VERTEBRA AND RIBS 49 



about thirty-five, there being twenty-six in some bats, forty in Cho- 

 Icspus and Hyrax and over eighty in some genera. As a rule the 

 centra are amphiplatyan, but the cervicals and some trunk vertebrae 

 of many Ungulates are opisthocoelous. Each centrum has an epi- 

 physis at either end (poorly developed in Monotremes and Sirenia), 

 and (except atlas and epistropheus), the centra do not articulate 

 directly, but a fibrocartilage disc (meniscus, intervertebral ligament) 

 intervenes, this containing the only remains of the notochord as its 

 nucleus pulposus. Intercentra are rare, occurring in the lumbar 

 region of some Insectivores. 



The neural arches, which arise from the whole length of the cen- 

 trum, are fused with the centra in the adult, the suture persisting 

 for some time in Monotremes. The spinal nerves make their exit 

 between the neural arches which are notched for their passage, the 

 notches of two adjacent vertebrae forming the intervertebral 

 foramina. 



Neural arches are present upon all except the posterior caudals, 

 while haemal arches (chev bones) occur on the anterior caudals, 

 and their homologues occur as hasmapophyses on some presacral 

 vertebrae in a few forms. The chevrons sometimes are fused with 

 their centra, sometimes articulate 

 with reduced haemapophyses and 

 occasionally are incomplete arches, 

 the haemal spine being absent. 



Excepting the caudal vertebrae 

 and passing by for the moment, the 

 cervicals, the vertebrae bear dia- 

 pophyses (transverse processes), but ^ , j, u .u 



i^ f J \ r- 'J Fig. 54. — .4, second lumbar vertebra 



the parapophysis is represented by a of dog; B, second and third lumbars of 



1 ,, J- ^ .1 , r iU Myrmecophaga (Zittel, '92). a, anapo- 



shallow facet on the centrum for the p^ysig. ,, centrum; d. diapophysis; m. 



capitular head of the rib. In some metapophysis; pz, postzygapophysis; s, 



1 /J • spinous process; z, prezvgapophvsis. 



cases the facet is divided (demi- 



facets) between the centra of two adjacent vertebrae. In the 

 presacrals the neural arch bears both pre- and postzygapophyses, 

 and in a few groups the articulation of the vertebrae is strengthened 

 by other processes, most common being a metapophysis above the 

 prezygapophysis, an anapophysis below the postzygapophysis. 

 These are most frequent in Carnivora and Edentates (fig. 54); 

 American members of the latter group often have other articulating 



