6o COMPARATIVE MORPHOLOGY OF VERTEBRATES 



noted that the 'transverse processes* of the cervical vertebrae are, as in birds, 

 composed in part of reduced ribs, as will be shown below. 



Ribs 



Two different structures are included under the common name 

 of rib or costa, both connected at one end with a vertebra, the other 

 supporting the body walls around the viscera. In following for- 

 ward the haemal arches in the skeleton of a bony fish (fig. 4^, A, 

 B) it is seen that when the body cavity is reached the arch becomes 

 incomplete below, the two haemapophyses separating and coming 

 to lie just beneath the peritoneum in the walls of the coelom. Above, 

 each is either united directly to the centrum or is jointed to a small 

 process of it. More careful study shows that this fish rib (haema- 



FiG. 57. — Vertebrae and ribs of (/) anterior and (//) posterior trunk region of Polyplerus, 

 after Gegenbaur. p, pleural rib; h, haemapophysial rib. 



pophysial rib) lies in the intersection of a myoseptum with the 

 median partition of the skeletogenous tissue (p. 41) and is medial 

 to the hypaxial muscles. In the higher vertebrates the rib is formed 

 in the intersection of the myosepta with the horizontal plate, and 

 thus is lateral to the hypaxial muscles and between them and the 

 epaxial series. This is the true or pleural rib. Any vertebra may 

 bear ribs of either kind (including haemal arches) and the two kinds 

 frequently coexist on the same vertebra in the trunk of salmonids, 

 clupeids and Folypterus (fig. 57), and in the caudal region of urodeles 

 and some reptiles. Their possible occurrence in all parts of the body 

 is explained by the existence of the myosepta and other skeletogenous 

 structures in all regions. 



Another view regards haemal and pleural ribs as homologous 

 structures, but this does not commend itself. It would explain the 

 coexistence of the two kinds of ribs in the same individual by sup- 

 posing that the haemal rib had shifted dorsally and that another 

 structure arising from the stump or from a process on the medial 

 side of the haemapophysis had developed into the lower rib (fig. 58). 



