114 FATS 



istic instability of the unsaturated fatty acids. This instability is in 

 fact enhanced in the phospholipins generally to a remarkable degree, 

 and the difficulties attending their preparation and purification are 

 rendered exceptionally great by their extreme susceptibility to oxi- 

 dation. It is a fact which is doubtless of very great significance that 

 the tissues of the Brain are notably rich in phospholipins, while the 

 activities of the brain are exceptionally dependent upon an abundant 

 and continuous supply of oxygen, the first bodily activities to dis- 

 appear in asphyxia being those of the higher cerebral centers. 



The various members of the phospholipin group resemble one 

 another very closely in physical and chemical behavior. They differ 

 among themselves mostly markedly, first in the proportion of phos- 

 phorus to nitrogen which they contain, and secondly in their solubilities 

 in certain organic solvents. 



Those phospholipins which, like lecithin, contain one atom of 

 phosphorus (i. e., one molecule of phosphoric acid) for every atom of 

 nitrogen, are termed Monoamino-monophosphatids ; those which con- 

 tain two molecules of phosphoric acid for every atom of nitrogen 

 (P : N = 2 : 1), are termed Monoamino-diphosphatids ; those which 

 contain two atoms of nitrogen for every atom of phosphorus are 

 termed Diamino-monophosphatids (P : N = 1 : 2), and so forth. 

 The highest proportion of nitrogen to phosphorus which has been 

 found to occur in a phosphatid is that of four atoms of nitrogen for 

 every atom of phosphorus. 



The majority of the phospholipins are soluble in alcohol and in 

 ether, but some of them are insoluble in ether, and others, while 

 soluble in alcohol or in ether alone, are insoluble, or but sparingly 

 soluble in certain mixtures of the two. The great majority of the 

 phospholipins, but not all of them, are precipitated from ether solu- 

 tions by the addition of acetone, a fact which is utilized very frequently 

 in their preparation. They are also precipitated by a variety of 

 metallic salts, and platinum chloride, and particularly cadmium chloride 

 are frequently employed for their separation and purification. 



The phospholipins are amorphous substances which are generally 

 white or cream-colored when pure, but darken rapidly on exposure 

 to the air. The iodine-number simultaneously diminishes, indicating 

 that the unsaturated linkages have been partially neutralized by com- 

 bination with oxygen. This oxidation is particularly accelerated by 

 heat and by traces of moisture, and the dried or partially dried phos- 

 pholipins are unfortunately extremely hygroscopic, rapidly attracting 

 and condensing moisture when exposed to the air. The drying of 

 phospholipins without decomposition can therefore only be achieved 

 at low temperatures, and in vacuo or in an atmosphere composed of 

 some indifferent gas. There seems to be some reason for supposing 

 that the lability of the phospholipins may be greatly enhanced by 

 impurities which are commonly associated with them, and that in the 

 absence of these, they may be comparatively stable. 



