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SECT. XIX: PHYSIOLOGY. 157 
the ingesta upon each other, like fermentation, — 
In their time, chemistry was in its infancy, and 
fermentation little understood, but this opinion 
was refuted by facts of which they were even in 
possession. Digestion is usually accomplished in 
four or five hours ; fermentation, whether vinous, 
acetous, or putrefactive, requires days, even in the 
most favourable circumstances; and its products 
are either alcoholic and acid fluids, or carbonic 
acid gas, substances not very liable to be con- 
founded with chyle and feces. 
cccLxxx1x. Other physiologists referred diges- 
tion to the mechanical friction of the parietes of 
the stomach on the alimentary mass. The argu- 
ments in favour of this theory were chiefly drawn 
from observing the power with which the gizzards 
of gallinaceous birds pulverize hard substances. 
The trituration of birds gizzards is, more properly 
speaking, mastication than digestion of aliment, 
for no’ modification of mechanical force can con- 
vert it into chyle. 
ccexc. In 1752, Reaumeur published two 
papers in the Memoirs of the Royal Academy of 
Paris, which introduced new and sounder views 
of the digestive process. He put pieces of flesh 
into tubes open at both ends, for the purpose of 
admitting the free access of the gastric juice: 
birds of prey were forced to swallow the tubes 
thus prepared; and the pieces of flesh having in 
