1020 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Nudeohiston, soluble in H 2 0, 

 decomposed by HC1 or Ba(OH) 2 into 



Histon, a proteid. Leuconuclein, an acid ; 



decomposed by strong alkali into 



Proteid. Adenylic add (nucleic acid), which on heating 



with mineral acids yields adenin, thymin, 

 levulic acid, and phosphoric acid. 



(For the respective offices of histon and leuconuclein in the coagulation of the blood, 

 see section on the Blood. ) 



In the sperm of salmon is found only free nucleic acid uncombined with proteid. 

 According to Kossel other nuclei may at times contain free nucleic acids. 



Phospho-glyco-proteids. This class is represented by Hammarsten's helico-proteid, 

 which yields paranuclein, and, unlike other nucleo-proteids of the paranuclein class, it 

 yields a reducing carbohydrate on boiling with acids. 



The Albuminoids. These are bodies derived from true proteid in the body, but not 

 themselves convertible into proteid. They are resistant to the ordinary proteid solvents, 

 and as a rule exist in the solid state when in the body. 



Collagen. This is the chief constituent of the fibres of connective tissue, of the 

 organic matter of bone (osse'in) and is likewise one of the constituents of cartilage. Col- 

 lagen is insoluble in water, dilute acids and alkalies. On boiling with water it forms 

 gelatin through hydration, which is soluble in hot water, but gelatinizes on cooling (as in 

 bouillon). Dry gelatin swells when brought into cold water. By continuous boiling or by 

 gastric or tryptic digestion further hydration takes place with the formation of soluble 

 gelatin peptone. Gelatin fed will not take the place of proteid, but, like sugar, only more 

 effectively, it may prevent proteid waste by being burned in its stead. 1 Gelatin yields leucin 

 and glycocoll on decomposition, but no tyrosin. It therefore gives the biuret reaction, but 

 none with Millon's reagent. It contains but little sulphur. It yields about the same 

 amido- acids as ordinary proteid. 



Elastin. This is very insoluble in almost all reagents and in boiling water. On 

 decomposition it yields leucin, tyrosin, glycocoll, and lysatin. It is slowly hydrated by 

 boiling with dilute acids, and by pepsin hydrochloric acid. It contains very little sulphur, 

 and gives Millon's test. It is found in various connective tissues, and especially in the 

 cervical ligament. 



Keratin and N euro-keratin. These are insoluble in water, dilute acids and alkalies ; 

 insoluble in pepsin hydrochloric acid, and alkaline solutions of trypsin. Keratin is found 

 in all horny structures, in epidermis, hair, wool, nails, hoofs, horn, feathers, tortoise-shell, 

 whalebone, etc. Nenro-keratin has been discovered in the brain, and in the medullary 

 sheath of nerve-fibres. 2 On decomposition with hydrochloric acid keratin yields all the 

 products given by simple proteids. It contains more sulphur than simple proteid and 

 yields more tyrosin. Drechsel 3 believes that it is transformed from simple proteid by the 

 substitution of sulphur for some of the oxygen and of tyrosin for leucin or other amido- 

 acid. Part of the sulphur is loosely combined, and a lead comb turns hair black, due to 

 the formation of lead sulphide. There are different keratins, and their sulphur content 

 varies greatly. 



GENERAL REMARKS ON THE PROTEIDS. It has been impossible within 

 the limits set to more than glance at the proteid bodies. Many facts concern- 

 ing the behavior of proteids have been mentioned throughout the text, and 



1 Voit: Zeitschrift filr Biologic, 1872, Bd. 8, p. 297. 



2 Kiihne and Chittenden : Zeitschrift fur Biologic, 1890, Bd. 26, p. 291. 

 8 Ladenburg's Handworlerbuch der Ch&nie, 1885, Bd. 3, p. 571. 



