xm PHYLUM CHORDATA 435 



tion for the vertebral artery, and in the presence on the posterior 

 edge of the centrum of a little concave semi-lunar facet. 



The thoracic vertebrae (C) have elongated spines which are mostly 

 directed backwards as well as upwards. The transverse processes 

 are short and stout ; each bears near its extremity a small smooth 

 articular surface or tubercular facet for the tubercle of a rib. On 

 the anterior and posterior borders of each vertebra is a little semi- 

 lunar facet, the capitular facet (fac.), situated at the junction of 

 the centrum and the neural arch. The two contiguous semi-lunar 

 facets of successive vertebras form between them a little cup-like 

 concavity into which the head or capitulum of a rib is received. 

 The semi-lunar facet on the last cervical vertebra forms, with that 

 on the anterior border of the first thoracic, the concavity for the 

 head of the first rib. 



In the lumbar region the spines are comparatively short, and 

 both transverse processes and bodies are devoid of facets. From 

 the centrum of each of the first two (or three) projects downwards 

 a short flattened process the liypapophysis. Certain accessory 

 processes the metapophyses (met.} and anapophyses are well- 

 developed, the former being extremely long in the posterior lumbar 

 region. The metapophyses are situated in front, projecting forwards 

 and outwards over the pre-zygapophyses ; and the anapophyses 

 are situated below the post-zygapophyses and project backwards. 

 The transverse processes are long, and are directed forwards and 

 outwards ; that of the last lumbar is bifurcated. 



The sacral vertebrae are firmly ankylosed together to form a 

 single composite bone, the sacrum. The vertebrae bear a close 

 resemblance to those of the lumbar region, but the hypapophyses 

 and anapophyses are wanting, and the metapophyses are com- 

 paratively small. The first and second bear great expanded lateral 

 processes, or sacral ribs, with roughened external surfaces for 

 articulation with the ilia. These are the only sacral vertebrae in 

 the strict sense of the term, the following two being in reality 

 anterior caudal. 



Of the caudal vertebrae the more anterior resemble those of the 

 sacral region, and have similar processes ; but as we pass back- 

 wards in the caudal region all the processes gradually diminish in 

 size, the most posterior vertebra being represented merely by nearly 

 cylindrical centra. 



There are twelve pairs of ribs, of which the first seven are known 

 as true ribs, i.e. are connected by their cartilaginous sternal parts 

 with the sternum ; while the remaining five, the so-called false or 

 floating ribs, are not directly connected with the sternum. All, 

 except the last four, bear two articular facets, one on the vertebral 

 extremity or capitulum, and the other on a little elevation "or 

 tubercle situated at a little distance from this the former for 

 the bodies, the latter for the transverss prosssses of the verbebraa. 



