60 PHYSIOLOGY 



B. The unsaturated fatty acids, namely : 



(1) Acrylic series, e.g. oleic acid (C n H 2n _ 2 2 ) 



(2) Linoleic series, e.g. linoleic acid (C n H 2n _ 4 2 ) 



(3) Linolenic series, e.g. linolenic acid (C n H 2n _ 6 2 ) 



Of the long list of fatty acids given above only a few occur to any 

 extent in the animal body. In milk, although the greater part of the 

 fat consists of the triglycerides of oleic, palmitic, and stearic acids, 

 other members of the series given above are present in small amounts. 

 On the other hand, the adipose tissue, strictly so called, consists almost 

 exclusively of the fats derived from the fatty acids palmitic, stearic, 

 and oleic, i.e. tripalmitin, tristearin, and triolein. The great differences 

 in the appearance of the fat of different animals are due to the 

 varying amounts in the relative quantities of these three fats which 

 may be present. While triolein is liquid at C., tristearin and 

 tripalmitin are solid at the temperature of the body. According 

 to the relative amounts of these three substances, therefore, we 

 may have a fat which, like mutton suet, is solid at the body tem- 

 perature, or a fat containing much olein which is still fluid and runs 

 away when the body is opened after death, even when it has already 

 cooled. 



PROPERTIES OF THE FATS. The fats are colourless substances 

 devoid of smell. They are insoluble in water, in which they float. 

 They are soluble in warm absolute alcohol, but separate out into crystal- 

 line form on cooling. They are easily soluble in ether. If they are 

 strongly heated with potassium bisulphate they give off pungent 

 vapours of acrolein derived from the decomposition of the glycerin 

 of their molecule. 



C 3 H 5 (OH) 3 - 2H 2 = C 3 H 4 



If they are heated with water or steam or submitted to the action of 

 certain ferments, they undergo hydrolysis, taking up three molecules 

 of water, and are split into three molecules of fatty acid and one 

 molecule of glycerin, e.g., 



C 3 H 6 (C 16 H 31 2 ) 3 + 3H 2 = 3HC 16 H 31 2 + C 3 H 5 (OH) 3 



(neutral fat tripalmitin) (palmitic acid) (glycerin) 



This process may occur spontaneously when fat is left exposed to 

 the air. Fat which has thus been artificially split in this way is said 

 to be rancid. Most natural fats generally contain a small amount 

 of fatty acid which gives them an acid reaction. 



On boiling a neutral fat for a long time with an aqueous solution 

 of potassium or sodium hydrate, or better still with an alcoholic 

 solution of potassium or sodium ethylate, the fat undergoes saponifica- 



