36 HUMAN PHYSIOLOGY 



Some nucleo-proteids also contain iron, e.g. the hcemalogen of 

 the egg-yolk, so called because haemoglobin is supposed to origi- 

 nate from it. 



The best-known nucleo-proteid is caseinogen of milk. 

 It is formed during secretion by the milk-gland. It is in- 

 soluble in water, forms soluble compounds with alkalies and 

 alkaline earths, and is split up by acids, yielding paranuclein. 

 Caseinogen is not coagulated by boiling, but it is precipi- 

 tated by weak acids. By the action of the ferment rennin 

 it yields casein, a proteid which forms an insoluble compound 

 with calcium. 



III. Proteoses. The proteoses are the products of the 

 splitting up of the simple and combined proteids. They are 

 formed during digestion of proteids or by the action of dilute 

 acids upon proteids, and they differ but little in elementary 

 composition from each other and from the proteids out of 

 which they are formed. Their formation does not depend 

 upon deep-seated chemical changes of the proteids, but only 

 upon the splitting up of a large molecule into many similar 

 smaller molecules, under the assumption of water. It is 

 only in the amount of sulphur they contain that they differ 

 from each other and from the mother-substances. 



In the splitting up of simple proteids many intermediate 

 products are formed which are called the albumoses, while 

 the end-products are called peptones. According to their 

 origin, the albumoses are called fibrinoses, globuloses, vitel- 

 loses, caseoses, and myosinoses. 



The proteoses (albumoses and peptones) are all readily 

 soluble in water, except heteroalbumose, and many of them 

 (peptones and some albumoses) are dialyzable. They are 

 not coagulated by heat; alcohol precipitates them with 

 difficulty but does not coagulate them. They are all levo- 

 rotatory. The rotatory power of all the proteoses formed 

 by gastric digestion from a simple proteid body is greater 

 than that of the undigested proteid, but the rotatory power 

 of the products formed by pancreatic digestion is smaller 

 than that of the original proteid. 



