THE INTERNAL SKELETON, I7I 
tail, or sacral vertebre ; and (4) those of the tail, or caudal 
vertebre. 
The cervical vertebre vary in number from eight or nine, 
as in the Sparrow, to twenty-three, as in the Swans—some- 
times, it is said, even twenty-four. The usual number is 
thirteen, or one more or less. An ordinary cervical vertebra 
has only a very short neural spine. Each centrum is shaped 
somewhat like an hour-glass. Anteriorly it is concave from side 
to side, and convex from above downwards; posteriorly the 
reverse. Thus either end is saddle-shaped, but the saddles 
being differently disposed, each saddle-shaped surface of a ver- 
tebra fits admirably into that of the vertebra it adjoins both in 
front and behind. Vertebre the centra of which articulate 
together in this manner are spoken of as heterocelous. The 
transverse processes of either side are connected by a bony 
bridge (the pleurapophysis), whence a styliform process projects 
backwards. Thus the series of these arches on either side 
forms a lateral canal which shelters blood-vessels, and is called 
the vertebrarterial canal. The bony-bridge, with its styliform 
process which thus encloses it, is at first a separate bone, and 
is called a cervical rib. Those of the more posteriorly situated 
cervical vertebra are the more prolonged, and the last is a very 
long rib-like bone. It always, however, ends freely, and does 
not join the breast-bone. The first two vertebree differ from all 
the others and from each other. 
The first of these, the atlas, is a short ring of bone. The 
anterior surface of the ventral portion of the ring presents a 
cup into which the hindermost portion of the base of the skull 
is received. Above this there is a lateral prominence on either 
side jutting inwards, and the apices of these prominences are 
connected by a strong membranous cord, or ligament, which 
separates off the lower part of its cavity from the larger cavity 
above it for the spinal cord. This ligament may become ossified, 
and then the vertebra forms two superimposed rings, greatly 
differing in size. 
The second vertebra is called the aavs vertebra, and bears a 
strong peg-like process on the front of its centrum. This is 
called the odontoid process, and it is received into the small ring 
of the atlas just described as bounded above by a ligament. 
Round it, as on an axis, the atlas turns—bearing the head with it 
——a circumstance which gives its name to the vertebra, which is 
also known, from the process it bears, as the Os odontoideum. 
