DIGESTIVE SYSTEM. OSp 
vr pounding, performed by the strong muscular gizzard, 
assisted by stones which have been swallowed, and.are re- 
tained on purpose. In cther animals this trituration «is 
accomplished by hard bony parts in the stomach, as among: 
some of the crustacea. These expedients supply the de- 
ficiencies of imperfect mastication, and enable the gastric 
juice, and the other secretions, to become more intimately 
mixed with the food. What, then, are the changes which 
the food undergoes in the stomach ? 
Without attempting to give a history of the opinions 
which have been advanced .on this subject, and expressed 
by the terms Concoction, Putrefaction, Fermentation, Disso- 
lution, and Maceration, let us attend to the phenomena. The 
food, after having been mixed with the saliva and gastric 
juice, and exposed to the heat and motion of the stomach, m 
-a short time begins to exhibit changes in its mechanical pre- 
perties. That some changes likewise take place of a che- 
mical nature appears obvious, from the circumstance that 
an acid is produced. This has been considered by some 
as the phosphoric, by others as the acetic or carbonic acid. 
This much seems to be determined that it is a volatile 
acid ; asits effects on litmus paper are temporary. But the 
mechanical effects are the most remarkable. The appear- 
ance of the food by degrees changes, and the whole is re- 
-duced to a somewhat uniform and pultaceous mass, termed 
chyme. This mechanical change takes place from the sur- 
face to the centre of the contents of the stomach; and by 
means of the movements of the organ, the reduced portion 
is withdrawn as it is prepared, and passes out at the pyloric 
opening of the stomach. 
After all this preparation, we might expect that the .nu- 
tritious portion should now be withdrawn and enter the sys- 
tem. But other changes are still necessary. ‘The pylo- 
rus opens into the duodenwm, which constitutes the com~ 
