48 PHYSIOLOGY CHAP. 



in what quantities they are taken. This is tantamount to saying 

 that they are incomplete foods, i.e. they are lacking in one or other of 

 the properties necessary for life which protein substances possess. 



They are not histogenic foods, that is, they cannot form or 

 repair the losses sustained by living protoplasm during functional 

 activity. We have seen that gelatine and the collagenous tissues in 

 general, carbohydrates and fats, can only partly take the place of 

 proteins as combustible material, as a source of heat and work. 

 The saving in the consumption of nitrogen effected by them is 

 only partial. 



Gelatine and collagenous substances burn rapidly during a 

 normal alimentary regimen, but less so during fasting. They are 

 good foods for producing heat (thermogenic foods), but cannot be 

 stored up as reserve material. 



Saccharides are excellent for the supply of energy (dynamo- 

 genie foods'). During muscular work they may be substituted 

 for the non-nitrogenous group of the protein molecule which is 

 being burnt, and can preserve from consumption the nitrogenous 

 nucleus by reconstituting and reintegrating the muscular proto- 

 plasm. This is why work does not cause any notable or propor- 

 tionate increase in the nitrogenous products found in the urine 

 (Vol. III. Chap. I.). Saccharides can also be partly stored, either 

 in the form of hepatic and muscular glycogen or in that of fat. 



Fats are eminently thermogenic, because their combustion 

 produces a great deal of heat ; in the organism, however, they burn 

 more slowly than collagenous substances and saccharides. They 

 further possess to a high degree the power of acting as reserve 

 materials, accumulating in the various organs, and being deposited 

 in large quantities inside the cells of the adipose tissue. 



Condiments (substances extracted from meat, alcoholic drinks, 

 spices, and aromatic substances) are necessary ingredients in a 

 good diet, since by stimulating the senses of taste and smell 

 they promote the secretion of the digestive juices, and, by raising 

 the tone of the nervous system as a whole, increase the direct or 

 indirect trophic influence exercised by it on the various tissues. 

 Certain condiments are nutritious (cocoa, or thermogenic alcoholic 

 drinks) ; but all of them act as nerve stimulants, and may as such 

 be regarded as indirect sources of energy. 



Mineral salts, chemically combined with natural protein food- 

 stuffs, serve as simple building or supporting materials in the 

 structure of the tissues in general, and the osseous system in 

 particular. 



Water is the common solvent, which is indispensable in every 

 diet. It may be that naturally contained in articles of food, or 

 added to it in more or less large quantities in proportion to the 

 daily loss in the urine, the air expired from the lungs, or the 

 perspiration and sweat. 



