64 INTRODUCTION TO GENERAL PHYSIOLOGY 



be composed of it alone. But it would be wasteful, because the 

 greater part of the nitrogenous component is excreted unused. 

 There is some popular misconception implied in the name some- 

 times given to proteins as being " flesh-formers," as distinguished 

 from carbohydrates, which are said to be "heat-givers." It is 

 unnecessary to say that protein must be supplied when new tissue 

 is being formed ; but this is a very different thing from the sugges- 

 tion that it will form flesh (f.e. t muscle) of itself alone. If the 

 muscles are exercised, they may increase in bulk, and to do this a 

 small amount of nitrogen is required. On the other hand, a food 

 giving heat is equivalent to saying that it gives energy in general, 

 and the name applies to protein as well as to non-nitrogenous food. 

 It was, indeed, supposed at one time that there was some special 

 value in protein as a source of energy, but exact observations have 

 been unable to confirm this view. The names " flesh-formers " and 

 " heat-givers " are quite unscientific, and do not correspond to any 

 real distinctions. They should be given up altogether. 



Not very much has been said, as yet, with regard to the third 

 class of substances used for food. These are the fats. They con- 

 sist of carbon, hydrogen, and oxygen, like the carbohydrates, but 

 the hydrogen is present in larger proportion than required to com- 

 bine with the oxygen to form water. Accordingly they afford, 

 when equal weights are oxidised, more energy than carbohydrates 

 do. Otherwise there does not seem, so far as can be made out, 

 any physiological necessity for fat as there is for protein and 

 carbohydrate. There is, undoubtedly, a desire for it, but this may 

 be for reasons of making dishes attractive to the palate. It has 

 been found possible for strong, healthy men to live without fat for 

 two years. The presence of a particular accessory factor, the " fat- 

 soluble A," in some fats makes their use advisable, although the 

 factor is not confined to what are generally called fats. 



As to the chemical nature of fats, they are what are known as 

 " esters" a large class of compounds in which an alcohol residue is 

 united with an acid residue. Alcohols are characterised by the 

 presence of a CH 2 OH group, united with carbon and hydrogen. If 

 the alcohol group is combined with hydrogen alone, we have methyl 

 alcohol, CH 3 OH ; adding CH 2 , we get ethyl alcohol, C 2 H 5 OH, and 

 so on up to a large number, when the compounds become solid. 

 The additions are not necessarily made in a way to form a straight 

 chain, hence we have different alcohols with the same number of 

 carbon atoms. The group CH 2 OH may be attached to other more 

 complex groups than in the fatty acid series above referred to, but 

 it is this series that interests us more especially here. There may 

 also be more than one alcohol group, as in glycerin (more correctly, 

 glycerol, since the termination ol has been agreed upon as that of 



