[ 3*2 3 
fpecifically belong, as, in any other cafe, to diftin- 
guifh proximate fteps in the How and impercepti- 
ble gradations of nature. 
The two extremes of true gizzard, and membra- 
nous ftomach, are eafily defined; but they run fointo 
«ach other, that the end of one and the beginning 
of the other are quite imperceptible. Similar gra- 
dations are obferved in the food ; the kinds fuited 
to the two extremes mixing together in different 
proportions, adapted to the intermediate ftates of 
ftomach. 
A true gizzard is compofed of two ftrong muf- 
cles placed oppofite, and adting upon each other, 
as two broad grindftones. Thefe mufcles are 
joined together at their fides by a middle tendon, 
into which the mufcular fibres are inferted, and 
which forms the narrow anterior and pofterior 
ftdes of the flat quadrangular cavity, in which the 
grinding is performed. The upper end of this 
cavity is filled up by the termination of the ccfo- 
phagus , and the beginning of the inteftine. The 
lower end confifts of a thin mufcular bag con- 
necting the edges of the two mufcles together. 
By a foft flexible fubftance being thus interpofed 
between the two ftrong grinding mufcles, a double 
advantage is gained ; for whilft it gives an eafy 
pafifage to the cefophagus and gut above, it allows 
of that free motion of the grinding furfaces on 
each other, which is neceffary for the commi- 
nution of the food. The half- mufcular half- 
membranous bag, which is fitted on to the lower 
end of the cavity, at the fame time that it ferves for 
a refervoir, is perhaps the only part which has the 
power 
