NUTRITION AND THK CHANGES IN THE TISSUES. 369 



either stored up or in action. When, now, these tissues 

 are called upon for work of any kind the energy to accom- 

 plish this is derived from the chemical disintegration of a 

 part of its living tissue, just as energy might be derived 

 by the chemical disintegration of a bit of nitro-glycerine. 

 We have therefore to look upon all manifestations of the 

 energy in the body, be it muscle, or nerve, or gland, as 

 a disintegration and burning up and dying of a part of 

 this tissue. In such a chemical disintegration a good deal 

 of energy is liberated, partly in heat to maintain the tem- 

 perature of the tissues, or in the case of the muscles, in 

 additional energy to contract them. The products of such 

 a chemical disintegration are mainly as follows: 



The carbon of the living tissue appears as carbon dioxide COz 



The hydrogen as water HaO, 



And the nitrogen in the form of a compound closely related 

 to urea. 



The carbon dioxide is of course at once removed through 

 the lungs in the manner explained at great length in the 

 chapter on respiration. The water drops into the blood and 

 is so lost track of, but the urea-like product, this remnant 

 which contains the nitrogen of the living molecule, is not 

 lost from the tissue, but is retained. The living cell which 

 retains this nitrogenous remnant of the disintegration is 

 able to use this remnant again to build up 'new living tis- 

 sue. It is able to use the same nitrogen over again and 

 needs only a new supply of carbon, hydrogen and oxygen 

 to replace the amounts of these substances lost in the car- 

 bon dioxide and the water. This supply of carbon, hydro- 

 gen and oxygen it is able to take from either the fats or 

 sugars of the blood and the oxygen constantly brought from the 

 lungs. The living cell is able to re-combine the nitrogenous 

 remnant with the carbon, oxygen and hydrogen derived 

 from the sugars or fats, and an added amount of oxygen 

 from the lungs into a new living tissue molecule having the 

 same composition as the original. This molecule may 

 again under nervous or other influences be dissociated and 

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