108 GENERAL PHYSIOLOGY 



coagulates spontaneously upon standing — a phenomenon that 

 appears in dying muscle in rigor mortis ; and, finally, plant- 

 globulin, which gives to kernels of grain their glutinous quality, 

 and hence has been termed glutin. 



The vitellins are likewise soluble in neutral salt solutions only, 

 but, in contrast to the globulins, they are not precipitated by satura- 

 tion of the solution with salts. Among them are the so-called yolk- 

 plates of the yolk of eggs, and the already-mentioned aleurone 

 grains of plant seeds, both of which are proteids capable of crystal- 

 lization. 



The above-mentioned proteids occur in a free state in living 

 substance. A very large number of proteids, however, are not free, 

 but are chemically combined with other substances. In these com- 

 pounds, which have been termed combined proteids in distinction 

 from the simple proteids, the proteid molecule behaves in general 

 like a feeble acid, and by the addition of stronger acids it can 

 frequently be forced out of its compounds,- the stronger acid taking 

 its place. The proteid then becomes free. We have already become 

 acquainted with one of these compounds, hasmoglobin, which plays 

 in blood so important a role and is a compound of proteid and iron. 

 But the most important compounds, in which proteids appear with- 

 out exception in every cell, are the nucleina. The nucleins, as 

 Altmann ('89) has shown,are compounds of proteid with nucleic acid, 

 an acid which is itself a compound of phosphoric acid with peculiar 

 basic bodies, the so-called nuclein bases — guanin, adenin, xanthin 

 and hypoxanthin. The nucleins are capable of entering into 

 further combinations with a second proteid molecule, and these ex- 

 tremely complex compounds are termed nudeo-proteids or nucleo- 

 alhumins. Casein, a body which for a long time has presented 

 difficulties to the physiological chemists, is such a nucleo-proteid, 

 combined with calcium. Casein is the calcareous nucleo-proteid 

 of milk that is manufactured into cheese ; it has the peculiarity 

 of not coagulating when the milk is boiled, while it is immediately 

 precipitated when separated, as by acetic acid, from the calcium. 

 A fourth group of combined proteids is that of the glyco-proteids, in 

 which proteid is combined with a carbohydrate ; prominent among 

 these is mucin, which is contained in the cells of mucous glands. 



Besides the genuine proteids which we have just described, 

 there exist a number of bodies which behave in manj' ways 

 similar to proteids and, therefore, have been termed albuminoids. 

 The group of albuminoids is a true omnium gatherum ; it contains 

 a very large variety of bodies. These are partly compounds of pro- 

 teids and partly bodies of similar constitution to the proteids, but 

 which show in their chemical behaviour much less similarity and 

 are much less known than the proteids themselves. Especially 

 prominent among albuminoids are many of those substances that 

 are produced by cells to serve as skeletal substances for the support 



