54 



COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



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 Polypterus, 

 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. 



The hasmapophysial ribs end freely below, never being connected with a sternum. 

 In some aberrant fishes they are lacking, while in the ostariophysi they play a part 

 in the 'Weberian apparatus' connecting the swim bladder with the ear (see ear). 

 The teleosts have, in addition, numerous rib-like structures which are not preformed 

 in cartilage (epineurals, epimerals, epipleurals) which are formed in the epaxial 

 or hypaxial regions or in the horizontal partition. 



Y \ 



V s ) 



\ ^- ^ (>! 



c 



FIG. 52. Front and side views of cervical vertebra of fowl, showing the cervical rib. 

 c, centrum; cs, spinal canal; d, diapophysis; p, parapophysis; r, rib; va, vertebrarterial 

 canal; the arrow in the side view passes through the canal. 



The typical rib (it is not certain whether this is the primitive form) 

 has two heads for articulation with the vertebra, a capitular head 

 connecting with the parapophysis, a tubercular head joining the 

 diapophysis. Between the two heads and the centrum is a space, the 

 vertebrarterial canal, through which the vertebral artery passes 

 (fig. 39, C.) The true ribs, which are preformed in cartilage, have 

 various extents in the different regions of the body. In the thoracic 

 region, where they have the greatest extension, the ribs have to allow 

 for changes in size of the contained cavity, and hence parts of them 

 are frequently left unossified, or at least they are jointed, the two parts 

 being called vertebral and sternal ribs. 



In the cervical region the true ribs are usually greatly reduced and are lacking 

 in the turtles. In many reptiles they clearly show their nature, being short, 

 bicipital and with their heads articulated to dia- and parapophyses (fig. 45). In 

 the birds they may be recognized (fig. 52), their distal ends being bent inward to 



