CHAPTER XII. 

 SPECIAL METABOLISM (Cont'd). 



Metabolism of Fats. Fats are absorbed into the lacteals and 

 discharged into the blood of the left subclavian vein through 

 the thoracic duct. They are carried to various parts of the body 

 and gain entry into the cells, in the protoplasm of which they 

 become deposited. This process occurs extensively in the sub- 

 cutaneous connective tissues, between the muscles, and retroperi- 

 toneally around the kidney (the suet). The fat which is thus 

 deposited possesses more or less the same qualities as the fat of 

 the food. Thus, when the only fat taken over a long period of 

 time is one with a very low melting-point, such an oil, the fat 

 deposited in the tissues is likely to be oily in character, whereas 

 it is stiff after feeding with a high melting-point fat, such as 

 mutton fat. This similarity between the tissue fat and that of 

 the food becomes very striking when the animal has been sub- 

 jected to a preliminary period of starvation and then fed for 

 some weeks with a large excess of the particular fat and as little 

 carbohydrate and protein as possible. Fat in the food is of 

 course not the only source of the fat in the tissues. It also be- 

 comes formed out of carbohydrates, a fact which is well known 

 to farmers, who fatten their stock by feeding them with maize 

 and other starchy grains, and to physicians, who reduce their 

 corpulent patients by restricting carbohydrate foods. The fat 

 thus deposited has the chemical characteristics of the fat which 

 is peculiar to that animal. It is almost certain that there is ordi- 

 narily no formation of fat out of protein in the higher animals. 



The fat thus deposited in the tissues may remain for a long 

 time, but ultimately it is again taken up by the blood and car- 

 ried to whatever active tissue requires it as fuel. Before being 

 thus burnt, it splits into glycerine and fat acid (see p. 75). The 

 fat acid possibly undergoes some preliminary change in the 



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