THE SKELETAL SYSTEM 



153 



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. Epiphyses at the capitulum and tuberculum make 

 possible the elongation of thoracic ribs. The ventral extremities of the 

 ribs do not ossify but remain throughout Kfe as the costal cartilages. 



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 

 cartilaginous processes lying in the 

 horizontal septum separating 

 epaxial and hypaxial muscles. 

 Such true ribs should not be con- 

 fused 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. 143. 



The ribs of modern Amphibia 

 show little advance above those of 

 the elasmobranchs, and in many 

 Anura continue as short cartilagin- 

 ous 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 vertebrates, by means 

 of tubercular and capitular proc- 

 esses. 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. 



In reptiles, ribs increase in number, and in some forms encircle the 

 abdominal cavity. Abdominal ribs are common. The ribs of snakes are 

 especially numerous. In mammals and man, ribs which articulate with 

 the vertebrae and extend around the body-cavity are limited to the thoracic 

 region. 



The Sternum. The sternum is a flat, dagger-shaped bone lying mid- 

 ventrally of the chest. 



Three parts are distinguished, i. The manubrium or presternum, 

 triangular, the widest portion and the most anterior. It articulates with 

 the clavicle. 2. The gladiolus or mesosternum, the longest portion. 



Fig. 143. — Diagrammatic section of a 

 vertebrate to show the relation of ribs 

 to the muscles of the body wall, av, 

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

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

 hypaxial muscles; i, intestine; mes, 

 mesentery; n, nephridium; o, omentum; 

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

 pleure; v, vertebra. (From Kingsley's 

 ' ' Comparative Anatomy of Vertebrates. ' ' ) 



