SKELETON 851 
thickest on the ventral side, becoming thinner dorsally and enclosing 
a central opening, through which passes the ligamentum suspensorium 
—hbeing the remnant of the notochord (see page 205) and its sheath 
—connecting the several vertebre together. In well-macerated pre- 
parations its former existence is indicated by a pinhole-like pit 
exactly in the middle of the articular surface. The pad is frequently 
incomplete dorsally, and then being half-moon shaped, has obtained 
the name of meniscus, by which it is often known. It is morpho- 
logically the homologue of the pair of basiventral elements, which by 
their lateral extension give origin to the corresponding ribs. As in 
Birds, however, the ribs are removed backward on the centrum, and, 
attached also by the tubercle to the dorsilateral process of the dorsal 
arch, these basiventrals are relieved, so to speak, of their original 
function, and are reduced to intervertebral pads. This explains 
why these pads fuse with the anterior end of the vertebra to which 
they belong, forming there in fresh or imperfectly macerated skeletons 
a fibrous or cartilaginous non-ossified covering. Often, however, 
especially when the flexibility of the vertebral column is reduced or 
lost, the pads fuse with both the apposed surfaces of the adjoining 
vertebre and then resemble the annulus fibrosus of the Mammalian 
vertebra. Lastly, when as in the sacrum the vertebre are wholly 
ossified together, all trace of the intervertebral disks is lost. 
Besides these primary ligaments, there is a considerable number 
of additional bands (probably produced by the muscles which move 
the vertebral column) connecting the various bony processes of 
successive vertebrae with each other. It is chiefly owing to them 
that Birds can retain the neck in the well-known S-shaped curve 
without muscular exertion. 
In this place it may be more useful to treat specially the several 
vertebre in succession than to enter further upon generalities 
respecting them. 
The First Cervical, called, as in other Classes of Vertebrates, the 
Atlas—since it bears that important portion the Head—is the only 
one that retains very primitive features. It consists of three 
elements, each ossifying from its own centre. These are a pair of 
lateral pieces joining above the spinal cord to form a simple neural 
arch, without any spinous process, and a single ventral piece, 
morphologically equivalent to the pair of basiventral elements. 
The Atlas has no ribs, and with rare exceptions has no transverse 
foramina for the passage of vertebral arteries. The unpaired 
median piece is incompletely ossified, the rest of it standing up as a 
halfmoon-shaped cartilage, called the ligamentum transversum atlantis. 
It is really the first intervertebral meniscus clinging round the 
centrum of the Atlas, and fused with the two portions of the 
neural arch. These last display on their anterior surface a cup- 
shaped cavity which receives the occipital condyle of the head. 
