40 LABORATORY WORK IN PHYSIOLOGICAL CHEMISTRY. 



allowed to stand for three days in cold ether. The solution 

 is then filtered, and to the filtrate, which contains the lecithins 

 and cholesterol, acetone is added. The precipitated leci- 

 thins may be removed by filtration and the ether-acetone 

 filtrate evaporated to dryness. (Keep this cholesterol.) 

 Try the following reactions with the lecithin: 

 (a) Place a particle of the substance on a glass slide and 

 add a drop or two of water. Examine this under the micro- 

 scope. 



(6) Shake some of the substance in water and examine 

 microscopically. 



(c) To some of the substance add a drop of osmic acid. 

 What is the effect? 



(d) Test the substance for phosphorus by fusion. 



(e) Test for the glycerol radical. 



CEREBROSIDES. 



This class of non-phosphorized nitrogenous substances, 

 whose exact composition is not known, is characterized by a 

 glucoside constitution yielding upon decomposition a sugar 

 which is identical with galactose. Fatty acids in relatively 

 large quantities can also be isolated from the cleavage prod- 

 ucts. 



CEREBRIN. 



This is the most important substance in this group. 

 Whether it exists uncombined in the tissue is still an un- 

 decided question. It is insoluble in water, dilute alkalies, 

 or baryta-water. It dissolves in boiling alcohol, from which 

 solution it separates as a flaky precipitate upon cooling. 

 This precipitate is made up of microscopic globules. 



