BIRDS. 
311 
the food is swallowed entire. When the food is flesh, the 
process of digestion is sufficiently simple and rapid to need 
no preparation ; but in the case of the hard grains and 
seeds that constitute the staple diet of so large a number 
of species, a peculiar provision is requisite for grinding — a 
sort of internal mill. 
This organ, well known as the gizzard, is endowed with 
immense power for grinding and crushing; it is almost 
wholly made up of two semi-globular masses of dense 
muscle, the two opposing faces of which are coated with a 
layer of thick leathery skin. Between these the vegetable 
substances to be ground are dropped from the crop, just as 
the corn is dropped from the hopper between the mill- 
stones; and the force exerted when these faces work on 
each other is immense, and all but irresistible, lhe faci- 
lity with which substances the most hard, angular, and 
even acute, are ground down, and that with perfect impu- 
nity to the coats of the gizzard, is proved by the researches 
of Plater, Reaumur, Redi, and Spallanzani. The experi- 
ments of the last named philosopher possess the highest 
interest : he introduced tin tubes variously strengthened 
with wire, into the stomachs of Turkeys, and invariably 
found them crushed, flattened, broken, and variously dis- 
torted. Thick halls of glass were broken, ground down, 
and in a few hours completely pulverised. Pieces of glass 
with sharp, jagged edges, shared the same fate, without in 
the least wounding the callous skin of the gizzard. Needles 
were cast into a ball of lead, so that their points projected 
a quarter of an inch, and, being encased in a soft substance, 
were thrust down the throat of a Turkey; in twenty-four 
hours the points were broken off close or rubbed down, and 
