_a_, vertebral foramen; _b_, centrum; _c_, caudal, and _d_, cranial,
costal demifacets; _e_, radix or pedicle; _f_, lamina; _g_, transverse
process; _h_, cranial articular facet; _i_, caudal articular facet;
_j_, caudal articular process; _k_, spinous process.]
At the caudal end of the centrum, at its dorsolateral angle, is a smooth
area on each side continuous with the surface of the epiphysis and
bounded dorsolaterally by a sharp ridge of bone (_c_). It is a =costal
demifacet=. In corresponding positions at the cranial end of the centrum
are two demifacets not limited by bony ridges (_d_). When the centra of
two contiguous thoracic vertebræ are placed together in the natural
position the cranial costal demifacets of one together with the caudal
demifacets of the other form two =costal= facets (Fig. 4, _e_), one on
each side, and each receives the head of a rib.
The dorsal two-thirds of the vertebral ring forms the =vertebral arch=
which is continued dorsally into the long, bluntly pointed =spinous
process= (Figs. 2 and 3, _k_) for attachment of muscles.
The vertebral arch (each half of which is sometimes called a
=neurapophysis=) rises on each side from the cranial two-thirds of the
dorsolateral angle of the centrum, as a thickened portion, the radix or
pedicle (Figs. 2 and 3, _e_), which forms the ventral half of the
lateral boundary of the vertebral canal. From the dorsal end of each
radix a flat plate of bone, the =lamina= (_f_), extends caudomediad to
join its fellow of the opposite side and form the vertebral arch. Owing
to the fact that the radix rises from only the cranial two-thirds of the
centrum there is left in the caudal border of the vertebral arch a notch
bounded by the radix, the lamina, and the centrum. There is also a
slight excavation of the cranial border of the radix. When the vertebræ
are articulated in the natural position, these notches form the
=intervertebral foramina= (Fig. 4, _d_), for the exit of the spinal
nerves.
At the junction of radix and lamina the arch is produced craniolaterad
into a short process, the =transverse process= (_g_), knobbed at the
end. On the ventral face of its free end the transverse process bears a
smooth facet, the transverse costal facet or tubercular facet (Fig. 4,
_c_), for articulation with the tubercle of a rib.
On the dorsal face of each lamina at its cranial border is a smooth oval
area, the =cranial articular facet= (superior articular facet of human
anatomy) (Figs. 2 and 3, _h_). Its long axis is oblique and it looks
dorsolaterad. The slight projections of the cranial edge of the laminæ
on which the facets are situated are the inconspicuous =cranial
articular processes= (prezygapophyses).
On the ventral surface of each lamina at the caudal border, near the
middle line is a similar area, the =caudal articular= =facet= (inferior
articular facet of human anatomy) (_i_); these occupy the ventral
surfaces of two projections which form the =caudal= (inferior)
=articular processes= (postzygapophyses) (_j_). These are separated by a
median notch. When the vertebræ are in their natural position the caudal
articular facets lie dorsad of the cranial facets and fit against them.
They thus strengthen the joint between contiguous vertebræ, while
permitting slight rotary motion.
[Illustration: FIG. 4.--THORACIC VERTEBRÆ, SIDE VIEW.
_a_, spinous processes; _b_, cranial articular processes; _c_,
transverse costal facets; _d_, intervertebral foramina; _e_, costal
facets; _f_, accessory processes; _g_, mammillary processes; _h_,
caudal articular processes.]
=Differential Characters of the Thoracic Vertebræ= (Fig. 4).--Following
the thoracic vertebræ caudad there is to be seen a gradual increase in
the size of the centra brought about by an increase in their
craniocaudal and transverse measurements. The dorsoventral measurements
remain nearly the same. The costal facets (Fig. 4, _e_) shift caudad so
that on the eleventh, twelfth, and thirteenth thoracic vertebræ each
lies entirely on the cranial end of its centrum, while the caudal end of
the centrum immediately preceding is not marked by any part of it. In
the eleventh thoracic vertebra each costal facet is usually still
confluent with the smooth cranial end of the centrum. In the twelfth
vertebra the facets are separated by smooth ridges from the cranial end
of the vertebra, while in the thirteenth vertebra they are separated by
rough ridges.
The spinous processes (_a_) of the first four are of about the same
length. They then decrease in length to the twelfth, while the twelfth
and thirteenth are slightly longer than the eleventh. The first ten
slope more or less caudad, while the spinous process of the tenth
(=anticlinal=) vertebra is vertical and those of the eleventh, twelfth,
and thirteenth point craniad.
