=Fibro-cartilagines intervertebrales.=--The separate vertebræ (except
the atlas and axis) are united by the disk-shaped =intervertebral
fibro-cartilages=, which are situated between the centra of the
vertebræ. Each consists of a central pulpy portion and a fibrous outer
portion, covered by strong intercrossing tendinous fibers which unite
with the periosteum of the vertebræ.
=Ligamentum longitudinale anterius.=--On the ventral face of the centra
of the vertebræ, from the atlas to the sacrum, lies a longitudinal
ligament, the =anterior longitudinal ligament=. It is very small, almost
rudimentary, in the cervical region: large and strong in the thoracic
and lumbar regions.
=Ligamentum longitudinale posterius= (Fig. 14, _a_).--A corresponding
ligament (=posterior longitudinal ligament=) lies on the dorsal surface
of the centra (therefore within the vertebral canal). It is enlarged
between each pair of vertebræ and closely united to the intervertebral
fibro-cartilages.
=Ligamentum supraspinale.=--Between the tips of the spinous processes of
the thoracic and lumbar vertebræ extend ligamentous fibers. They are not
united to form a distinct band, and can hardly be distinguished from the
numerous tendinous fibers of the supraspinous muscles. Together they
represent the =supraspinous ligament=. From the tip of the spinous
process of the first thoracic vertebra to the caudal end of the spine of
the axis extends a slender strand representing the =ligamentum nuchæ= or
=cervical supraspinous ligament=. It is imbedded in the superficial
muscles of this region, some of which take origin from it.
Ligamentous fibers are also present between the spinous processes of the
vertebræ (=ligamenta interspinalia=): between the transverse processes
(=ligamenta intertransversaria=), and between the vertebral arches
(=ligamenta flava=).
=Capsulæ articulares.=--The joints between the articular processes are
furnished with =articular capsules= attached about the edges of the
articular surfaces. These are larger and looser in the cervical region.
=Atlanto-occipital Articulation.=--The joint between the atlas and the
occipital condyles has a single articular capsule, which is attached
about the borders of the articular surfaces of the two bones. This
capsule is of course widest laterally, forming indeed two partially
separated sacs, which are, however, continuous by a narrow portion
across the ventral middle line. This capsule communicates with that
which covers the articular surface of the dens, and through this with
the capsule between the atlas and axis. That portion of the capsule
which covers the space between the ventral arch of the atlas and the
occipital bone represents the =anterior atlanto-occipital membrane=; it
is strengthened by a slender median ligamentous strand. The =posterior
atlanto-occipital= membrane covers in the same way the space between the
dorsal arch of the atlas and the dorsal edge of the foramen magnum. In
it a number of different sets of fibers, with regard to direction and to
degree of development, may be distinguished; these have sometimes been
considered separate ligaments.
The =lateral ligaments= of the atlas begin at the lateral angle of the
cranial margin of the atlas, at about the junction of its dorsal and
ventral arches, and pass cranioventrad to the jugular processes.
=Articulation between the Axis and Atlas.=--The =articular capsule= is
large and loose, being attached to dorsal and ventral borders of the
atlas, about the articular surfaces of the axis, and to the cranial
projection of the spine of the atlas. It also passes craniad along the
ventral side of the dens and communicates here with the capsule of the
atlanto-occipital articulation. In the dorsal part of the capsule a
short strong ligamentous strand is developed, connecting the caudal
border of the dorsal arch of the atlas with the tip of the cranial
projection of the spinous process of the axis.
[Illustration: FIG. 14.--LIGAMENTS OF THE ODONTOID PROCESS OR DENS.
First three cervical vertebræ and base of the skull, with dorsal
surface removed. _a_, ligamentum longitudinale posterius; _b_,
transverse ligament of the atlas; _c_, ligamenta alaria; _d_, odontoid
process; _e_, occipital condyles; 1, 2, 3, the first three cervical
vertebræ; 4, basal portion of the occipital bone.]
The dens or odontoid process is held in place by the =transverse
ligament= (Fig. 14, _b_) of the atlas, which passes across the process
as it lies within the vertebral canal of the atlas. The transverse
ligament is attached at its two ends to the medial surface of the sides
of the atlas at about the region where the dorsal and ventral arches of
the atlas unite.
From the cranial end of the odontoid process the two =ligamenta alaria=
(Fig. 14, _c_) diverge craniolaterad to the rough ventromedial angle of
the condyles of the occipital bone.
=II. RIBS. COSTÆ= (Figs. 1 and 15.)
