THE NITROGENOUS SUBSTANCES 



75 



moses and peptones which will be fully treated in our study of digestion 

 (Chapter VII). 



E. Finally, we should mention the compounds arising by coagulation of 

 the soluble proteids, whose properties are less well known than those of other 

 proteid substances. To this group belongs also the fibrin formed in the coagu- 

 lation of the blood by splitting of fibrinogen (see Chapter V). 



In order to give a more exact idea of the quantitative composition of the 

 simple proteids, we have brought together in the following table the results of 

 analyses by Aberhalden. 





* Present. 



B. THE COMPOUND PROTEIDS 



Simple proteids are distinguished largely by the fact that they can unite 

 with each other as well as with other substances to form new compounds. 

 The atomic group conjoined with the proteid in the latter case is described 

 by Kossel as the prosthetic group. Such conjugated proteids can be isolated 

 from the animal fluids and tissues in great numbers. They are classified 

 according to the nature of the conjugant into the following groups : 



A. Haemoglobins: These represent compounds of a basic proteid body, 

 globin, with the acid, iron-containing, pigment haemochromogen, and will be 

 discussed at length in Chapter Y. 



B. Nucleo-albumins : phosphorus-containing proteids which are character- 

 ized by the fact that on digestion with pepsin-HCl a portion remains tem- 

 porarily insoluble. This portion, like the soluble portion, contains phosphorus, 

 and is described as pseudonuclein or paranuclein. It is distinguished from 

 the true nucleins (see below) by not containing any purin bases. 



To the nucleo-albumins belong the casein of milk, whose properties will be 

 fully considered later (Chapter XXVI) ; vitellin found in the yolk of a bird's 

 egg; various proteids of the bile, the kidneys, the mucous membrane of the 

 bladder, etc. 



7 



