Physiology of the Metabolic Phenomena. 



By the term metabolism we mean those phenomena, whereby all even the 

 most lowly living organisms are capable of incorporating the substances obtained 

 from their food into their tissues, and making them an integral part of their own 

 bodies. This part of the process is known as assimilation. Further, the organism 

 in virtue of its metabolism forms a store of potential energy, which it can trans- 

 form into kinetic energy \ and which, in the higher animals at least, appears most 

 obvious in the form of muscular work and heat. The changes of the constituents 

 of the tissues, by which these transformations of the potential energy are accom- 

 panied, result in the formation of excretory products, which is another part of the 

 process of metabolism. The normal metabolism requires the supply of food 

 quantitatively and qualitatively of the proper kind, the laying up of this food 

 within the body, a regular chemical transformation of the tissues, and the forma- 

 tion of the effete products which have to be given out through the excretory 

 organs. [Synthetic or constructive metabolism is spoken of as anabolic, and de- 

 structive or analytical metabolism as katabolic, metabolism.] 



229. THE MOST IMPORTANT SUBSTANCES USED AS FOOD. Water. 



When we remember that 58*5 per cent, of the body consists of water, that water 

 is being continually given off by the urine and fasces, as well as through the skin 

 and lungs, that the processes of digestion and absorption require water for the 

 solution of most of the substances used as food, and that numerous substances 

 excreted from the body require water for their solution, especially in the urine, the 

 great importance of water and its continual renewal within the organism are at 

 once apparent. As put by Hoppe-Seyler, all organisms live in water, and even in 

 running water, a saying which ranks with the old saying " Corpora non agunt 

 nisi fluida." 



Water as far as it is not a constituent of all fluid foods occurs in different forms as drink : 

 (1) Eain water, which most closely resembles distilled or chemically pure water, always 

 contains minute quantities of C0 2 , NH 3 , nitrous and nitric acids. (2) Spring water usually 

 contains much mineral substance. It is formed from the deposition of watery vapour or rain 

 from the air, which permeates the soil, containing much C0 2 ; the C0 2 is dissolved by the 

 water, and aids in dissolving the alkalies, alkaline earths, and metals, which appear in solution 

 as bicarbonates, e.g., of lime or iron oxide. The water is removed from the spring by proper 

 mechanical appliances, or it bubbles up on the surface in tlie form of a " spring. " (3) River 

 water usually contains much less mineral matter than spring water. Spring water floating 

 on the surface rapidly gives off its C0 2 whereby many substances e.g., lime are thrown out 

 of solution, and deposited as insoluble precipitates. 



Gases. Spring water contains little 0, but much C0 2 , the latter giving to it its fresh taste. 

 Hence, vegetable organisms flourish in spring water, while animals requiring, as they do, much 

 0, are but poorly represented in such water. Water flowing freely gives up C0 2 , and absorbs 

 from the air, and thus affords the necessary conditions for the existence of fishes and other 

 marine animals. River water contains ^ to -^V f its volume of absorbed gases, which may be 

 expelled by boiling or freezing. 



