THE ENDOSKELETON: VERTEBRAL COLUMN AND RIBS 77 



The former serve for the articulation of the ribs and are probably not homologous in different 

 vertebrates. The zygapophyses yoke successive vertebrae together. 



9. The haemal arches tend to disappear or become reduced in vertebrates. They persist 

 chiefly in the tail region and may also contribute to the axis and the atlas. 



10. The vertebral column in fishes is divided into trunk and tail regions only. In 

 Amphibia a very short cervical region and a sacral region are added. In reptiles, birds, and 

 mammals, the cervical region is longer, and the trunk region is divided into an anterior thoracic 

 region, bearing long ribs, and a posterior lumbar region with reduced or no ribs. The differ- 

 ences between the vertebrae of the different regions become more and more marked the higher 

 one ascends in the vertebrate scale. 



11. Originally each vertebra was provided with a pair of ribs, but in the higher verte- 

 brates these are reduced or absent except in the trunk or thoracic regions. Reduced ribs 

 are generally present on the cervical vertebrae and always on the sacral vertebrae. 



12. Ribs are of two kinds: those that arise at the intersection of the myosepta with the 

 horizontal skeletogenous septum, known as true or intermuscular ribs; and those that arise 

 at the intersection of myosepta with the ventral skeletogenous septum or its derivatives, 

 known as false or subperitoneal ribs. The latter are characteristic of teleosts, the former of 

 all other vertebrates. Some fishes possess both kinds of ribs simultaneously and may also 

 develop additional ribs at other levels of the myosepta. 



13. Ribs typically articulate with the vertebrae by two heads, the space between the 

 heads forming a vertebrarterial canal for the passage of blood vessels. 



