136 The Bird 
in a chicken, is in shape like a double convex lens. The 
cavity in the centre is lined with a tough yellow membrane, 
sometimes almost as hard as bone. Two great tendons 
spread over the outer surface on each side, and although 
in life forever buried in the absolute darkness of the 
bird’s body, yet when brought into sunlight they shine 
with an iridescence like the beam from a spectrum. 
It is hardly possible for the gizzard to grind up food 
in the sense of having much lateral motion, like the move- 
ment of the jaws in chewing, but it shuts together again 
and again with great force. Gravel and sharp stones are 
swallowed by many birds, and are of great importance 
in helping to grind the food. The number and size of 
these stones are sometimes almost beyond belief. I 
have known a cassowary to swallow over a quart of rubble 
in one day, and have given a quartz pebble twice as large 
as a hen’s egg to one of these birds and watched it slip 
down the bird’s throat as easily as a cube of carrot. This 
particular bird preferred smooth white quartz pebbles, 
and would search through a whole heap, picking out stones 
of this character. The same preference was exhibited 
by the gigantic extinct birds of New Zealand called moas. 
Mr. Frederick Chapman, writing of a portion of New 
Zealand where the skeletons of moas were found in great 
abundance, says: “When we came upon the ground 
disturbed by the wind (the soil being shifting sand) 
we soon found a number of distinct groups of gigantic 
gizzard-stones. It was impossible to mistake them. In 
several cases they lay with a few fragments of the heavier 
bones. In all cases they were in distinct groups; even 
