172 ELEMENTS OF ORNITHOLOGY. 
The cervical vertebrae are the most moveable, and are so formed 
as to produce by their junction that sigmoid flexure of the neck 
before described *. 
The dorsal vertebre are very much less moveable, and generally 
some of them are anchylosed together. They are usually about 
seven or eight in number, but there may be only five or as many 
as eleven. They are very different in shape from the cervicals. 
They consist of the first vertebra which bears a jointed rib 
directly articulating with the breast-bone, together with all the 
vertebre behind it to which ribs belong in whatever way such 
ribs may terminate at either end. When we consider the 
dorsal vertebree with the ribs attached to them, we see at once 
the rib-like nature of the cervical ribs—the hindmost of which 
is very much prolonged, though it does not attain the breast- 
bone. Each such vertebra is, generally, shorter than a cervical 
vertebra, they are also broader as seen dorsally, but their centra 
are much narrower from side to side than those of the cervical 
vertebra, being compressed so as to form a median ventral ridge. 
Above, they bear high, broad, and thin neural spines in the 
form of squarish plates, which very often become anchylosed 
together. 
There is a broad upper transverse process—* diapophysis "— 
to which part of the rib (called the “tubercle”) is attached, 
and there is below it an articular surface for the proximal end 
of the rib (called the ‘“ head”), which surface may be taken to 
represent the lower transverse process—‘ parapophysis ”—of 
the cervical vertebree. ‘lhe centra articulate together by saddle- 
shaped surfaces in all existing birds save the Penguins, in which, 
from the third dorsal backwards, each centrum exhibits behind 
a hemispherical cup, into which is received a hemispherical ball 
belonging to the anterior surface of the centrum of the vertebra 
next behind it. Such vertebra are termed “ hollow-behind ” or 
opisthocelous. 
There are often median inferior processes or hypapophyses, 
and these may bifurcate distally, as in the Penguin and Diver. 
The sacral vertebre of birds are very numerous. They con- 
stitute what is called the sacrum, and it is a very extensive 
structure. A “sacrum” is that part of a vertebral column 
where the vertebre are anchylosed together in order to form a 
firm point of support, or fulcrum, for the hinder or pelvic limbs. 
* See ante, p. 148. 
