THE EXDOSKELETON: VERTEBRAL COLUMN AND RIBS 71 



fused interventrals. It projects forward into the ring formed by the atlas as a 

 pointed projection, the odontoid process. The atlas therefore possesses all of 

 the parts of a primitive vertebra except the interdorsal. The axis (also called 

 the epistropheus) has a large centrum, composed of the fused interventrals and 

 bears at its anterior end the odontoid process, which, as already explained, is 

 really the centrum of the atlas. The place of union of the odontoid process with 

 the centrum of the axis represents the reduced basiventral of the axis. The axis 

 has a strong neural arch (basidorsals) and broad neural spine. It bears a pair 

 of ribs, which, curiously enough, have moved forward so that they are attached 

 to the odontoid process. Hence it might be said that the atlas has two pairs of 

 ribs and the axis none. 



The vertebrae of the alligator are composed in general of the following arcualia. The 

 neural arch consists of the fused basidorsals; the centrum of the fused interventrals; basi- 

 ventrals are present only in the atlas, the axis, and the caudal vertebrae, where they form the 

 chevron bones; but they are probably represented throughout by the intervertebral cartilages 

 (absent, of course, on dried skeletons). The basiventrals are present as distinct bones, the 

 intercentra, wedged in between the vertebrae in the primitive reptile Sphenodon, which is 

 sometimes called a "living fossil" on account of its primitive characteristics, in some lizards 

 and in many extinct reptiles. Interdorsals are absent. Vertebrae like those of the alligator 

 in which the centrum is formed of the interventrals are named gastrocentrous vertebrae. 

 Gastrocentrous vertebrae are characteristic of reptiles, birds, and mammals (Fig. 22E, p. 66). 



2. Study of the ribs. — The ribs may be studied on the cervical or first 

 thoracic vertebrae. Observe that each rib is attached to the vertebra by two 

 processes, which are called heads. The upper head, or tuberculum, is attached 

 to a short blunt transverse process arising from the neural spine. The lower 

 head, or capitirtum, is attached to the side of the anterior end of the centrum. A 

 large opening is naturally inclosed by the two heads of the ribs. The successive 

 openings form a canal called the vertebr arterial canal in which blood vessels to 

 the head are located. The ribs of the third to seventh cervical vertebrae are 

 short and blunted, those of the eight and ninth cervicals considerably longer, 

 and those of the first eight thoracic vertebrae very long and curved, reaching 

 to the ventral side. Observe in the series of the thoracic vertebrae that the 

 capitular head of the rib gradually moves dorsally until it finally comes to be 

 attached to a little projection on the transverse process. The little smooth 

 places on the vertebrae to which the heads of the ribs are articulated are 

 known as facets, and rib-bearing vertebrae can always be recognized by such 

 facets. 



The thoracic ribs consist of three parts. The upper third which has the 

 heads is composed of bone and is known as the vertebral rib. The middle third 

 is partly cartilaginous and is called the intermediate rib. The lower third is 

 also partly cartilaginous and is the sternal rib or costal cartilage. The last two 

 thoracic ribs consist of vertebral ribs only. 



