THE ANATOMY OF BIRDS. — OSTEOLOGY. 139 
stylets or riblets, completely anchylosed with the neural arches in adult life, and lying parallel 
with the long axes of the bones. The anchylosis of pleuropophyses distinguishes most cervical 
vertebre in another way: for from it results, on each side of the neural arch, a foramen 
(Lat. foramen, a hole, pl. foramina), through which blood-vessels (vertebral artery and vein) 
pass to and from the skull. The series of these foramina is called the vertebrarterial canal ; 
none such exist in those posterior cervical vertebree which bear free ribs; thus, in the raven the 
canal begins abruptly at the fowrth from the last cervical. But, as in Rhea for instance (and 
doubtless in many other cases), the vertebrarterial canal shades visibly into the series of 
foramina formed by the spaces between the head and shouldér of any rib and the side of the 
vertebra to which it is attached; such being, as I suppose, the true morphology of the canal. 
The cervical is the most flexible region of a bird’s spine; the articular ends of the vertebral bodies 
are the most completely saddle-shaped (heteroccelous); the zygapophyses are large and flaring, 
overriding each other extensively ; the largest processes are at the fore ends of the bones; the ap- 
positions of the central and zygapophysial articular surfaces are collectively such, that the column 
tends to bend in an S-shape or sigmoid curve. The vertebral bodies are more or less contracted 
in the middle, or somewhat hour-glass-shaped; on several lower cervicals, hypapophyses are 
likely to be well developed; as are neural spines toward both the beginning and end of the 
series. The vertebra on the whole are large; their neural canal is also of ample calibre. The 
first two cervicals are so peculiarly modified for the articulation of the skull as to have received 
special names. The first one, fig. 56, at, the atlas (so called because it bears up the head, as 
the giant Atlas was fabled to support the firmament), is a simple ring, apparently without a 
centrum. The lower part of the ring is deeply cupped to receive the condyle of the occiput 
into ball-and-socket joint. The second cervical is the axis, ax, which subserves rotary move- 
ments of the skull. It has a peculiar tooth-like odontoid (Gr. ddovs, d8évros, odous, odontos, 
tooth ; ides, eidos, form) process, borne upon the anterior end of its body, fitting into the lower 
part of the atlantal ring; about which pivot the atlas, bearing the head, revolves like a wheel 
upon an eccentric axis. The cervicals of birds vary greatly in number; according to Huxley 
there are never fewer than eight, and there may be as many as twenty-three; Stejneger gives 
twenty-four for some of the swans. Twelve to fourteen may be about an average number. 
Thoracic or Dorsal Vertebre (fig. 56, dv) extend from the cervical to or into the 
pelvic region of the spine. In most animals, and in ordinary anatomical language, a ‘ dorsal” 
is one which bears a distinct free rib, and is therefore truly thoracic, since ‘‘ ribs” are the side- 
‘walls of the chest. But in birds, as we have seen, certain cervicals have distinct elongate 
ribs; and, as will be seen soon, long jointed pleurapophyses are usually found in that region 
commonly called ‘ sacral.” The first dorsal, in birds, is arbitrarily considered to be that one 
which bears the first rib which is jointed, and which reaches the sternum by its lower (heema- 
pophysial) half. Five or six vertebra of birds commonly answer this description; though the 
last one which bears a long free jointed rib (which may or may not reach the sternum) is coin- 
monly anchylosed with the sacrum, as sr. So few as only three hemapophysis-bearing ribs nay 
reach the sternum. There may also be a long free-jointed rib which “floats” at both ends ; 
i. é., is articulated neither with the sternum uor with the vertebra to which it belongs as in the 
loon, for example. As the dorsal series thus shades insensibly behind into another series, the- 
lumbar (which has no free, nor any distinct ribs, — ribs that one would not hesitate to call 
such), it is best to consider as dorsal or thoracic all those vertebra, succeeding the last 
cervical (which is to be determined as explained in the last paragraph), which have distinct 
jointed ribs, whatever the connection or disconnection é6f such pleurapophyses at either end. 
On this understanding, one, sometimes two or even three ‘‘ dorsal” vertebrae anchylose with 
the pelvic region of the spine. Fixity of the dorsal region being of advantage to flight, these 
vertebre are very tightly locked together; not only by the close apposition or even 
