Fig. 8.--Caudal Vertebra, from near the caudal end of the tail.
Fig. 9.--Fourth Caudal Vertebra, ventral view. _a_, transverse
processes; _b_, cranial articular processes; _c_, hæmal processes;
_d_, chevron bone.]
The parts of a typical vertebra--vertebral arch, transverse processes,
cranial and caudal articular processes--may be recognized in the
vertebræ as far back as the eighth or ninth. The transverse processes
(Fig. 9, _a_) are directed caudad and decrease rapidly in length. They
are very small on the ninth vertebra, but may be recognized for a
considerable distance back of this. The spinous process disappears at
about the fourth caudal vertebra, and the vertebral canal becomes
gradually smaller caudad, until on the eighth or ninth vertebra it
becomes merely a groove open dorsad.
Caudad of the third vertebra for a considerable distance, each centrum
bears on each lateral face at its cranial end a short =anterior
transverse= process, and on its ventral face at its cranial end a pair
of rounded tubercles, =hæmal processes= (_c_), which articulate with a
small pyramidal =chevron= bone (_d_) so as to enclose a canal. These
structures disappear caudad.
=Cervical Vertebræ.= =Vertebræ cervicales= (Fig. 10).--The cervical
vertebræ number seven. The first two of these are so peculiar as to
require a separate description, so that the last five may be first
considered.
Passing craniad from the fourth thoracic vertebra to the third cervical
there is a gradual transition. The centra of the cervical vertebræ are
broader and thinner than those of the thoracic vertebræ, while the
vertebral arches and vertebral canal are larger (Fig. 11). The caudal
end of each centrum is concave and looks dorsocaudad when the centrum is
held with its long axis horizontal. The cranial end of the centrum is
convex and looks ventrocraniad when the centrum is horizontal. These
peculiarities are more marked in the third vertebra than in the seventh.
The spinous processes grow rapidly shorter as we pass craniad; the
fifth, sixth, and seventh are directed dorsocraniad, the third and
fourth dorsad.
[Illustration: FIG. 10.--CERVICAL VERTEBRÆ, SIDE VIEW.
_a_, spinous processes; _b_, cranial articular processes; _c_, caudal
articular facet; _d_, intervertebral foramina; _e_, transverse process
proper; _f_, processus costarius; _g_, wing of the atlas; _h_, dorsal
arch of the atlas; _i_, atlantal foramen.]
The caudal articular processes are situated at the junction of the
radices and laminæ; their facets (Fig. 10, _c_) look ventrocaudolaterad.
The cranial articular processes also become more prominent than is the
rule in the thoracic vertebræ; they are borne at the junction of radix
and lamina and have their facets (Fig. 11, _b_) directed dorsomediad.
The cranial and caudal articular processes of each side are joined by a
prominent ridge which is most pronounced in the third, fourth, and fifth
vertebræ.
The characteristic feature of the cervical vertebræ is their transverse
process, so called. In each of them it arises by two roots, one from the
centrum and one from the arch. These two roots, which are broad and
thin, converge and unite so as to enclose a canal or foramen, the
=foramen transversarium= (Fig. 11, _g_), for the vertebral artery.
Laterad of the foramen the two parts of the process are, in the third
cervical, almost completely united, the dorsal part being, however,
distinguishable as a tubercle at the caudolateral angle of the thin
plate formed by the process as a whole. This dorsal component is the
=transverse process proper= (Figs. 10 and 11, _e_), while the ventral
portion represents a rib, and is hence known as the =processus
costarius= (_f_). The expanded plate formed by the union of these two
processes is directed nearly ventrad and somewhat craniad in the third,
fourth, and fifth vertebræ. The two components of the process gradually
separate as we pass caudad; in the fourth and fifth vertebræ the part
which represents the transverse process proper forms a very prominent
tubercle at the caudolateral angle of the plate formed by the processus
costarius. In the sixth (Fig. 11) the two parts are almost completely
separated; the dorsal part forms (_e_) a slender knobbed process, while
the processus costarius is divided into two portions (_f_ and _f′_) by a
broad lateral notch. In the seventh the ventral part (processus
costarius) is usually quite lacking, though sometimes represented by a
slender spicule of bone. In the former case the foramen transversarium
is of course likewise lacking.
[Illustration: FIG. 11.--SIXTH CERVICAL VERTEBRA, CRANIAL END.