THE INTEENAL SKELETON. 17I 



tail, or sacral vertebrae ; and (4) those of the tail, or caudal 

 vertebrsB. 



The ce'i-vical vertebra vary in number from eight or nine, 

 as in the Sparrow, to twenty-three, as in the Swans — some- 

 times, it is said, even twentj'-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. Vertebrae the centra of which articulate 

 together in this manner are spoken of as heterocoelous. 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 vertehrarterial 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 vertebrae 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 vertebrae 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 axis 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. 

 Eound 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. 



