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keel very much developed in the Carinatge, the better to permit the 

 insertion of the large muscles used in flight, The small hones of 

 the back, vertebrae, 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 vertebrae 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. Comiug 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 not 

 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 Uke a bullseye from the centre. 



