MOEPHOLOGT OP THE OWLS. 23 



case : sometimes, however, the capitulum and tuberculum and a remnant of the shaft 

 remain, and fusing with the di- and pav-apophyses of the vertebra form a bridge for the 

 vertebral artery, thus converting the cervico-thoracic into a " true " cervical vertebra. 



The sharp distinction maintained between the cervical and cervico-thoracic vertebrae, 

 by reason of the fusion of the rib-element of the former with the centrum, is a point of 

 some interest. This fusion maybe ascribed, perhaps, to kinetogenesis ; or, more vaguely, 

 to the adaptation to the peculiar and characteristic movements of the neck in birds. 

 Only in very yoving birds are the riblets of the cervical vertebrae free, but in Arcliceo- 

 pteryx they appear to have been free throughout life. The number of these cervical 

 vertebrae is, as we have already pointed out, being constantly augmented by additions 

 from the cervico-thoracic series. The latter series is fed by the thoracic, as the 

 sternum shifts further and further backwards. 



The posterior thoracic vertebrae become severed from the sternum by the reduction 

 of the articular surface for the sternal ribs, a reduction sometimes associated with a 

 reduction of the sternal plate itself. Severance from the sternum brings about, as in 

 the case of the anterior ribs, first a reduction of the sternal, and finally of the vertebral 

 segments ; the amount of the reduction being proportionate to the period of their 

 isolation. The result of the final disappearance of the last vestiges of these ribs is to 

 leave the vertebra to which they belonged indistinguishable in appearance from the 

 vertebrae of the lumbar series, with which it is generally reckoned. Occasionally, small 

 spicules of the sternal ribs remain after the complete disappearance of their vertebral 

 segments. 



The fate of this hindmost vertebra brings us to the consideration of the question of 

 reduction by excalation. 



Excalation appears to occur most frequently in the lumbar region, but also in the 

 cervical, lumbo-sacral, and caudal regions. 



In the cervical region the number of vertebrae is constantly 14. So far I have 

 found only one exception to the rule, when the number was reduced to 13 {Athene, 

 Brit. Mus. n. 1095 e). Of these 14 vertebrae the number bearing fused riblets varies 

 between 11 and 13, the remainder being cervico-thoracics bearing free ribs or vestiges of 

 them. Since in the skeleton of Athene the maximum number of thoracic and lumbar 

 vertebrae are pi'esent, there can be no doubt but that the reduction in the cervical region 

 is a real, not an apparent one. 



In the lumbar region the maximum number of vertebrae is four (fig. A, p. 28). But 

 for the correct determination of the vertebrae in this region it is necessary carefully to 

 determine the number and limits of the thoracic series ; inasmuch as the last thoracic, 

 by the loss of the vertebral segments of the ribs, become indistinguishable from the 

 lumbar series (*7, fig. B, p. 28). 



The number of thoracic vertebrae appears invariably to be 7, six of which may be 

 attached to the sternum, though frequently only four succeed in effecting such an 

 attachment. Earlier, in more primitive ancestral forms, as we have already pointed 

 out, probably as many as ten vertebrae reached the sternum ; severance from it has 

 taken place at both ends of the sternal plate. 



