70 CHEMICAL CONSTITUENTS OF BODY AND FOOD. 



include keratin, elastin, collagen, gelatin, reticulin, amyloid substance, 

 and a group of materials called skeletins. 



Collagen. Collagen is the mother substance of gelatin. It is the 

 material of which the white fibres of connective tissue are made, and is 

 the principal constituent of which the organic substratum of bone is 

 composed; it is there called ossein. In cartilage the material 'called 

 chondrigen is collagen mixed with the mucinoid materials of the carti- 

 laginous matrix. Collagen has also been obtained from the flesh of 

 cephalopods. 1 



By boiling with water, especially if it is faintly acidified, collagen is 

 converted into gelatin ; and gelatin is reconverted into collagen by 

 heating it to 130 C. Hence collagen is regarded as the anhydride of 

 gelatin (Hofmeister) ; 2 the reaction* may be represented by the equation 



CK.HU.N.A.-H, o = c 10 . 2 H 110 N 31 o 28 



(gelatin) (collagen) 



The above formulae, however, cannot be regarded as more than provisional, 

 for we are as ignorant of the molecular constitution of the albuminoids 

 as of the proteids. Schiitzenberger attributes the formula C 70 H 124 ]Sr 34 29 

 to gelatin, and regards the sulphur described by other investigators as 

 due to admixture with proteid impurities. Hainmarsten, 8 on the other 

 hand, regards the sulphur, of which there is 0*6 per cent., as an integral 

 part of collagen and gelatin. 



Collagen is insoluble in water, alcohol, salt solutions, and dilute acids, 

 and alkalis. It swells with dilute acids. Its decomposition products 

 are the same as those of gelatin. 



Gelatin. Gelatin is a colourless, amorphous, and translucent sub- 

 stance ; it swells but does not dissolve in cold water ; it readily dissolves 

 in hot water, and on cooling the solution, if its concentration is greater 

 than 1 per cent., it sets into a jelly. It contains a considerable amount 

 of ash, the removal of which lessens its power of gelatinising. 4 



Gelatin is precipitated by saturating its solution with neutral salts, 

 like magnesium sulphate and ammonium sulphate. 5 This is also true for 

 gelatin which has been altered by the action of hot water so as to be no 

 longer or only partially gelatinisable. 



Gelatin is not precipitated by acetic acid, nor by acetic acid and ferro- 

 cyanide of potassium, nor by most of the heavy metallic salts that precipi- 

 tate proteids. It gives a violet colour with copper sulphate and caustic 

 potash ; it gives Millon's reaction, but only a faint xanthoproteic reaction. 6 



It is precipitated by mercuric chloride, and also, as in the process of 

 tanning, by tannic acid. Gelatin is levorotatory. 7 



Derivatives of gelatin. The prolonged action (twenty-four hours) of 

 boiling water, or the shorter action of water heated above the boiling 

 point, destroys the gelatinising power of gelatin. Gelatin, in fact, under- 

 goes hydrolysis, being converted into the so-called gelatin peptones. 

 Similar substances are formed during digestion. Hofmeister distinguished 



1 Hoppe-Seyler, "Physiol. Clieni.," S. 97. 



2 Ztschr. f. physiol. Chem., Strassburg, Bd. ii. S. 315. 



3 " Physiol. Chem.," 3rd German edition, S. 46. Analyses of gelatin were made in addi- 

 tion to those quoted above by Mulder, Ann. d. Chem., Leipzig, Bd. xlv. ; Fremy, Jahresb. 

 d. Chem., 1854 ; and Paal, Ber. d. deutsch. diem. Gescllsch., Berlin, Bd. x'xv. S. 1208. 



4 Nasse and Kriiger, Jahresb. n. d. Fortschr. d. Thier-Chem., Wiesbaden, Bd. xix. S. 29. 



5 Nasse, Arch. f. d. ges. Physiol., Bonn, Bd. xli. S. 504. 



6 Salkowski, Ztschr. f. physiol. Chem., Strassburg. Bd. xii. S. 215 ; Berl. Tclin. Wchnschr., 

 1885, No. 2. 



7 Hoppe-Seyler gives () p = - 130at 30C, Nasseand Kriiger give () p = - 136t<s - 167 '5, 