Each of the transverse processes of the seventh thoracic vertebra shows
a tendency to divide into three tubercles; one of these is directed
craniad, the mammillary process (or metapophysis), one caudad, the
accessory process (or anapophysis), while the third (transverse process
proper) looks ventrad and bears the transverse costal facet. This
division becomes more prominent in the succeeding vertebræ, being most
marked in the ninth and tenth. In the eleventh, twelfth, and thirteenth
vertebræ the mammillary (_g_) and accessory (_f_) processes are very
pronounced, while the transverse costal facet and that part of the
transverse process which bears it have disappeared. The ribs of the
eleventh, twelfth, and thirteenth vertebræ are thus attached to their
respective centra by their heads alone.
The cranial articular processes (_b_) are prominent on the first two
thoracic vertebræ; back of these they are very small as far as the
eleventh, so that the articular facets seem to be borne merely upon the
dorsal surface of the cranial edge of the laminæ. In the eleventh,
twelfth, and thirteenth the cranial articular processes are large,
bearing the articular facets on their medial surfaces, while the
mammillary processes appear as tubercles on the lateral surfaces of the
articular processes. The caudal articular processes (_h_) are prominent
in the first thoracic, then smaller until the tenth is reached; in the
tenth, eleventh, twelfth, and thirteenth they are large and their facets
are borne laterally, so as to face the corresponding cranial facets.
Thus from the tenth to the thirteenth thoracic vertebra rotary motion is
very limited, owing to the interlocking of the articular processes.
=The Lumbar Vertebræ.= =Vertebræ lumbales= (Fig. 5).--The last thoracic
vertebræ form the transition to the typical lumbar vertebræ. These are
larger than the thoracic vertebræ. The centra are of the form of the
centra of the thoracic vertebræ, and increase in length to the sixth,
but the seventh is about the length of the first. They increase in
breadth to the last.
[Illustration: FIG. 5.--LUMBAR VERTEBRÆ.
_a_, cranial articular processes; _b_, mammillary processes; _c_,
caudal articular processes; _d_, accessory processes; _e_, transverse
processes; _f_, spinous processes.]
The cranial articular processes (Fig. 5, _a_) are prominent and directed
craniodorsad; they have the facets on their medial surfaces, while
their dorsolateral surfaces bear the mammillary processes (_b_) as
prominent tubercles. The caudal articular processes (_c_) are likewise
large; their facets look laterad. When the vertebræ are articulated they
are received between the medially directed cranial processes.
The accessory processes (_d_) are well developed on the first vertebra,
diminish in size to the fifth or sixth, and are absent on the seventh
and sometimes on the sixth.
The transverse processes (more properly pseudo-transverse processes)
(_e_) arise from the lateral surface of the centra; are flat and are
directed ventrocraniolaterad. The first is small, and they increase in
length and breadth from the first to the sixth, those of the last being
slightly smaller than in the sixth. The free ends of the last four are
curved craniad.
The spinous processes (_f_) are flat and directed craniodorsad. They
increase in length to the fifth and then decrease. The first five are
knobbed at the end. In a dorsal view the spinous process and cranial
articular processes of each vertebra are seen to interlock with the
caudal articular processes and accessory processes of the preceding
vertebra in such a way as to prevent rotary motion, and this arrangement
may be traced craniad as far as the eleventh thoracic vertebra.
=Sacral Vertebræ.= =Vertebræ sacrales= (Figs. 6 and 7).--The three
sacral vertebræ are united in the adult into a single bone, the =os
sacrum=, or =sacrum=. In a kitten the three vertebræ are separate, while
in an animal almost mature the first two are united and the third is
still separate. The sacrum lies between the last lumbar and the first
caudal vertebræ and articulates laterally with the two innominate bones.
It is pyramidal, with the base of the pyramid directed craniad, and is
perforated by a depressed longitudinal canal, the =sacral canal=, which
is a continuation of the vertebral canal, and by four large foramina
dorsally and four ventrally. It may be described as having a cranial end
or base and a caudal end or apex, a dorsal, a ventral, and two lateral
surfaces.
The =base= is slightly oblique and presents a smooth transversely oval
articular facet (the cranial end of the centrum of the first sacral
vertebra), for articulation with the centrum of the last lumbar
vertebra. Dorsad of this is the sacral canal, more depressed than the
vertebral arch craniad of it. It supports a spinous process (Fig. 6,
_a_) which is directed dorsad. At the junction of its lamina and radix
is seen the prominent cranial articular process (_b_) with sometimes
slight indications of a mammillary process on its lateral surface.
Laterad of the articular facet is seen the cranial face of the expanded
“pseudo-transverse process” (_c_) of the first sacral vertebra. The
ventral border of the base is concave ventrad, forming an arc of about
120 degrees. The =apex= shows the caudal end of the last sacral centrum.
Dorsad of this are the vertebral arch with a very short spinous process
(_a′_), and the caudal articular processes (_d_). Laterad of the centrum
appears the laterally directed thin transverse process (_e_).
[Illustration: FIG. 6.--SACRUM, DORSAL SURFACE.