CHAPTER XIV 

 EXCRETION 



The mathematically accurate end-reaction of a chain of known and 

 unknown causes and effects. — Noyes. 



Provisions for eliminating from the organism the waste products 

 of metabolism are not less important than those for supplying the 

 matter and energy by which the vital processes are carried on. 

 In many of the unicellular forms the whole surface of the organism 

 functions as an excretory organ, but it will be recalled that even 

 in some of these, such as Amoeba and Paramecium, contractile 

 vacuoles facilitate the removal of useless metabolic products. In- 

 deed, in all but a few of the lowest Metazoa there are highly spe- 

 cialized organs for excretion. In the Vertebrates we find the 

 kidneys and the gills or the lungs devoted largely to excretion, 

 and the skin and liver acting in subsidiary capacities. Each 

 receives a profuse blood-supply from which it takes the waste 

 products that are to leave the body as an excretion. Usually 

 the effective surface of an excretory organ consists of gland cells. 

 (Figs. 6, 27, 111.) 



There is therefore an essential distinction between an excre- 

 tion, which represents chemical waste from the vital processes, 

 and the major part of the material which is ejected from the di- 

 gestive tract as feces. The latter is almost entirely indigestible 

 material taken in with the food which has not directly contrib- 

 uted to the metabolic processes of the organism, though some of it 

 may have acted temporarily in an accessory capacity. Accord- 

 ingly the digestive tract is not included in the list of excretory 

 organs, though it will be recalled that certain waste products 

 excreted by the liver reach the outside world with the feces. 



The nature and amount of the material eliminated by the excre- 

 tory organs is, of course, determined by what is brought to them 

 by the blood, and this, in final analysis, is dependent upon the 

 food - - the fuel that has been burned — and the disintegration 

 of protoplasm from the wear and tear of life. Carbohydrates and 

 fats yield carbon dioxide and water, while proteins give in addi- 



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