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92 THE CONDOR Vol. XIX of which the most important are those anatomical peculiarities, which give the bird a long and flexible neck. In birds there are generally more neck vertebrae than among many other vertebrates, and these are longer than usual and have Fig. 40. NECK VERTEBRA OF CROW SHOWING THE SADDLE' SHAPED ARTICULATION. remarkably flexible joints. Fig. 40 shows the articulatory surfaces of a crow's vertebra. Th?se do not form a ball and socket joint, which theo- retically would be the most flexible possible artic- ulation, but each is convex in one direction and concave, or saddle-shaped, in the other. Several such joints in series make a neck as flexible as a similar number of ball and socket joints. Next to the flexibility of the neck as a factor in increasing the usefulness of the bill, is the rath- er flexible articulation of the mandible itself. In reptiles and birds an extra bone, the quadrate, is inserted between the articu- latory tip of the mandibular ramus and the base of the cranium. This bone in the mammalia forms the incus of the middle ear according to a generally ac- l?i. 41. a, DIAGRA? OF THE i?ONOCULAR VISION OF A CHICKEN. DIAGRA1V[ OF THE BINOCULAR VISION OF AN OWL. C. ?)IAGRA?I OF A HU?Ii?INGBIRD'S HEAD. EACH EYE WITH TWO CENTERS OF ACUTE SIGHT GIVING BOTH ?ONOCULAI? AND BINOCULAR VISION. cepted view. In most reptiles the insertion of this quadrate in the articulation of the mandible makes a true double joint, but in the birds it is so firmly fused to the cranium that the hinge of the jaw lies between the quadrate and mandi- ble. Nevertheless, the joint is perhaps freer, at least than in mammals, as it is farther removed from the base of the cranium. With this, but of considerably more importance, is the length of the bill, which removes the grasping tip far enough from the eyes at its base to bring the work of the tip within the range of vision. As one result of this we find that the very finest nests are built by such birds as hummingbirds and the longer-billed insectivorous species, rather than by the conicaLbilled sparrows, whose nests are usually loosely constructed as compared with the nests of the preceding. The smooth horniness of the bill, and the lack of teeth, are characters that permit the use of the bill as a shuttle or needle, making it an organ that can pro- duce the beautiful woven nests of such forms as the orioles, or the interesting nests of the tailor bird, with leaves sewed about them. But even with these char-