ENERGY RELATIONS OF PLANTS AND ANIMALS 25 



Next in amount to the salts in the body of average build, 

 and not uncommonly exceeding them, are the fats. 1 The 

 word " fat " is used sometimes in a chemicaTand some- 

 times in an anatomic sense. In the first case it denotes a 

 compound of carbon, hydrogen, and oxygen, having a 

 formula of a certain type. In the other usage the mean- 

 ing is " adipose tissue," a form of connective tissue rich in 

 such compounds. Fats have familiar physical characters. 

 They are not soluble in water or to any great extent in the 

 fluids of the body. They pass from solid to liquid form 

 at moderate temperatures; the fats of the human body 

 are regarded as in a fluid state when under the influence of 

 its warmth. No other common physiologic compounds 

 have so much latent energy awaiting release by oxidation. 

 Fats are more plentiful in apparently lean individuals than 

 might be judged. A considerable store of adipose tissue 

 is to be found in any condition short of imminent starva- 

 tion. 



It has been said above that carbohydrates usually have 

 the leading place among the. solid matters of the diet. 

 This is owing to the large proportion of vegetable foods 

 generally consumed. In the animal body the occurrence 

 of carbohydrate is rather scant, and it is one of the chief 

 problems of the physiologist to account for the daily dis- 

 appearance of a great quantity of these compounds in the 

 economy of the organism. The reader may already foresee 

 what we shall later explain in detail, that this disappear- 

 ance of carbohydrate is due in part to the fact that it is 

 the fuel most constantly called upon to evolve energy, and 

 in part to the ease with which the tissues transform to fat 

 a surplus of these substances. Under the head of carbo- 

 hydrates we distinguish the starches and the sugars. 



1 Fats are compounds which can be resolved into glycerin and 

 organic acids. Those of chief interest in nutrition are the glycerids 

 of palmitic, stearic, and oleic acids. The first mentioned has a com- 

 position indicated by the formula C 3 H 6 (OOCH 31 C 15 )3. The others 

 are nearly related. The three common fats differ in their melting- 

 points and in other respects. They are mingled in definite propor- 

 tions to form the body fat of each animal species. 



