212 



comparativp: anatomy 



culum, which articulates with the transverse process. As the rib basket 



rises and falls in breathing, each rib rotates on an axis running through 



the tuberculum and the capitulum. 



Each rib has a costal groove extending along its lower or posterior 



border. To the ridges which border this groove are attached the external 



and internal intercostal muscles. 



The Development of Ribs. Ribs develop in the embryo as costal 



processes of the vertebrae in the intermuscular septa or myocommata. 



Primarily, the cartilaginous anlagen 

 of the ribs are continuous with the 

 cartilaginous vertebrae. The short 

 costal processes in the cervical, 

 lumbar, and sacral regions unite 

 with the transverse processes and 

 are indistinguishable from them in 

 the adult. In the thoracic region, 

 separate centers of ossification in the 

 ribs are formed and articulations 

 with the vertebrae develop. Epi- 

 physes at the capitulum and tuber- 

 culum make possible the elongation 

 of thoracic ribs. The ventral extrem- 

 ities of the ribs do not ossify but 

 remain throughout life as the costal 



-Diagrammatic section of a 

 vertebrate to show the relation of ribs CartilagCS 

 to the muscles of the body wall, av, 

 aorta; c, coelom; e, ectoderm; ep, epaxial 

 muscles; g, gonads; ha, hemal rib; hp, 

 hypaxial muscles; /, intestine; mes, 

 mesentery; n, nephridium; o, omentum; 

 r, true rib; p, somatopleure; sp, splanchno- 

 pleure; v, vertebra. (From Kingsley's 

 "Comparative Anatomy of Vertebrates.") 



The Evolution of Ribs. Ribs are 

 wanting in chordates below the 

 elasmobranchs, and even in elasmo- 

 branchs they occur only in the 

 anterior trunk region as short carti- 

 laginous processes lying in the 

 horizontal septum separating epaxial and hypaxial muscles. Such true 

 ribs should not be confused with the hemal arches of fishes which are 

 median to the lateral trunk muscles and adjacent to the peritoneal lining 

 of the body cavity. See Fig. 169. 



The ribs of modern amphibia show little advance above those of the 

 elasmobranchs, and in many Anura continue as short cartilaginous 

 processes of the vertebrae. But bony ribs are present in urodeles such as 

 Necturus and the attachment to the vertebrae is, as in the higher verte- 

 brates, by means of tubercular and capitular processes. In some fossil 

 amphibia the ribs were elongated and extended around the body to the 

 ventral side. Abdominal ribs were also present, as in some modern 

 reptiles. , 



