 

 Section 73). 



33. ON ORGANS OF SPECIAL METABOLISM 

 By " metabolism " is meant, in general, the chemical transforma- 

 tions undergone by the materials of the living organism. It 

 includes, for instance, the disintegration of carbohydrate in the 

 muscle-fibre and the oxidation of the resulting lactic acid. But 

 there are organs where quite special changes occur and it will be 

 convenient to consider these separately. 



The liver is a metabolic organ that has many functions : it 

 transforms sugars (from the intestinal blood) into glycogen, 

 liberates and hydrolyzes glycogen into sugars which are discharged 



