SECTION XXVIII 

 EXCRETION 



CHAPTER LXXXIX 

 THE SECRETION OF URINE 



General Discussion. — The term excretion is commonly applied to 

 that process which purposes to remove the waste products from the 

 body. Living matter undergoes constant metabolic changes, and it is 

 • essential that the substances formed in the course of these processes 

 be removed as quickly and thoroughly as possible. But this state- 

 ment does not imply that the substances previously taken into the 

 body, are simply split into their components and excreted, because in 

 several instances the end-products are first converted into by-products 

 by synthesis. In other words, excretion should not be thought of as 

 a passive elimination of* the simple constituents of the food, but rather 

 as an active cellular synthesis. It is also evident that this process 

 must concern itself with the elimination not only of fluids, but also of 

 semi-solid material as well as of gases, and this is true of each in- 

 dividual cell as well as of the body as a whole. Thus, cellular dis- 

 similation counterbalances cellular assimilation, whereas excretion 

 counterbalances the nutritive material ingested. 



On the excretory side of metabolism matters are relatively simple, 

 because, while gases and liquids of varying composition are in- 

 volved, the number of the excretory channels may really be reduced 

 to four, namely, the skin, lungs, alimentary canal and kidneys. The 

 chief gaseous excretion is furnished by the lungs in the form of carbon 

 dioxid. It constitutes the final stage in the elimination of the carbon 

 of the absorbed food. The principal fluid excretion is furnished by the 

 kidneys in the form of the urine, which contains the hydrogen and 

 unchanged water of the food as well as the various end-products of 

 protein metabolism. But the hydrogen and unchanged water of the 

 food are also eliminated by the skin, .lungs, intestinal canal and, in a 

 small measure, also by the nasal mucosa, lacrimal glands and mucous 

 glands. The undigested and unabsorbed portions of the food, as well 

 as certain true excretory materials, are eliminated in the feces. 



The Structure of the Kidney. — The urinary organs embrace the 

 two kidneys, the two ureters, and the urinary bladder with the urethra. 

 In man each kidney is invested by a fibrous capsule and is deeply 

 imbedded in the fatty tissue of the lumbar region. Its capsule is only 



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