176 BIOLOGICAL CHEMISTRY 



alveoli is equal to the vapour pressure of the same substances 

 in the blood. Usually the volatile substances are so small in 

 amount that they are negligible. Volatile anaesthetics such 

 as chloroform, acetone and possibly traces of ammonia may be 

 found at times in the expired air. The diffusion of soluble 

 substances from gills is also a method by which a certain 

 amount of easily diffusible substance can escape from the 

 blood. 



The alimentary canal has well-marked excretory functions. 

 Bile pigments and cholesterol are excreted from the liver. 

 Calcium and iron are two substances excreted by the alimen- 

 tary canal. Iron is known to have a beneficial effect in 

 restoring haemoglobin in some forms of anaemia. This is 

 easily understood, as iron is necessary for the formation of 

 haemoglobin, it being a constituent portion of the haematin. 

 When iron salts are given by the mouth it is found that the 

 iron apparently passes through the alimentary canal without 

 being absorbed. In order to explain the beneficial effect of 

 iron it was assumed that only organic forms of iron can be 

 absorbed and that inorganic iron was beneficial by preventing 

 the destruction of the organic compounds containing iron. 

 The real explanation is that iron is excreted by the alimentary 

 canal and that the beneficial effect of iron is due to its absorp- 

 tion, but that owing to simultaneous excretion of iron there is 

 apparently no iron absorbed.* 



Calcium behaves like iron in being absorbed and excreted 

 by the alimentary canal. It has an additional importance as 

 it may form insoluble soaps, with fatty acids, which cannot be 

 absorbed, and these may give rise to concretions in the ali- 

 mentary canal, f 



r The skin in some animals is of importance both for excretion 

 and respiration (frog), but in mammals the skin is mainly 

 protective. Loss of heat by evaporation is important in 

 some animals and the sweat may sometimes contain waste 

 products such as urea. 



SPECIAL EXCRETORY ORGANS B 



In most animals the excretory mechanism is built on the 

 plan of the nephridia found in worms. These consist of a tube 

 with a bulbous extremity opening to the ccelom by the bulbous 

 extremity and to the exterior by the opposite end of the tube. 



The mammalian kidneys consist of a series of long tubes 



* E. Meyer, Ergebnisse der Physiol., 1906, vol. 5, p. 698. 

 f O. T. Williams, Biochem. Journ,, 1907, vol. 2, p. 395. 



