24 



VERTEBRATE SKELETON 



In a number of fishes (Crossopterygii, fig. 23, Salmonids, Clu- 

 peids) another type of ribs may occur along with the haemal ribs. 

 These lie in the intersection of myosepta and horizontal septum (fig. 

 22, Bpr) and hence between epaxial and hypaxial muscles. These 

 are the true or pleural ribs, the only kind occurring in Tetrapoda. 



Fig. 23. 



-Vertebrae of anterior and posterior trunk regions of Polypterus (Gegenbaur). 

 h, haemal rib; p, pleural rib. 



These pleural ribs occur in the tails of Urodeles (fig. 22, ^) and many 

 reptiles, attached (sometimes ankylosed) to the centra which also 

 bear haemal arches. In Urodeles these caudal ribs lie in the hori- 

 zontal septum and are connected with the vertebrae by two heads, a 

 dorsal connected with the neurapophysis, the 

 ventral to the centrum, so that an opening 

 (vertebrarterial canal) through which runs 

 the vertebral artery, lies between centrum 

 and rib. 



In the higher Vertebrates the same relations 

 of the pleural ribs occur, except that in the 

 anterior part of the trunk the hypaxial muscles 

 are lacking, though often present further back 

 ('tenderloin' of mammals). In Amniotes (fig. 

 ,24) the ribs are articulated with the vertebrae, 



Fig. 24. — Schematic ^-' _ ' 



vertebra of Amniote. usually by two heads, and with the same 



bs, basal stump; c, cap- , , , . , , . .-, tt j i ^ •! 



ituiar head of rib; d, vertebrartcriai canal as m the Urodele tail, 

 diapophysis; h, haemal jhe dorsal of thesc hcads is the tubercular head 



arch; n, neural arch; p, i i • i 



parapophysis; r, pleural of human anatomy, the ventral the capitular 

 Hi ;• l"'l"".'e"raner°a'; head. These articulate with processes from the 

 canal with vertebral vertebra, a dorsal diapophysis and a ventral 



artery. , 



parapophysis. 



In mammals, where the parapophysis is reduced, the diapophysis is called 

 the transverse process and the same term is applied to the united diapophysis, 

 parapophysis and rib in the cervical region of many forms. There is the same 

 confusion of terms in the caudal region of some other Tetrapoda. 



