4OO THE CHEMICAL COMPOSITION OF THE BODY. [CH. xxv. 



3. NUCLEINS AND NucLBO-PROTEiDS. These are compounds of 

 proteid with a complex organic acid called nucleic acid, which 

 contains phosphorus. 



Nucleo-proteids. Compounds of proteids with nuclein. 

 They are found in the nuclei and protoplasm of cells. Casei- 

 nogen of milk and vitellin of egg-yolk are similar substances. 

 In physical characters they often closely simulate mucin ; in fact, 

 the substance called mucin in the bile is in some animals a 

 nucleo-proteid. They may be distinguished from mucin by the 

 fact that they yield on gastric digestion not only peptone but 

 also an insoluble residue of nuclein which is soluble in alkalis, 

 is precipitable by acetic acid from such a solution, and contains a 

 high percentage ( i o- 1 1 ) of phosphorus. 



Some of the nucleo-proteids also contain iron, and it is probable 

 that the normal supply of iron to the body is contained in the 

 nucleo-proteids, or hsematogens (Bunge), of plant and animal cells. 



The relationship of nucleo-proteids to the coagulation of the 

 blood is described in the next chapter. 



Nucleo-proteids may be prepared from cellular structures like 

 testis, thymus, kidney, <fec., by two methods : 



1. Wooldridge's method. The organ is minced and soaked in 

 water for twenty-four hours. Acetic acid added to the aqueous 

 extract precipitates the nucleo-proteid, or, as Wooldridge called 

 it, tissue fibrinogen. 



2. Sodium chloride method. The minced organ is ground up 

 in a mortar with solid sodium chloride ; the resulting viscous 

 mass is poured into excess of distilled water, and the nucleo- 

 proteid rises in strings to the top of the water.- 



The solvent usually employed for a nucleo-proteid, whichever 

 method it is prepared by, is a i per cent, solution of sodium 

 carbonate. 



Nuclein is the chief constituent of cell-nuclei. Its physical 

 characters are somewhat like those of mucin, but it differs 

 chemically in its high percentage of phosphorus. It is identical 

 with the chromatin of histologists (see p. 11). On decomposi- 

 tion, it yields an organic acid called nucleic acid, together with 

 a variable amount of proteid. In the nucleus which composes 

 the head of a spermatozoon proteid is practically absent. Nucleic 

 acid on decomposition yields phosphoric acid and various bases 

 of the xanthiiie group. Some forms of nuclein, called pseudo- 

 nuclein, such as are obtained from casein and vitellin, differ 

 from the true nucleins in not yielding these xanthine com- 

 pounds, or, as they are sometimes termed, alloxuric or purine 

 bases. The purine bases are closely allied chemically to uric 



