EXPERIMENTS ON DIGESTION. 
523 
salts which it contains, the chemical property of dissolving simple 
aliments, it is clear that it can exercise the same power over the 
compound aliments, in which chemistry shews us nothing more 
than the simple nutritive materials differently combined with one 
another. 
As to what relates to the digestibility of the different aliments, 
according to this theory they are more easily and quickly digested 
in proportion as their peculiar composition renders them more 
soluble in the gastric juice. Then sugar, gum, liquid albumen, 
and gelatine, are easy of digestion, because they most readily dis- 
solve in warm water. Aliments that require the aid of acids to 
dissolve, as those which contain much gluten, concrete albumen, 
fibrine, and caseum, are difficult of digestion ; while substances 
which the gastric juice cannot dissolve, as the hard fibres of plants, 
or of wood, the envelopes of certain leguminous seeds, the ker- 
nels and the stones of fruit, hair, feathers, &c. are indigestible. 
The absolute digestibility of aliments depends on their proper- 
ties and their composition : but there is a relative digestibility 
also to be distinguished, as it regards the composition and solvent 
power of the gastric juice of different animals. In general the 
gastric juice of the carnivora is much less active than that of the 
herbivora. This enables us to understand why the former can 
well digest animal substances, and vegetable substances which are 
easily dissolved, but cannot digest the grosser vegetable produc- 
tions, such as uncooked herbs, grasses, and straw. On the other 
hand, these latter substances are digested by herbivorous animals, 
who have, for the most part, a more complicated digestive ap- 
paratus. 
But although the gastric juice be, in virtue of its chemical com- 
position, the solvent of both simple and compound aliments, and 
its action on these substances is chemical, digestion is nevertheless 
a vital operation — one that which has for its object the growth and 
life of the animal, while the stomach secretes the solvent, the gastric 
juice, by means of the vital power with which it is endued. In 
order that this secretion shall take place, it is necessary that the 
stomach should be in a healthy state, and possess those cer- 
tain particular conditions of form, and of organic composition, 
which alone will permit the full exercise of its functions. It is 
also necessary that it shall possess the faculty of being affected or 
excited by the aliment which it receives into its interior, so that it 
shall have power, in consequence of this stimulation, to extract 
from the blood which is alkaline an acid and solvent gastric juice. 
Finally, it is necessary that this organ should have the power, by 
its own proper muscular movement, to pass the aliment which has 
been dissolved and digested — the chyme — through the pylorus 
