IMPORTANT FOODSTUFFS, IMPORTANT TISSUES 119 



needed by individuals of different size or different habits of life. 

 For an average sized (70 kilos) city dweller, perhaps 90 grams 

 protein and enough fats and carbohydrates to make the total 

 energy intake about 2,500 calories is about the right amount, 

 but these figures will vary greatly with changing conditions. 

 The diet must contain certain unknown substances called 

 vitamines, which will be discussed in the chapter on metabolism. 



Some Important Tissues 



Although the substances occurring in the different tissues have 

 been discussed under the various groups of which they are 

 members, it may be useful from the student's point of view to 

 include here a brief summary or survey of some of the im- 

 portant tissues, and to discuss some additional points of inter- 

 est. 



Muscle. The muscles make up about % of the body weight 

 in adults. They contain about 18-20% protein, 72-78% water 

 and from 0.15-0.3% of glycogen. If muscle tissue is subjected 

 to a very high pressure, a liquid known as the plasma is squeezed 

 out. This represents about 60% of the total muscle weight. 

 The remaining material is called the stroma. The plasma has 

 the power of clotting. The chief proteins of the plasma are 

 myosin and myosinogen (or myogen). Most authors have con- 

 sidered myosinogen to be the mother substance of myosin. This 

 has been disputed, however; it is considered by some investiga- 

 tors to be an albumin. The stroma contains a protein resembling 

 an albuminoid. In addition to the above substances, muscle 

 contains various extractives and salts, such as creatine, urea, 

 inosite, taurine, and also some lipins. After activity, particu- 

 larly if the muscle supply of blood or oxygen is low, lactic acid 

 is found. This is believed to be a product of the partial oxida- 

 tion of glucose. The lactic acid may be further oxidized, or re- 

 built into something else. 



The important property of muscles is their power of contract- 

 ing. The process has been extensively studied, but as yet it is 

 but imperfectly understood. Heat is liberated, glucose is used 



