THE CHEMICAL BASIS OF THE BODY 757 



The Non-Nitrogenous Bodies. 



Fats. — The fats found in body-fat, milk, and the marrow of bones, 

 are compounds formed by the union of fatty acids with glycerine. 

 These fats are palmitin, stearin, and olein. 



Fatty acids are formed by the oxidation of alcohols, the group 

 being a large one. Some of the earlier members of the series, such 

 as acetic, propionic, valeric, caproic, have been referred to in speak- 

 ing of the amino acids, for these form glycine, alanine, valine, and 

 leucine, in the manner already described. The acids responsible 

 for the body fats — palmitic and stearic acids — are the sixteenth 

 and eighteenth in the acetic series, while oleic is the eighteenth in 

 the acrylic series. A certain proportion of the fats in milk, and hence 

 in butter, is formed from acids lower down in the acetic series, 

 such as caproic, caprylic, and capric acids. 



Fat is insoluble in water, and only slightly so in alcohol, but 

 freely soluble in ether, chloroform, and benzene. When pure, it 

 is neutral in reaction, tasteless and colourless, and by the action of 

 caustic alkalies or superheated steam may be decomposed into its 

 respective fatty acid and glycerine. When this splitting is brought 

 about by an alkali, the base, sodium or potassium, at once unites 

 with the free fatty acid and forms a salt (soap). This decomposi- 

 tion and saponification takes place to a greater or less extent in 

 the intestine under the influence of the pancreatic juice and bile. 



The solid fat of the body is composed principally of stearin, 

 such as is found in the ox and sheep. The more liquid fat, such as 

 is found in the horse and carnivora, contains palmitin, but in all 

 cases a mixture of the three fats is obtained. Fat as it exists in 

 the cells of the living body is, of course, in a liquid condition. Since 

 the melting-point of palmitin is 45 C, and that of stearin 55 to 

 6o° C, it is evident that the fluidity of living fat is due to the olein 

 it contains, the melting-point of which is - 5 C. The amount of 

 fat in the body must depend upon the feeding of the animal, and will 

 obviously vary within extreme limits. In individual tissues marrow 

 has the largest amount ; nerve, brain, milk, muscle, liver, bone, 

 bile, and blood, have proportions which decrease in the order given. 

 The change which the fats undergo in the alimentary canal has been 

 discussed in the chapter on the Pancreas (p. 256), while the origin 

 of fat in the body, and its function is dealt with under Nutrition 

 (P-358). 



Lipoids. — This term is applied to a group of bodies found mixed 

 with fat in various tissues and organs of the body, especially with 

 protoplasm, cellular structures, and nervous tissue. The groups 

 are distinguished by their chemical composition. One important 

 member (cholesterin) contains neither nitrogen nor phosphorus ; 

 another contains nitrogen, but no phosphorus ; a third both nitrogen 

 and phosphorus. The three to be considered are lecithin, choles- 

 terin, and a group known as galactosides. Lecithin is a wax-like 

 body consisting of glycerine, stearic acid, phosphoric acid, and a 

 nitrogenous base known as choline. It occurs abundantly in the white 

 matter of the nervous system, and is also found in cellular structures 

 and in bile. In the latter some of it is united to a carbohydrate 

 residue, and forms Jecorin. The function of lecithin in the system 

 is unknown, but its wide distribution in cellular structures points to its 

 being concerned in the processes of metabolism. The base choline men- 

 tioned above is poisonous, and on oxidation with nitric acid yields the 

 extremely poisonous substance muscarine. Lecithin enters the body 



