172 ELEMENTS OF OENITHOLOGT. 



The cervical vertebrse are the most moveable, and are so formed 

 as to produce by their junction that sigmoid flexure of the neck 

 before described *. 



The dorsal vertebrae 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 

 vertebrae behind it to which ribs belong in whatever way such 

 ribs may terminate at either end. "When we consider the 

 dorsal vertebrae 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 

 vertebrae, 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 vertebrae. The 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 vertebrae are termed " hollow-behind " or 

 opistkoeoehus. 



There are often median inferior processes or hypapophyses, 

 and these may bifurcate distally, as in the Penguin and Diver. 



The sacral vertebrae 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 vertebrae are anchylosed together in order to form a 

 firm point of support, or fulcrum, for the hinder or pelvic limbs. 



* See aide, p. 148. 



