138 GENERAL ORNITHOLOGY. 
of the centra of such vertebre as are freely movable upon each other is highly characteristic 
of birds, in so far as the shapes of the articular ends of the vertebral centra are concerned. 
In eenlonny at large, a vertebral centrum which is cupped or hollowed at both ends, is of 
course bi-concave. Such a vertebra is called amphicelous (Gr. dui, amphi, on both sides; 
xoidos, koilos, hollowed) ; this is the rule in fishes, and obtained in some extinct Cretaceous birds, 
as Ichthyornis ; it is unknown in recent birds.1_ A centrum cupped in front only is procelous ; 
one cupped only behind is opisthocelous (Gr. émiade, opisthe, behind). Such structure neces- 
sarily results in a ball-and-socket jointing of vertebree. In those vertebree of birds in which 
this arrangement obtains, it is always the posterior face of a centrum which is cupped, the- 
anterior one being balled; such vertebra are therefore opisthocelous. But in the freest 
vertebral articulation of birds, that existing in the region of the neck, another modification 
oceurs. Both ends of each vertebra are saddle-shaped ; 1. e., concave in one direction, convex 
in the other; a condition which may be called heterocelous (Gr. érepos, heteros, contrary). 
The concavo-convexity of any one vertebra fits the reciprocal concavo-convexity of the next. 
Anterior faces of heteroccelous vertebrae are concave crosswise, up-and-down convex ; posterior 
faces are the reverse; consequently, such vertebre are proccelous in horizontal section, but in 
vertical section opisthocelous. The various physical characters of vertebrae in different regions 
of the body, and their connections with and relations to other parts of the body, have caused 
their division into several sets, as cervical, dorsal, etc., which are best considered separately. 
Cervical Vertebre (fig. 56, cv) are those of the neck: all those in front of the thorax or 
chest, which do not bear free pleurapophyses in adult life, or the free pleurapophyses of which, 
if any, are not in two-jointed pieces and do not reach the breast-bone; ¢. e., have no hema- 
pophyses. It is advisable, in birds, to draw this line between cervical and succeeding vertebra, 
no other being equally practicable; for, on the one hand, one, two or more of the cervicals 
(recognizable as such by their general conformation and free articulation) may have long free 
ribs, movably articulated; and all the cervicals, excepting usually the first, or first and second, 
have short pleurapophyses, anchylosed in adult life, but free in the embryo; while, on the 
other hand, a vertebra, apparently dorsal by its configuration and even its anchylosis with the 
dorsal series, may be entirely cervical in its pleurapophysial character.? Thus, in fig. 56, of an 
owl’s trunk, the bone which is apparently first dorsal, and is so marked (dv), bears a free 
styliform ‘‘riblet” an inch long (c’), only it is not jointed, and does not reach the sternum ; 
while the next to the last cervical has a minute but still free rib (ce). In a raven’s neck before 
me, the last cervical rib is about two inches long, articulating by well-defined head and shoul- 
der to body and lateral process of the vertebra; the penultimate rib is about half an inch long, 
with one articulation to the lateral process; while the next anterior vertebra (third from the 
last) has a minute ossicle, as a free ‘‘riblet.” The rule is two such free pleurapophyses or 
cervical ribs of any considerable length: sometimes one; rarely three; in the cassowary four. 
Rudimentary pleurapophyses may usually be traced up to the second cervical vertebra, as slender 
, 1 Except to this statement, however, the oddly-massed pygostyle, which, in birds where a terminal disc 
develops inferiorly, may be distinctly cupped at both ends, as it is in a raven for example. 
2 The case is very puzzling; the more so because, viewing the whole series of birds, the ambiguous “ cervico- 
dorsal,’ or two such equivocal vertebra, may lean in different cases in opposite directions when the whole sum of 
characters is taken into account. Therefore it may be best, as already said, to make the possession of a jointed 
sternum-reaching rib the criterion of the jirst dorsal vertebra, even though an antecedent one may have the 
physical characters of a dorsal, and be anchylosed with the dorsal series. This is the view taken by Huxley, who 
says: ‘The first dorsal vertebra is defined as such by the union of its ribs with the sternum by means of a sternal 
rib.” (Anat. Vert. Anim., 1872, p. 237.) Owen appears to regard as dorsal any of the vertebrae in question which 
bear free ribs. The actual uticertainty in the case, and the discrepant reckoning by different authors, prevents us 
from making a satisfactory count of the numbers of the two series of vertebra in any given case. Thus, fig 56, as 
marked by Dr. Shufeldt, shows six dorsals (dv), to which is to be added the one under p, bearing the rib sr; and 
from which is to be subtracted the anterior one, bearing the rib c’, which is to be regarded as cervical, though its 
physical characters are evidently those of the dorsal series. 
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