314 THE HUMAN BODY. 



struction of new tissue elements. All the active tissues yield 

 on analysis large quantities of proteids which, as pointed out 

 in Chapter I, 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 compounds 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 down and eliminated in the form of urea and 

 other waste substances. Even albuminoids (p. 10), although 

 so nearly allied to proteids, will not serve to replace them 

 entirely in a diet; a man fed abundantly on gelatine, 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 *he 

 utilization of the carbon contained in more complex and less 

 stable compounds, such as proteids, fats or sugars. 



Nearly all the tissue-forming foods must therefore consist 

 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 chemical degradation is only postponed, 

 taking place in the body of the offspring instead of that of 

 the parent. In all cases animals are thus, essentially, proteid 

 consumers or wasters, and breakers down of complex bodies; 

 the carbon, hydrogen, and nitrogen which they take as foods 

 in the form of complex unstable bodies, ultimately leaving 



* There is some reason to believe tliat some few of the lower animals 

 which contain chlorophyl can manufacture proteids and utilize carbon 

 dioxide. 



