VERTEBRA AND RIBS 



25 



Another view of ribs is that they are homologous in all Vertebrata. In many 

 there is a haemal process (fig. 25) on the medial side of each haemapophysis, 

 extending between caudal artery and vein. Farther forwards, it is argued, the 

 haemapophysis divides, leaving the haemal process attached to the centrum, the 

 rest extending laterally as a transverse process. This gives the haemal process a 

 chance to expand as a haemal rib, while a segmentation of the transverse process 

 results in a pleural rib, explaining the presence of both kinds of ribs in the trunk 

 of forms like Polypterus (fig. 23). It does not explain the closed arch and the 

 pleural rib in the Urodele tail, nor how a sub-peritoneal rib can be the same as 

 one in the horizontal septum. 



Fig. 25. — Schauinsland's homology of ribs, based on Lamar gns. A, mid-caudal; 

 B, base of tail; C, middle of trunk; D, cervical, c, centrum; h, haemal processes; na, up, 

 neural arch; nc, notochord; p, haemapophysis; r, pleural rib. 



Regions of the Vertebral Column.^ — The number of vertebrae in 

 the column varies between wide limits and is correlated to some 

 extent with the length of the animal. It is smallest in the Aglossate 

 Anura where, not including the urostyle, there are eight (in other 

 Anura nine) while some limbless lizards and some snakes have nearly 

 five hundred. These vertebrae may be divided into groups or regions 

 by important characters. Fishes have two such regions, trunk, 

 where the vertebrae bear haemal ribs, and caudal where the centra, 

 at least anteriorly, support haemal arches. 



In Tetrapoda, except the limbless forms, the hind limbs are 

 attached by their girdles to the vertebral column at a point between 

 trunk and tail, the one or more vertebrae to which they are attached 

 forming a sacrum. All recent Amphibia have but one sacral verte- 

 bra. Higher in the scale (most reptiles) a second vertebra is added 

 to the sacrum, and, corresponding to the greater use of the hind 

 limbs and an increase in body size, still other vertebrae may be 

 included in the sacral region. The true sacral vertebras are not articu- 

 lated directly to the pelvis, a sacral rib intervening, which is usually 

 fused with the sacrum, but retains its individuality in a few forms 



