CHAPTER XI 



THE METABOLISM OF FAT 



IN the animal fats are found in all the organs and tissue, and this 

 is so before birth, so fat is a normal tissue component. The per- 

 centage amount in each tissue is, however, so variable that there 

 is little profit in giving figures. In the heart freed from connec- 

 tive tissue fat anything up to 16 per cent, of the dry matter may 

 be regarded as normal 1-2-1 '3 grm. fat to 1 grm. nitrogen (Rosen- 

 feld) ; in the muscles up to 4 per cent. There are three chief fat 

 depots the intra-muscular, the abdominal, and the subcutaneous 

 connective tissues. The connective tissue cells at the outset have 

 no particular character, but as the globules of fat accumulate and 

 coalesce within them, they enlarge until their protoplasm forms a 

 thin envelope and the nucleus is pushed to one side. In plants 

 fats occur, especially in seeds and fruits ; in some cases they are found 

 in the roots. The fat of animals consists of neutral fats with small 

 quantities of fatty acids. The neutral fats are esters of the 

 trihydroxyl alcohol glycerine with the mono-basic fatty acids. Three 

 hydrogen atoms of the hydroxyl groups of the glycerine are replaced 

 by a radical of fatty acid forming a triglyceride, C 3 H 5 3 R 3 . Animal 

 fat chiefly consists of the esters of stearic, palmitic, and oleic acids. 

 In addition, there are small quantities of esters of some other fatty 

 acids especially myristic. In milk there also occur glycerides 

 of volatile fatty acids butyric, caproic, caprylic, &c. In plants other 

 fatty acids are sometimes richly present, such as erucic in colza oil, 

 linolic in linseed-oil, &c. ; oxy-fatty acids, and waxes or alcohols of high 

 molecular structure, are also found in many plants, and may occur in 

 small amounts in animal fats. The average elementary composition 

 of animal fat is : 0, 76'5 ; H, 12'0 ; 0, 11-5. In different animals, and 

 even in different parts of the body of the same animal, the fat has a 

 different consistence. The more solid fat contains a greater pro- 

 portion of stearin and palmitin, the more fluid, a greater proportion 

 of olein. The latter kind of fat exists in cold-blooded animals, so 

 that their fat remains soft at ordinary temperatures. Human fat 

 contains 67-80 per cent, olein. Olein is the solvent of the other 

 fats, and the melting-point of a fat depends on the proportionate 

 composition of the mixture. Stearin, C 3 H 5 (C ]8 H 35 2 ) 8 , melts at 55-71. 



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