ESSENTIALS OF PHYSIOLOGY. 



Thus oleic acid has the formula, C 8 H 17 . CH : CH . C 7 H 14 . COOH, and 

 has one double bond. 



The fatty acids present in the liver contain two, three, or even four 

 double bonds, each of which represents a weak spot in the long chain 

 of carbon atoms ; and an unsaturated fatty acid tends to split at these 

 points with the formation of smaller molecules. The unstable fatty 

 acids formed by the liver are carried to the muscles and other tissues, 

 in which they enter the complex protoplasm of the living cells, and are 

 finally oxidised to carbonic acid and water. 



During starvation the fat in the body is used by the tissues as 

 their chief source of energy : the liver receives more fat than usual from 

 the fat depots, and is called upon to desaturate fat in larger amount and 

 thus render it available for use by the tissues. The fat passes to the 

 liver from the fat depots more rapidly than it can be desaturated, and 

 an accumulation of saturated fat takes place in the liver ; this accumula- 

 tion, which is termed fatty infiltration, readily occurs in starvation and 

 equally readily passes off again when food is given. It takes place, not 

 only in starvation, but whenever the tissues need a larger supply of 

 fat, for example in diabetes. 



The nature of the process whereby the subcutaneous tissues and 

 omentum take up fat after a meal and give it off to the blood again in 

 times of need is not known, though it has been supposed to be due to 

 the reversible action of an enzyme (lipase) in the fat cells. 



In a man whose weight remains steady the fat taken in the food is 

 rapidly used in supplying energy in the body, and little or none of it 

 is permanently stored in the fat depots. When weight is being put on, 

 a certain proportion of fat in the food is stored up instead of being- 

 oxidised, and fat may also be formed from carbohydrate. On the 

 contrary, loss of weight during starvation, or produced by some 

 other means, involves depletion of the store of fat in the body. 

 The fat normally stored represents a reserve of potential energy 

 which can be increased, or which in time of need can be drawn upon 

 for the use of the muscles and other tissues ; and owing to its 

 high calorie value, the energy which may be thus kept in reserve 

 is very considerable. 



The fats on leaving the liver pass to the tissues, in which they 

 become combined in such a way that they can no longer be recognised 

 histologically, although their presence is made manifest by chemical 

 analysis of the tissue. A healthy kidney, for instance, when stained 

 with Sudan III may show only a few specks of fat here and there, 

 although when analysed it is found that 18 to 20 per cent, of its dried 

 substance is fat. Under the influence of certain poisons (e.g. phos- 



