CHOLESTERIN. 101 



In like manner stearin would form a stearate, and olein an 

 oleate. The sodium soaps are "hard," and those of potassium are 

 " soft." In the discussion of intestinal digestion it will be seen that 

 the process of saponification takes place in the small intestine, and 

 that the soap there formed aids in the important functions of that 

 portion of the alimentary canal (p. 236). 



Emulsification. Besides being saponifiable, fats are also emul- 

 sifiable capable of forming an emulsion. If oil and water are 

 poured into a test-tube, they will at once separate, the oil floating 

 on the water. If the mouth of the tube is closed by the thumb 

 and the tube firmly shaken, the oil and water will form a milky 

 mixture, but will separate again when the agitation ceases ; if a 

 small amount of an alkali is added and the tube is again shaken, 

 separation will not take place as before, but the milky appear- 

 ance will continue for some considerable time. If a drop of the 

 mixture is placed under the microscope, it will be found that the 

 oil-globules have been broken up into an exceedingly fine state of 

 subdivision, some of the particles being too small to measure even 

 with a very high magnifying power. This more or less permanent 

 subdivision and suspension of the oil-globules constitutes an emul- 

 sion. The change is not a chemical one, but purely physical. A 

 similar process takes place in the small intestine during intestinal 

 digestion (p. 237) and is regarded by some as a necessary prelimi- 

 nary to the absorption of fat (p. 261). 



The fat in milk is in an emulsified condition ; consequently 

 milk may be regarded as a natural emulsion. 



I/ecithin. This substance may be regarded as a fat, and from 

 the fact that it contains phosphorus it has been spoken of as " phos- 

 phorized fat. 7 ' Its formula is C 42 II 84 NPO 9 . It is decomposable 

 into glycerin, stearic acid, phosphoric acid, and an alkaloid, 

 cholin. 



Lecithin occurs in the brain and other nervous tissues, consid- 

 ered by some authorities as here produced by decomposition of 

 protagon, in yolk of eggs, blood-corpuscles, semen, bile, and milk. 

 It is also one of the constituents of protoplasm. 



Cholesterin. This substance bears some resemblance to the 

 fats in that it is insoluble in water, but soluble in ether, hot alco- 

 hol, and chloroform. It is a constituent of protoplasm, and is also 

 found in blood-corpuscles, bile, serum, and white substance of 

 Schwann. In the blood it is in combination with oleic and pal- 

 mitic acids. It forms esters with fatty acids, and as such exists 

 in the fatty secretions of the skin. Lanolin, the fat obtained from 

 sheeps' wool, is said to be rich in esters, and these are very resist- 

 ant to the action of bacteria. 



