36 CHEMICAL CONSTITUENTS :—GELATIN, CHONDRIN. 
same is the case with the animal substance of bones from 
which the earthy matter has been removed ; and in each case 
the fibrous texture of the living tissue is but very imperfectly 
developed. For the extraction of gelatin from the skin, the 
ligaments, the tendons, and various internal membranes, 
whose fibrous texture is more pronounced (§ 29), a much 
longer action of boiling water is required. 
20. A peculiar modification of gelatin, which presents 
itself in Cartilage (or gristle), is distinguished as Chondrin. 
This requires longer boiling than gelatin for its solution in 
water; as is seen when a knuckle of veal or of mutton 
is cooked, the tendons and ligaments about the joint 
being almost reduced to pulp, whilst the cartilages are scarcely 
at all softened. The essential properties of chondrin are 
nearly the same as those of gelatin, and its composition 
seems nearly identical; but it is thrown down from its 
solution by muriatic and acetic acids' and some other reagents, 
which do not disturb a solution of gelatin. 
21. It is not yet fully known how the material of the 
gelatinous tissues is produced in the animal body. There 
can be no doubt of its being producible from albumen ; since 
we find it in large proportion in the tissues of animals that 
have never received gelatin into their bodies in any shape. 
And although carnivorous animals will receive it as part of 
their aliment, yet there is strong reason to believe that the 
gelatin which is thus supplied to them does not really serve 
to nourish their bodies, but that it is speedily decomposed 
and got rid of (§ 159). It may be considered as quite certain 
that the albuminous tissues cannot be formed by the meta¬ 
morphosis of gelatin ; whilst conversely, looking to the fact 
that in the egg and in milk no gelatin is provided for the 
young animal, although the gelatinous tissues form a yet 
larger proportion of its body than they do in the adult, we 
seem entitled to question whether it is possible that these 
tissues can be formed in any other way than at the expense 
of the albuminous constituents of the blood. 
Structure of the Primary Tissues . 
22. In considering the structure of the “ primary tissues,” 
of which the various organs of animals are composed, it will 
be convenient first to treat of those which are subservient 
