70 CHEMICAL CONSTITUENTS OF BODY AND FOOD. 
include keratin, elastin, collagen, gelatin, reticulin, amyloid substance, 
and a group of materials called skeletons. 
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 aciditied, collagen is 
converted into gelatin; and gelatin is reconverted into collagen by 
heating it to 130° 0. Hence collagen is regarded as the anhydride of 
gelatin (Hofmeister) ; 2 the reaction may be represented by the equation — 
( \<«H 151 JN 31 2a — H 2 = C^H^X,,* )„ 
(gelatin) (collagen) 
The above formula?, 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 76 H 1 . J4 X.. 4 2!) 
to gelatin, and regards the sulphur described by other investigators as 
due to admixture with proteid impurities. Hammarsten, :; on the other 
hand, regards the sulphur, of which there is 06 per cent., as an integral 
part of collagen and gelatin. 
( 'ollagen 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. Cheni.," S. 97. 
- 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, Jakresb. 
d. Chem., 1S54 ; and Paal, Ber. d. dcutsch. chem. GcscUsch., Berlin, Bd. xxv. S. i208. 
4 Nasse and Kriiger, Jahresb. u. d. Fortschr. d. Thier-Chem., Wiesbaden, Bd. xix. S. 29. 
3 Xasse, Arch. f. d. gcs. Physiol., Bonn, Bd. xli. S. 504. 
6 Salkowski, Ztschr. f. physiol. Chem., Strassburg, Bd. xii. S. 215 ; Berl. klin. Wchnschr., 
1885, No. 2. 
7 Hoppe-Seyler gives («V= - J30°at30°C, Nasseand Kriiger give («)p= - 136 : te> - 167 "5°, 
