CHEMICAL COMPOSITION OF THE HUMAN BODY 35 



epithelial cells of mucous membranes, and in tendons and 

 the umbilical cord. They are insoluble in water, but, because 

 of their acid properties, give a neutral, stringy solution with 

 weak alkalies. They are not coagulated by boiling and are 

 completely precipitated from salt-free solutions by acids as 

 well as by alcohol and most of the proteid precipitants (not 

 by nitric acid, nor by acetic acid and potassium ferro- 

 cyanide). They give all the color reactions of proteids. 

 By boiling with acids, they split up into proteid and a poly- 

 saccharide, the animal gum. They serve to. lubricate the 

 mucous membranes, and to shield them from mechanical and 

 chemical injuries. 



(3) Compounds of proteid with substances containing 

 phosphorus are nucleins and nucleo-albumins. 



(a) Nucleins, so called because they were first obtained 

 from the nuclei offish-blood corpuscles, are divided into: 



(a) Paranucleins, i.e. compounds of proteids and 

 phosphoric acid. 



(/#) The true nuclcins, i.e. compounds of proteids 

 and nucleic acids which are composed of phosphoric 

 acid and xanthin or nuclein bases. 



(b) Nucleo-albumins or nucleo-proteids are compounds of 

 nuclein and proteids. They are found in cells as the con- 

 stituents of the nucleus and of the protoplasm. The chro- 

 matin of the nucleus and probably the structure of the 

 protoplasm capable of staining, are composed of nucleo- 

 proteids. They are insoluble in water but soluble in dilute 

 alkalies, with which they form neutral compounds because 

 of their strong acidity. They are precipitated by acids and, 

 in their precipitated condition, they are coagulated by heat. 

 They also give most of the proteid reactions. Because of 

 their readiness to break up, it is difficult to isolate them. 



By boiling with dilute acids or alkalies the nuclein is split 

 from the nucleo-albumin and, by continued action of these 

 reagents, this breaks up into proteid, phosphoric acid, and, 

 eventually, the xanthin bases (xanthin, guanin, adenin, 

 hypoxanthin, etc.), which are closely related to uric acid. 