The cat has thirteen pairs of ribs. One of the fifth pair (Fig. 15) may
be taken as typical. It is a curved flattened rod of bone attached at
its dorsal end to the vertebral column, and at its ventral end to a
cartilage (=costal cartilage=, Fig. 15, _f_) which serves to unite it to
the sternum.
The most convex portion of the bone is known as the =angle= (_e_). Each
rib presents a convex lateral and a concave medial surface, a cranial
and a caudal border. The borders are broad dorsad and narrow ventrad,
while the surfaces are narrow dorsad and broad ventrad. The rib has thus
the appearance of having been twisted.
The rib ends dorsad in a globular head or =capitulum= (_a_), by which it
articulates with the costal demifacets of two contiguous thoracic
vertebræ. Between the capitulum and angle on the lateral surface is an
elevated area, the =tubercle=, marked by the smooth tubercular facet
(_c_) for articulation with the transverse process of a vertebra. The
constricted portion between the head and tubercle is known as the =neck=
(=collum=) (_d_). The angle is marked by a projecting process (_e_)
(=angular process=) on its lateral border, for attachment of a ligament.
[Illustration: FIG. 15.--FIFTH RIB OF LEFT SIDE, CRANIAL VIEW.
_a_, head; _b_, tubercle; _c_, tubercular facet; _d_, neck; _e_,
angle, with angular process; _f_, cartilage.]
The ribs increase in length to the ninth (the ninth and tenth are of the
same length) and then decrease to the last. They decrease in breadth
behind the fifth. The first is nearly in a dorsoventral plane, while the
others have their dorsal ends inclined slightly craniad.
[Illustration: FIG. 16.--STERNUM, VENTRAL VIEW.
_a_, manubrium; _b_, the separate pieces forming the body; _c_, bony
part of the xiphoid process (the expanded cartilaginous portion not
being shown); _d_, facet for attachment of first rib.]
The tubercles become less prominent as we pass caudad and are absent on
the last two or three ribs, which do not articulate with the transverse
process.
The first nine ribs (=true ribs= or =costæ veræ=) are attached
separately to the sternum by their costal cartilages. The last four
(false ribs or costæ spuriæ) are not attached separately to the sternum.
The costal cartilages of the tenth, eleventh, and twelfth are united to
one another at their sternal ends. They may be united also to the ninth
costal cartilage or to the sternum by a common cartilage of insertion,
or they may be quite free from the sternum. The thirteenth costal
cartilages are free (floating ribs).
=Ligaments of the Ribs.=--The articular surfaces between the head of the
rib and the centra, and between the tubercle and the transverse process
of the vertebra, have each an articular capsule. There are also a number
of small ligamentous bands from the tuberosity and the neck of the rib
to the transverse process of the vertebra.
=III. STERNUM.= (Fig. 16.)
The sternum consists of three portions, a cranial piece or =manubrium=
(_a_), a caudal piece or =xiphoid= process (_c_), and a middle portion
or =body= (=corpus=), which is divided into a number of segments (_b_).
To the sternum are united the ventral ends of the first nine ribs. It
thus forms the median ventral boundary of the thorax. Since the thorax
decreases in dorsoventral measurement craniad, the long axis of the
sternum is inclined from its caudal end dorsocraniad, and if continued
would strike the vertebral column in the region of the first cervical
vertebra.
The manubrium (_a_) makes up about one-fifth the whole length of the
sternum and projects craniad of the first rib. It has the form of a
dagger and presents a dorsal surface and two lateral surfaces, the
latter uniting ventrad to form a sharp angle. In the middle of the
lateral surface near the dorsal margin is an oval articular surface
(_d_) borne on a triangular projection. It looks caudodorsad and is for
the first costal cartilage.
The caudal end articulates with the body by a synchondrosis and presents
a slightly marked oval facet on each side for the second costal
cartilage.
The body consists of six cylindrical pieces (_b_) enlarged at their ends
and movably united by synchondroses. They increase in breadth from the
first, and decrease slightly in length and thickness. At the caudal end
of each near its ventral border there is a pair of facets looking
caudolaterad. They are for the costal cartilages.
The xiphoid process (_c_) is a broad thin plate of cartilage at its
caudal end; bony and cylindrical at its cranial end. It is attached by
its base to the last segment of the body by a considerable cartilaginous
interval, while the opposite end is free and directed caudoventrad.
The cartilage of the ninth rib is attached to the lateral face of the
cartilage between the xiphoid and the body, and just caudad of this the
common cartilage of insertion of the tenth, eleventh, and twelfth costal
cartilages is attached, if present.