NERVOUS TISSUES. 39 



stituents of cells in general. The cerebrosides are found in 

 plant as well as animal tissues. So little that is positive 

 is known about ' ' protagon " that no notice need be taken of it. 



LECITHINS. 



CH-O-R' 



CH 2 -0- 



PO 0-C 2 H 4 \ 



(CH 3 )=N 

 OH OH/ 



R and R'=the same or different fatty acid radicals. 



The structure of the lecithin molecule is proved by the 

 decomposition products which are obtained from it, viz., 

 chohne, glycerophosphoric acid, and fatty acids. From the 

 fact that various fatty acid radicals enter into the composi- 

 tion of the molecule and that the nitrogenous complex may 

 also differ, it follows that the existence of many lecithins is 

 possible. As a class they are soluble in ether and alcohol 

 and from such solutions may be precipitated by cooling to 

 C. or by the addition of acetone. The lecithins combine 

 with acids and bases as well as with proteins and other bodies 

 to form more or less loose combinations, such as the lecitho- 

 proteins. With water they swell up, giving off long fila- 

 ments (myelin forms), and finally pass into an emulsion. 



Various lecithins can be separated from the yolk of the 

 egg, from yeast, or from brain-tissue. The procedure for the 

 isolation of the brain-lecithins is as follows: The finely 

 divided brain-tissue, which may or may not have been dehy- 

 drated by boiling with acetone for some hours previously, is 



