. The structure and life of birds . ck ofthe alimentary canal, whereas the opening to thelungs is from the front. By an extension two organsin the same animal are said to be homologous ; forinstance, the Humerus is homologous to the Femur,the corresponding bone in the hind limb. To proverelationship we must look for true homologies, asmere analogies prove nothing. Here are some of the most striking features commonto birds and reptiles. (1) A single condyle or rounded projection in theskull fits into a cup-like hollow in the centrum orthickened base of the first or Atlas vertebra, whichis so sh
. The structure and life of birds . ck ofthe alimentary canal, whereas the opening to thelungs is from the front. By an extension two organsin the same animal are said to be homologous ; forinstance, the Humerus is homologous to the Femur,the corresponding bone in the hind limb. To proverelationship we must look for true homologies, asmere analogies prove nothing. Here are some of the most striking features commonto birds and reptiles. (1) A single condyle or rounded projection in theskull fits into a cup-like hollow in the centrum orthickened base of the first or Atlas vertebra, whichis so short as to be hardly more than a ring of one point in the rim of the cup there is a notch, and 32 THE STRUCTURE AND LIFE OF BIRDS CHAP. this is filled by a projecting tongue from the secondor Axis vertebra, called the odontoid process, whichthus completes the cup. All mammals have twocondyles. The great freedom with which a birdmoves its head is due to the way in which, by itssingle condyle, it articulates with the \ rA° J Fig. 9.—Skull of bird (Rhea) viewed from , condyle ; , entrance of spinal cord. (2) The lower jaw articulates with a bone calledthe Quadrate, which may be easily recognised. Itroughly resembles a St. Andrews cross. To the twolower and shorter arms the lower jaw is hinged. Tothe outside corner of the outer of these is attached along thin bone, which connects with the upper outer of the two upper arms fits into a hollowin the bone called the Squamosal. In mammals thequadrate is represented by an insignificant bone, theAnnulus of the ear (fig. 10, see p. 135). (3) In mammals the centra, the strong bases fromwhich spring the arches of the vertebrae, have betweenthem plates of bone, called Epiphyses, which are Ill RELATIONSHIP TO REPTILES 33 easily distinguishable. These are absent both inbirds and reptiles (fig. n).
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