294 THE HUMAN BODY. 



that this is not the case, but that the animal body requires,, 

 for the most part, highly complex compounds for the con- 

 struction of new tissue elements. All the active tissues 

 yield on analysis large quantities of proteids which, as 

 pointed out in Chapter L, enter always into the structure of 

 protoplasm. Now, so far as we know at present,* the animal 

 body is unable to build up proteids from simpler com- 

 pounds of nitrogen, although when given one variety of 

 them it can convert that one into others, and combine them 

 with other things to form protoplasm. Hence proteids are 

 an essential article of diet, in order to replace the proteid 

 of the living cells which is daily broken clown and elimi- 

 nated in the form of urea and other waste substances. 

 Even albuminoids (p. 11), although so nearly allied to pro- 

 teids, will not serve to replace them entirely in a diet; a. 

 man fed abundantly on gelatin, fats, and starches, would 

 starve as certainly, though not so quickly, as if he got no 

 nitrogenous food at all; his tissue waste would not be made- 

 good, and he would at last be no more able to utilize the 

 energy-yielding materials supplied to him, than a worn-out 

 steam-engine could employ the heat of a fire in its furnace. 

 So, too, the animal is unable to take the carbon for the 

 construction of its tissues, from such simple compounds as- 

 carbon dioxide.* Its constructive power is limited to the 

 utilization of the carbon contained in more complex and 

 lass stable compounds, such as proteids, fats, or sugars. 



Nearly all the tissue-forming foods must therefore con- 

 sist of complex substances, and of these a part must 

 be proteids, since the Body can utilize nitrogen for tissue 

 formation only when supplied with it in that form. The 

 bodies thus taken in are sooner or later broken down into 

 simpler ones and eliminated; some at once in order to yield 

 energy, others only after having first been built up into 

 part of a living cell. The partial exceptions afforded by 

 such losses to the Body as milk for suckling the young, 

 or the albuminous and fatty bodies stored for the same 

 purpose in the egg of a bird, are only apparent ; the chemi- 



* There is some reason to believe that some few of the lower ani- 

 mals which contain chlorophyl can manufacture proteids and utilize 

 carbon dioxide. 



