136 NUTRITIONAL PHYSIOLOGY 



stuffs, and we shall find that the story is most simple in the 

 case of the fats. 



Fat Metabolism. Broadly speaking, we can say that 

 fat never becomes anything else until it is decomposed with 

 release of energy. This statement may be too sweeping 

 to cover all the conditions which may arise in disease, 

 but it is substantially true in health. We have traced the 

 fat from the walls of the intestine into the lymphatics. 

 From the smaller branches it must find its way to the 

 thoracic duct, and through this vessel it goes to merge with 

 the general blood-stream. In the blood, the lymph, and 

 the tissues at large fat is present in a small percentage. 

 We must now give attention to the special provision made 

 for the storage of fat in what is called adipose tissue. 



The word "fat" is used in two senses. In its strict 

 chemical meaning it describes a certain type of compound, 

 and it is this usage which we have thus far employed. 

 But when we speak of the fat of meat we include something 

 more. We mean a form of connective tissue in which the 

 cells hold a large accumulation of fat in the chemical sense. 

 Under the microscope this tissue is seen to be composed of 

 a fibrous network holding within its meshes these distended 

 cells. The fat which they contain is in drops of such a size 

 that the protoplasmic portion of each cell seems a mere 

 envelope, while the nucleus is crowded to one side. So it 

 happens that while the fat is really an inclusion, it forms a 

 very large percentage of the whole mass. When a piece 

 of adipose tissue is subjected to the action of gastric juice 

 the fibers and protein of the cells are rapidly digested, and 

 the actual fat, -being liberated, rises to the top and floats 

 as a clear layer of oil. 



Of course, the amount of adipose tissue varies widely 

 with the individual. Still it is more abundant in subjects 

 of spare build than is usually supposed. The hollow shafts 

 of the long bones, such as those of the arms and legs, con- 

 tain what is called the white marrow. This is typical 

 adipose tissue. A large deposit is to be found at the back 

 of the abdominal cavity, where it closes round the upper 



