344 PHYSIOLOGY CHAP. 



(&) Carbonic acid and water, which are produced by oxidation 

 of all three groups of organic food-stuffs, and are formed in every 

 living tissue-cell ; 



(c) Salts, particularly phosphates and sulphates, which are 

 formed by oxidation of the sulphur of the proteins and the phos- 

 phorus of the nuclein, lecithin, and other phosphorated substances. 



These final products of metabolism, together with many other 

 intermediate products of oxidation, are expelled from the body as 

 fast as they are formed, by the various organs of excretion the 

 intestines, kidneys, lungs and skin. 



The kidneys are the principal organs for the elimination of 

 waste products. They secrete urine, which contains the greater 

 part of the urea and other nitrogenous products, the greater part 

 of the salts, a very large amount of water, and a little carbonic 

 acid. The lungs (as we saw in Vol. I. Chapter XL) eliminate 

 most of the carbonic acid formed in the tissues, along with a con- 

 siderable amount of water in the form of vapour. The intestine 

 and skin, although their excretory functions present some specific 

 characters, may undoubtedly be regarded as vicarious excreting 

 organs, which are complementary to the lungs and kidneys. 



Intestinal excretion must be treated first, since it is a necessary 

 complement to the study in the three preceding chapters of the 

 functions of the digestive canal. 



I. We have already considered the Intestine as an organ for 

 the digestion of foods, and absorption of the digestive products. 

 We now have to consider it as an organ for excretion, i.e. a canal 

 which collects a certain quantity 'of waste matter destined to be 

 periodically cast out of the body, in the form of the faeces or 

 excrements. 



The investigation of the faeces with as exact a determination 

 as is possible of their chemical constitution, their origin, and the 

 process by which the several products contained in them are 

 formed, together with the quantitative and qualitative variations of 

 the latter in relation to various forms of diet is a subject of 

 great scientific interest, which has not hitherto received adequate 

 treatment. 



The amount, physical characters, and composition of the 

 excreta differ widely in the different classes of animals, principally 

 in relation to the nature of their diet. Herbivora excrete a much 

 larger amount of faeces than carnivora, because vegetable foods 

 (in comparison with those of animal origin) are much richer in 

 substances which are indigestible or difficult of digestion, so that 

 larger quantities have to be taken in to satisfy the needs of the 

 body, and a larger residue is left in the intestine. Man, who is 

 omnivorous, produces a variable daily quantity of faeces, according 

 as his diet is mainly vegetable or animal : with the first he normally 

 excretes a larger amount, with the second, a less. 



