• somite - 



cranial sclerotomite 



caudal sclerotomite 



Figure 6-4. Sclerotomic composition of mouse vertebrae. (After 

 Remone, 1936) 



Alligafor The vertebral column of the alligator is observed 

 to be segmented much as in the mammal. There are 9 

 cervical vertebrae; 8 of these have small ribs attached to 

 them, virhile the ninth has a large rib. There are 8 thoracics 

 bearing ribs which are attached through ventral segments 

 to the membranous or bony sternum. The last cervical has 

 a long rib, but this is not attached ventrally. There are 2 

 lumbars with short ribs and 4 without ribs, plus 2 sacrals, 

 and about 40 caudals. 



The atlas (Figure 6-5 G) is composed of 3 units, 2 neural 

 arches and an intercentrum. The arches do not meet dor- 

 sally but are articulated with the fused neural arches of a 

 preatlas vertebra, which lies in contact with the back of the 

 skull. The neural arches of the atlas articulate through the 

 postzygapophyses with the axis. The intercentrum of the 

 atlas has splint-like and separate ribs extending back on 

 either side. 



The axis is composed of the second cervical plus the fairly 

 distinct centrum of the first (suture line evident). The first 

 centrum, or odontoid process, is partly enclosed by the ring 

 of the atlas and partly exposed behind that ring, where it 

 bears a two-headed rib on either side. The axis has a large 

 neural spine with pre- and postzygapophyses. Its tranverse 

 process is much reduced, the dorsal head of the rib appar- 

 ently having been transferred to the odontoid process. The 

 posterior articular surface is a ball that fits into the socket 

 surface (procoelous) of the third cervical. In the development 

 of the axis, a second intercentrum is not observed in the 

 young alligator or caiman. 



The third to ninth cervicals are much alike having strong 

 neural spines, pre- and postzygapophyses, and transverse 

 processes articulating with the dorsal head of the short ribs. 

 The neural arch is separated from the body by an indistinct 

 suture. The body has a distinct parapophysis for the ventral 

 head of the rib, and there is a ventrally projecting hypa- 

 pophysis. All the vertebrae, except the first 2 cervicals, the 

 last presacral, the sacrals, and the first caudal are pro- 

 coelous, that is, they have a socket anteriorly and a ball 

 posteriorly. 



The thoracic vertebrae are like the cervicals generally, 

 but have a broader dorsal spine and broad, flat transverse 

 processes. The ribs all appear to be two-headed, but only 

 the first two articulate with the diapophysis of the trans- 

 verse process above and the parapophysis below. The others 

 have both heads jointed with the elongate transverse process. 

 The most posterior ribs have only a single head. The first 

 three thoracics, along with the last cervical, have hypa- 

 pophyses of decreasing length. 



The lumbars show a continuation of the slight changes 

 observed in the thoracics. The sacral vertebrae have rela- 

 tively short transverse processes, closely sutured to thick- 

 ened rib heads. The rib head is also sutured to the body of 

 the vertebra along a line from the tip of the transverse proc- 

 ess to the general position of the parapophysis. There are 2 

 such ribs on either side that suture to the ilia of the pelvic 

 girdle. 



THE VERTEBRAL COLUMN • 145 



