EXCRETION 195 



capable of further oxidation), gives a day's income. Other 

 constituents of the diet are so small in quantity as to be negli- 

 gible in making up the body's accounts. The chemical changes 

 which they undergo add practically nothing to its capacity for 

 work. Yet some of them are essential to the maintenance of 

 health. Of such are common salt (sodic chloride), alkaline and 

 earthy carbonates, sulphur, phosphorus, etc. These things, 

 together with some products of action of the bacteria in the 

 alimentary canal, the final stage of haemoglobin, imperfectly 

 oxidized nitrogenous substances, and other soluble substances 

 which enter with, or are formed from the food, are removed 

 by the kidneys. We speak of the elimination of waste products, 

 as excretion. Not that there is any physiological distinction 

 between excretion and secretion. Both terms refer to the 

 selection or production and the discharge of materials by cells. 

 If the product discharged has a useful function to perform if 

 it be a digestive ferment, for example it is said to be secreted. 

 If it is of no further use to the economy, we say that it is ex- 

 creted got rid of. In some cases either term is equally appro- 

 priate. The sebum prepared by the sebaceous glands is useful 

 as a lubricant of the skin. It is thrown off. We may speak of 

 the glands as either secreting or as excreting this fatty sub- 

 stance. 



The Kidney. From worms upwards, all animals possess 

 organs for the removal of waste products in solution. This 

 statement might, indeed, be widened so as to include animals 

 even lower than worms. All animals which have a ccelomic 

 cavity a space between the alimentary canal and the body- 

 wall have organs for the removal of soluble waste. The seg- 

 mental organs of worms are obviously the same organs as the 

 kidneys of manlmals ; the latter are distinguished from their 

 prototypes by greater concentration of structure and specializa- 

 tion of function. The kidney is the oldest of organs, if its 

 antiquity be estimated as the length of time during which it has 

 had a form practically identical with that which it now pre- 

 sents. The lungs are of late appearance in the animal scale. 

 Alimentary canal, heart, brain, have passed through many 

 transformations. The kidney assumed its permanent form 

 very far back in the history of the animal kingdom. The 

 most primitive animal which has a digestive cavity, and 



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