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118 THE PROTEIN SUBSTANCES.
with water, and lastly with acetic acid. The residue is treated with cold
5 per cent hydrochloric acid for twenty-four hours, carefully washed
with water, boiled again with water, and then treated with alcohol and
ether.
In regard to the methods used by Scrwarz and by Richards and Gies, which
are somewhat different, we refer to the original publications.
Collagen, or gelatin-forming substance, occurs very extensively in
vertebrates. The flesh of cephalopods is also said to contain collagen. 1
Collagen is the chief constituent of the fibrils of the connective tissue and
(as ossein) of the organic substances of the bony structure. It also occurs
in the cartilaginous tissues as chief constituent; but it is here mixed
with other substances, producing what was formerly called chondrigen.
Collagen from different tissues has not quite the same composition, and
probably there are several varieties of collagen.
By continued boiling with wr
ater (more easily in the presence of a
little acid) collagen is converted into gelatin. Hofmeister 2
found that
gelatin on being heated to 130° C. is again transformed into collagen;
and this last may be considered as the anhydride of gelatin. Collagen
and gelatin have about the same composition.3
Collagen -50 .
75
Gelatin (commercial) .... 49.38
Gelatin from tendons.. . . 50.11
Gelatin from ligaments. . . 50.49
Fish glue (isinglass) 48.69
Gelatins of different origin show a somewhat variable composition,
which seems to indicate the occurrence of different collagens. It is diffi-
cult to say whether the variable content of sulphur is due to a contami-
nation with a substance rich in sulphur or to a splitting off of loosely
combined sulphur during the purification. C. Morner 4
has prepared
a typical gelatin containing only 0.2 per cent of sulphur by a method
which eliminated any possible changes due to reagents.
Sadikoff 5
has prepared gelatins by various methods from tendons and
from cartilage. Those from tendons, some of which were prepared after pre-
vious tryptic digestion, some after treatment with 0.25 per cent caustic potash,
and some after treatment with sodium hydroxide and then carbonate, showed
6.47
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