SKELETON. 141 



articular surfaces directed obliquely outwards and downwards. 

 This fits into a corresponding cavity (zygantrum) on the poste- 

 rior surface of the neural spine of the vertebra in front. 



In all forms above fishes, what are known as transverse 

 processes (pleurapophyses) occur. The homologies of these are 

 not settled. In general terms there may be said to be two of 

 these on either side, a diapophysis connected with the neural 

 process, and a parapophysis connected with the vertebral cen- 

 trum. One or the other of these may excel in development, 

 and occasionally either may be rudimentary. In addition the 

 names anapophysis and meta- 

 pophysis have been given to 

 certain projections upon the 

 neural processes which seem 

 to be without great morpho- 

 logical significance. 



The vertebral column or 

 backbone is built up of these 

 vertebrae, and in this struc- 



ture two or more regions can 



always be clearly distin- 

 guished. In the fishes there FlG - '49- Anterior thoracic vertebrae of 



alligator. C, canal ; (7/7, capitular head 



are two of these regions, trunk of rib; C r, centrum; D, diapophysis; />, 



and Caudal, the Caudal being parapophysis; PO, postzygapophysis ; /Yv', 



distinguished by the presence P*ygapophysis ; S, spinous process; 



TH, tubercular head of rib ; VC t vertebrar- 



of a complete haemal arch in terial canajt 

 connection with each verte- 

 bra, while in the trunk the haemal processes diverge and be- 

 come converted into so-called ribs (see below). In the am- 

 phibia two other vertebral regions cervical and sacral occur. 

 The sacrum intervenes between trunk and caudal vertebrae, and 

 gives support to the pelvic arch by which the hind limbs are 

 supported. The trunk vertebrae bear true ribs, while the cervi- 

 cal vertebra lacks ribs and transverse processes, or these are 

 present in a rudimentary condition. The line between cervical 

 and trunk vertebrae is also loosely drawn by the girdle of the 

 fore limb. In the sauropsida (except in the limbless forms) the 

 same regions can be traced as in the amphibia ; but it is to be 



