CHEMIC COMPOSITION OF THE HUMAN BODY. 21 



NUCLEO-PROTEmS. 



The nucleo-proteins are obtained from the nuclei and cell-substance 

 of tissue-cells. Chemically they are characterized by the presence of phos- 

 phorus in relatively large amounts. When hydrolyzed, they separate into a 

 protein and a nuclein. 



The nucleins derived from cell nuclei can be still further separated into 

 a simpler protein and nucleic acid, which latter in turn yields phosphoric 

 acid and the so-called purin bases, xanthin, hypoxanthin, adenin, and 

 guanin. All nucleins which yield the purin bases are termed true nucleins. 



PHOSPHO-PROTEINS. 



The two members of this group are distinguished by yielding on decom- 

 position a protein which contains phosphorus. They do not give rise to 

 purin bases as they at one time were supposed to do. 



(a) Caseinogen. This is the principal protein of milk, in which it 

 exists in association with an alkali, and hence was formerly regarded 

 as an alkali-albumin. It is precipitated by acetic acid and by mag- 

 nesium sulphate. It is coagulated by rennet that is, changed into 

 an insoluble protein, casein or tyrein. Calcium phosphate seems to 

 be the natural alkali necessary to this process, for if it be removed by 

 dialysis, or precipitated by the addition of potassium oxalate, coagula- 

 tion does not take place. 



(6) Vitellin. Vitellin is a constituent of the vitellis or yolk of eggs. 

 It differs from other proteids in the fact that it is semicrystalline 

 in character. Though usually regarded as a nucleo-proteid it is 

 not definitely known ^ whether or not it contains phosphorus in its 

 composition. 



SCLERO-PROTEINS (ALBUMINOIDS). 



The albuminoids constitute a group of substances similar to the proteins 

 in many respects, though differing from them in others. When obtained 

 from the tissues, in which they form the organic basis, they are found to be 

 amorphous, colloid, and when decomposed yield products similar to those 

 of the true proteins. The principal members of the group are as follows: 

 (a) Collagen, Ossein. These are two closely allied, if not identical, sub- 

 stances, found respectively in the white fibrous connective tissue and 

 in bone. When the tendons of muscles, the ligaments, or decalcified 

 bone are boiled for several hours, the collagen and ossein are converted 

 into soluble gelatin, which, when the solution cools, becomes solid. 

 (6) Chondrigen. This is supposed to be the organic basis of the 

 more permanent cartilages. When they are boiled, they yield a 



