These are the outcome of nutritive modification of protoplasm, and may be said to be directly manufactured by that substance, and to be specially adapted to meet the requirements of certain textures differing widely in function.

They are allied to one another and to the last group by - (a) their percentage composition; * (b) containing nitrogen; (V) being amorphous colloids.

They differ from albuminous bodies in - (a) their solubility; (b) their behavior to heat, acids, alkalies and the digestive fluids; and (V) their value as food stuffs.

1. Mucin is the characteristic ingredient of the mucus manufactured by epithelial cells, and is also found in connective tissue (abundantly in that of the foetus) and in some pathological growths. It gives a peculiar thick ropy consistence to the fluid containing it, enabling it to be drawn into threads. It is precipitated by mineral acids, alum and alcohol, and the precipitate swells in water and is redissolved in excess of the acid. With acetic acid a precipitate is formed which does not redissolve in excess of the acid. When boiled with sulphuric acid it yields leucin and tyrosin.

2. Chondrin is obtained by the prolonged boiling in water of slices of cartilage cleared of the perichondrium. On cooling, this solution forms a jelly. The jelly dissolves easily in hot water or alkalies, and can be precipitated by acetic or weak mineral acids, alum or acetate of lead. It gives only leucin on boiling with sulphuric acid.

3. Gelatin is produced by boiling fibrous connective tissues, such as ligaments, tendons, the true skin and bones in water. On cooling, the fluid forms a jelly, which can be dried to a colorless brittle body which swells in cold water and dissolves on being heated. It is not precipitated by acetic acid, but yields precipitates with mercuric chloride or with tannin, as seen in makingleather. On boiling with sulphuric acid it yields glycin and leucin but no tyrosin.

* The following Table gives the composition of the principal albuminoids and albumin: -

Gelatin.

Elastin.

Chondrin.

Mucin.

Keratin.

Albumin.

c

50%

55$

47%

50%

51%

51-54%

H

7

7

6

7

6

6-7

N

18

17

14

10

17

15-17

O

23

20

31

33

21

20-23

S

0.5

0.6

3

2-2.3

4. Elastin is obtained from yellow elastic tissue by boiling with caustic alkalies. It is little affected by boiling water, strong acetic acid, or weak alkalies, but dissolves in concentrated sulphuric acid. It is precipitated by tannin, and yields leucin when boiled with sulphuric acid.

5. Keratin exists in the epidermic appendages (hair, horn, nails, etc.). It resembles the albuminous bodies in containing a considerable quantity of sulphur, but differs from them and the other albuminoids in general properties. It is soluble in alkalies, swells in strong acetic acid, gives the xanthoproteic reaction, and is insoluble in the digestive juices.