246 



PROPERTIES OF FAT. 



from the air. If fermented in a large quantity of water with yeast, it is 

 converted into metacetonic acid. It occurs in the yolk of the egg, and 

 also in the fats of the brain. By gradual oxidation it can give rise to 

 lactic acid. 



The physical properties of the fats depend, for the most part, on the 

 nature of their acids. The fats derived from animals are of various de- 

 grees of consistency ; they are colorless or white, lighter than water, bad 

 conductors of heat. They are insoluble in water, and burn, in the pres- 

 ence of air, into carbonic acid and water, with the evolution of much heat. 

 By the action of certain nitrogenized ferments they may be separated into 

 their acid and glycerine, and by the action of pancreatic juice, as ex- 

 plained previously, may be brought into the condition of an emulsion. 

 The more important of the animal fats are stearine, margarine, and oleine. 

 Places of occur- They are inclosed in cells accumulated in various parts of 

 rence of fat. the system, such as in the orbit of the eye, around the heart, 

 and among the muscles of the face, under the cutis, and within the bones. 

 In morbid states they sometimes abound in the kidneys, liver, and spleen. 

 They are also discovered in some of the animal fluids : thus they commu- 

 nicate to the chyle its characteristic property, and therefore likewise oc- 

 cur in the blood. In their relative amount they vary at different periods 

 of life, being in a larger proportion in childhood, and again after the mid- 

 dle period. Their quantity likewise changes with physical changes, di- 

 minishing, for instance, after continued muscular exertion, and also by 

 long exposure to cold. 



Though the amount of fat in the blood varies with the nature of the 

 Quantity of fat f od > it can not, however, be increased, in a state of health, 

 in the blood, beyond a certain point, owing to the inability of the absorb- 

 ents to receive more than a definite quantity. The serum of arterial con- 

 tains less fat than the serum of venous blood; the blood of women more 

 than that of men. 



The manner of occurrence of fat in 



organized structures is twofold : oft- 



en it occurs in the free state, but also 



is very commonly inclosed in the in- 



terior of cells, as shown in Fig. 107, 



which is a fat-cell, a being the adi- 



pose membrane, and b the nucleus. 

 Fig. 108, adipose and areolar tissue: a, #, fat- 

 cells ; &, &, fibres of areolar tissue. 



Respecting the origin of the fat substances in 

 Fats arise from plants there can be no question. They 



are derived fr o m the decomposition of Adi P se and areolar tissue ' 



pig. ios. 



Fat-ceii. 



turning thereto, carbonic acid and water by those organisms under the in- 



