STRUCTURE OF BIRDS. 145 



another by a septum (fig. 81). The bones of the palate (fig. 62) 

 present four distinct types of structure, which are of considerable 

 importance for purposes of classification. The differences between the 

 four types may be studied in the recess No. 4 of the Central Hall and 

 in the wiudow-case illustrating the classification of the Carinata. The 

 lower jaw is suspended from the skull by means of a movable bone known 

 as the quadrate (fig. 5 2). The two halves of the lower jaw (fig. 5s) 

 are united in front, forming a symphysis, and are covered with a bony 

 sheath in front, like the upper jaw. 



In the vertebral column four divisions may be distinguished, viz. 

 the cervical, thoracic, syn sacral, and caudal. (1) The cervical or neck 

 division (fig. 5 5) possesses an extraordinary degree of flexibility, 

 which is necessary owing to the anterior limbs having become exclusively 

 organs of locomotion, and most of the complicated movements executed 

 by those limbs in Mammals and Reptiles have to be performed in Birds 

 by the bill. The number of vertebra; of this division varies from eight 

 to twenty-three. (2) In many birds the thoracic vertebrae (fig. 5 e) are 

 fused and form a solid bar of bone. The condition is not necessarily con- 

 nected with strong powers of flight, for though found in all Falcons it does 

 not occur in Eagles, in which the vertebrse are free. (3) Thesynsacrum 

 (fig. 5 7) is a long bone, generally formed of the last thoracic vertebra 

 and the united lumbar, sacral, and anterior caudal vertebrae, to which the 

 iliac bones are immovably attached. Although only two of the segments 

 of this series of fused vertebrae can be regarded as true sacrals, the whole 

 series is sometimes described as the sacrum and varies in number from 

 eleven to twenty. (4) Of free or movable caudal vertebra; (5 g) 

 there are eight or ten, the last being remarkable for its size, shape, and 

 function. From its shape it has been called the ploughshare-bone 

 (pygostijle). An examination of very young birds shows that in early 

 life it is composed of from four to ten free vertebrse which become 

 completely fused together in the adult, and form the single bone which 

 supports the tail-feathers and to which strong muscles are attached. 

 Thus, although the tail of a typical adult bird is apparently reduced to 

 a short series of a few segments well adapted for the execution of 

 powerful movements, embryology shows that originally this portion of 

 the vertebral column is very much longer. For instance, in the young 

 Swanit consists of not less than twenty-seven vertebrae, a condition which 

 considerably lessens the apparently great difference between the tail of 

 an adult Swan and that of a Reptile or Archaopteryx. 



All the thoracic and some of the posterior cervical vertebrae bear 

 movable ribs (fig. 5 9), their connection with the sternum bei ng effected 

 by means of short sternal ribs (fig. 5 10). In order to strengthen the 

 thorax, the ribs are connected with one another by bony spurs known 



