HUMAN PHYSIOLOGY 



of ;i. week or two, an animal has received 20 grams more 

 t,h,-i,n it has given back to its environment the 



p. is t,ha,t it has synthesized from the products 

 of digestion (20 X 6.25) or 125 grams of new protein. 

 If this were all represented by muscle the total would 

 he ;i.loul, f>2r ^ni.rns, for muscle is about 75 per cent. 

 ;i,iid e.onla.ins perhaps 5 per cent, of other non- 



m.'i.Uer, in other words, one-fifth of it is protein. 

 The increase, in weight of a growing organism will ac- 

 cordingly be several times as great as the protein storage. 

 If must. add wa.fer and mineral salts to make new tissue 

 Jind it is likely to ;i,dd SOUK; fat. 



H. will he apparent, also that when nitrogen is lost 

 I Von i 1 1 ic body, as in fasting, the weight must fall by 

 :m amount many limes gn-:if,or than that of the nitrogen 

 oxerolrd. The reasoning is parallel to that employed 

 il><>v(i: (i loss of 1 jj;i;im of nitrogen means a loss of 

 (i.'J. r > vJ-.'ims n f pi-olciii, ,-i.nd this in its turn stands for the 

 drslniclion of prrlui-ps 30 grams of average tissue. 

 A fasting nnini.Ml will thrivfom excrete an amount of 

 \v.M<(M- which ( > :iuin)( ln .-iccounlcd for on the basis of 

 income or o\id;i!ion; il is wnlrr set free by the dissolu- 

 tion of < issue. This is n. f;icl of! en overlooked. 



The composition of the urine depends more upon the 

 protein intnke th;m upon any other factor. It will be 

 well to emphasise just here n. nei^ntive statement, 

 n.-niu'lv. that (hi 4 proportion and quantity of the urinary 

 constituents ; nv but slightly influenced by muscular 

 activity, This is the same as saying that protein de- 

 composition is not materially increased by exercise. 

 The observation is an interest!- uise muscle 



is-SO largely made of pro! cm that early writers naturally 

 assumed thai this kind of material must be sacrificed in 

 the act of cont ract ion. It has become clear that in our 

 Consideration of muscle we must distinguish between 

 the machine and the fuel. The inaehinc 

 chietly of proteins v \vith water and salts\ but the pre- 

 ferred fuel is su.car. with fat also available. 



