854 SKELETON 
canals for their arteries. Movable ribs are borne by one or more of 
the lower Cervical Vertebre, thus forming a gradual transition to 
The Dorsal Vertebrx, which are composed of the same elements, 
but are marked by the high longitudinal crest into which their 
spinous process rises, while each of the dorsilateral processes of the 
neural arch (which are mostly large) possesses an articular facet for 
the tubercle of the rib, and a short ventrilateral knob near the 
anterior end of the centrum has a similar facet for the attachment 
of the head of the rib, Between the head and the tubercle is a 
large foramen, the serial homologue of the f. transversarium, protecting 
a continuation of the deep lateral strand of the Sympathetic Nervous 
System (page 626). Dorsal vertebra frequently exhibit a ventral 
outgrowth of the centrum, very variable in shape and extent. 
These outgrowths (Hypapophyses) may be simple vertical.blades, or 
i-shaped, or paired knobs—such modifications often occurring in 
the same bird—and they serve for the attachment of the thoracic 
origin of the m. longus colli anticus, reaching their greatest develop- 
ment in Sphenisci and the Colymbide. In many birds the thoracic 
vertebrae shew a tendency to more rigid junction, which is often 
effected in old individuals by the ossification of the various ligaments 
connecting the processes of adjoining vertebra, or even by the 
ossification of the attached tendons of the spinal muscles. In other 
cases consolidation is carried further by the co-ossification not only 
of the centra but also of the spinous, tranverse and zygapophysial 
processes of adjoining vertebra, so that in extreme cases the whole 
dorsal region may become one continuous mass of bone. The 
number of such synosteotically-connected vertebra varies consider- 
ably not only in closely-allied Families, genera and species, but 
even in individuals. It is however a character that with care will 
yield good taxonomic results, and thus may be depended upon as a 
common feature in many Ciconix, Gruidx, Rallide and Podicipedide. 
In most Columbxe the 15th, 16th and 17th vertebra, being gener- 
ally the three middle thoracics, are consolidated. In Crypturi and 
most Galline, in Phenicopterus and Pterocles the last cervical and 
the first 3 or 4 thoracics coalesce, and in many Accipitres the first 
4 thoracics. 
The Sacral Vertebre in the widest sense are all those that are 
overlaid by and partly fused with the iliac bones (cf. IL1UM, PELVIS) 
which are originally attached to not more than two of them situated 
just behind the Acetabulum, and are the primitive or true Sacrals. 
The iliac bones, however, during development extend considerably 
both forwards and backwards, gradually coming into contact with a 
variable number of others, which thus become presacral or post- 
sacral vertebrae, while all those that are not reached by the anterior 
extension of the ilia remain as Dorsals. Thus it follows that no 
absolute line of demarcation can be drawn in regard to these 
