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keel very much developed in the Carinatz, the better to permit the 
insertion of the large muscles used in flight, The small hones of 
the back, vertebre, are fixed firmly to make the thorax almost 
immovable, and thus enable the great pectoral muscles to act at 
the greatest advantage. As the fore limb is used for flying, the 
beak, the flexible neck, and partly the toes are modified for seizing 
and holding. As stated before, many of the bones of birds are not 
filled with marrow as in the mammals, but with air, which is in 
communication with the lungs. This obviously lightens the weight. 
In the skull after the great size of the orbit, the most noticeable 
feature is that it articulates with the first vertebra by only one 
protuberance or condyle instead of two, which are found in mammals. 
Then the lower jaw as you see here is not articulated directly with 
the skull, but this bone called the quadrate bone intervenes. In 
the spine we have mentioned that the vertebre of the neck are very 
movable, and those of the thorax firmly fixed, each region being 
exactly suited to the requirements of a bird. As to the ribs, notice 
these hooked processes, which serve to give them still greater 
firmness, somewhat similar to the hooked barbules in the feather. 
The chest bone sometimes has deep clefts in its sides, sometimes 
not. In front of it notice the collar bones or clavicles united to 
form the merrythought or furcula. Traces of three fingers are seen, 
and no bird has more than four toes. 
In the hind limb we have the hip joint, the knee joint, and what 
we may call the ankle joint, which is a joint between the two rows 
of tarsal or ankle bones as found in man, the first row being joined 
to the leg bones, the second row to the metatarsal or foot bones, 
this joint being between the two rows. [If the leg of a bird is put 
in the position of perching, the toes are completely bent, chiefly 
because bending the ankle joint stretches the tendons that bend the 
toes, and the bending of the knee joint stretches another tendon, 
which helps to bend the toes. In this way the weight of the body, 
keeping the knee and ankle joints bent, keeps the toes tightly con- 
tracted round the perch, whilst the animal is asleep; another of 
the beautiful adaptations found in nature. Coming now to the 
eyes ; in all birds except the night birds of prey, they are placed so 
as to look right and left and not forwards. The eyeball is noi 
globular but conical, the cornea or clear part being a segment of a 
much smaller sphere than the sclerotic. This sclerotic (the white 
of the eye) is a strong and firm membrane in which a number of 
- bony plates are developed, and which encloses the more delicate 
parts of the eye. In the pigeon, which we may take as a good 
example of the average shape, the sclerotic is much the shape of 
a kettledrum, and the cornea is like a cannon ball let half into it 
on its flat side, so that it projects like a bullseye from the centre. 
