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 window-case illustrating the classification of the Carinate. ‘The 
lower jaw is suspended from the skull by means of a movable bone known 
as the quadrate (fig. 52). The two halves of the lower jaw (fig. 5 3) 
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, syusacral. and caudal. (1) The cervical or neck 
division (fig. 55) 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 vertebree of this division varies from eight 
to twenty-three. (2) In many birds the thoracic vertebre (fig. 5 6) are 
fused and form a solid bar of bone. ‘The condition is not necessarily con- 
nected with strong powers of flight, for though found inall Falcons it does 
not occur in Eagles, in which the vertebree are free. (3) The synsacrum 
(fig. 5 7) is a long bone, generally formed of the last thoracic vertebra 
and the united lumbar, sacral, and anterior caudal vertebre, to which the 
iliac bones are immovably attached. Although only two of the segments 
of this series of fused vertebree can be regarded as true sacrals, the whole 
series 1s sometimes described as the sacrum and varies in number from 
eleven to twenty. (4) Of free or movable caudal vertebra (5 8) 
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 
(pygostyle). An examination of very young birds shows that in early 
life it is composed of from four to ten free vertebree 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 
Swan it consists of not less than twenty-seven vertebra, a condition which 
considerably lessens the apparently great difference between the tail of 
an adult Swan and that of a Reptile or Archeopteryx. 
All the thoracic and some of the posterior cervical vertebree bear 
movable ribs (fig. 5 9), their connection with the sternum being effected 
by means of short sternal ribs (fig. 510). In order to strengthen the 
thorax, the ribs are connected with one another by bony spurs known 
L 
