THE CHEMICAL COMPOSITION OF THE BODY. 119 



When treated with sulphuric acid and then neutralized with solid potas- 

 sium hydrate, it will give both the Biuret-test, denoting the presence of 

 proteid matter, arid also Fehling's test, showing the presence of a sugar: 

 the acid splits it into a globulin and a carbohydrate. 



Nucleins. The substance known as nudein and found in all cells 

 as well as in milk (caseinogen) and the yolk of egg (vitellin) is really a 

 compound proteid and consists of a whole series of bodies made up of 

 proteid and nucleic acid in varying proportions ; there is almost no limit 

 to the possible variations. At one end of the series is nucleic acid (C 30 - 

 H M N B P 3 17 , according to Kossel), a body containing the maximum (9 to 

 11 per cent) of phosphorus, but without any proteid, and found as such 

 only in spermatozoa; in the middle are the nucleins proper; and at the 

 other end are the nucleo-proteids, containing the minimum of phosphorus. 

 As phosphorus is the characteristic component of nucleic acid, its amount 

 will measure the amount of the acid present in any molecule. 



The karyoplasm (nucleus) of every cell is richer in the nucleins, while 

 the cytoplasm (cell body) is richer in the nucleo-proteids which contain a 

 smaller proportion of nucleic acid and, therefore, of phosphorus. The 

 difference in staining power of the nucleus and cell body is thus explained 

 as the relative affinity of these substances for a basic dye is proportional 

 to the amount of nucleic acid they contain. The chemical differences in 

 the action of cytoplasm and karyoplasm toward solvents are due also to 

 the proportion of nucleic acid and proteid which they contain. These 

 differences are quantitative and not qualitative. All of the nucleo-pro- 

 teids in the cell body are true ones in that they yield nuclein bases. 



Nudein Bases. These are xanthin, hypoxanthin, adenin, and guanin; 

 they are closely related nitrogenous bodies which are always present in 

 every chemical change in the cell, and one may be transformed into an- 

 other. They are also known as xanthin or purin bases, and all can be 

 derived from the so-called purin nucleus C 5 N 4 by substitution of atoms; 

 the purin base, as isolated by Emil Fischer, is C.H 4 N 4 . 



Hypoxanthin or oxypurin is . . . CsEUN^O 



Xanthin or dioxypurin is . . % . C 5 H 4 N4O 2 



Adenin or amino-purin is .. _-.' . C 5 H 5 N5 



Guanin or amino-oxypurin is V . . C 5 H 5 N 5 O. 



Uric acid is closely related, though not one of the nuclein bases, being 

 trioxypurin, C 5 H 4 lSr 4 3 . Caffeine, the active principle of coffee, is tri- 

 methyl dioxypurin, C 8 H ]o N 4 2 simply xanthin with three atoms re- 

 placed by the methyl group CH 3 . 



Caseinogen. Caseinogen, the chief proteid of milk, is strictly a 

 nucleo-albumin and does not yield the nuclein bases ; it bears the same 

 relation to casein that nbrinogen does to fibrin. When acted on by ren- 

 nin it splits into two parts of which one, the smaller, is peptone-like in 



