SKELETON 851 



thickest on the ventral side, becoming thinner dorsally and enclosing 

 a central opening, through which passes the ligamentum suspenswium 

 — being the remnant of the notochord (see page 205) and its sheath 

 — connecting the several vertebrae 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 

 vertebrae and then resemble the annuhis fibrosus of the Mammalian 

 vertebra. Lastly, when as in the sacrum the vertebrae 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 

 vertebrae 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. 



